Why Ventilate?

– Create a comfortable environment in terms of CO₂ & humidity.

– Purge pollutants such as VOC and NOX.

– Combat condensation.

Moisture Production:

Our home is an important part of our everyday life, and good air quality contributes to a healthy living environment.

Good air quality is achieved through effective ventilation . An effective ventilation strategy incorporates extract ventilation from “wet rooms” (such as a kitchen or utility) to remove stale air, supply ventilation to habitable spaces (living rooms or bedrooms) to provide fresh air, as well as purge ventilation (achieved by opening a window e.g.) to rapidly remove stale air and odours.

Until recently, the structure of the house itself would allow enough air to infiltrate through gaps in windows and wall, in addition to passive wall vents. However, as building methods and materials improved,  and energy efficiency standards became tighter, this infiltration was reduced to a minimum.

Excessive ventilation greatly reduces the energy efficiency of a building by increasing the load on the heating system.

Fresh air drawn in from the outside must be heated to a comfortable temperature by the building’s heating system. Stale air, exhausted to the outside, carries heat energy away from the building. Excess ventilation therefore, can reduce the energy efficiency of a building.

New methods of air supply are required. These methods of ventilation must balance competing for the demands of comfort and hygiene, with the requirement to reduce energy consumption.

System 1:

Intermittent Extract Fans & Background Ventilation.

How does it work?

This system comprises of background ventilators such as trickle ventilators fitted to windows, or standard hole-in-the-wall ventilators, with extract fans fitted in wet rooms. The background ventilators act to supply air to habitable spaces, while intermittently operated extract fans provide the extract ventilation removing odors and excessive humidity from wet rooms.

System 2: 

Passive Stack Ventilation and background ventilators.

How does it work?

A passive stack is a non-mechanical approach to ventilation, where air vents are located in various locations around the dwelling. Using the principle of convection, currents allow the movement of air through the ducts.

With passive stack system, airflow rates are very much weather dependant. Strong gusts can cause over ventilation and surges in ventilation rates; little or no wind may result in inadequate ventilation.

The system is completely uncontrolled. Large amounts of cold air can be drawn into the building, increasing the heat load.

System 3:

Demand Control Ventilation.

Demand-controlled ventilation (DCV)  automatically regulates ventilation based on actual demand using a suitable sensor. DCV can be triggered by occupancy sensors, moisture sensors or air-quality sensors detecting carbon dioxide or other pollutants, or a combination of these.

The difference between DCV and basic intermittent ventilation is that DCV operates automatically, without requiring any manual user intervention.

How does it work?

A central fan provides a continuous background extraction rate. A controller in the fan maintains a constant air pressure in the system, increasing or decreasing the fan speed as required. Humidity sensitive extracts located in wet rooms open and close depending on room humidity. If a particular room has high humidity, the extract opens.

When the extract opens, this causes a pressure drop in the system. The fan automatically increases speed to return to the target pressure.  The more extracts open, the higher the extract rate is.

Other models of extractor can be triggered by occupancy, carbon dioxide or other air contaminants.

Coupled with the boosted fan speed, this combination means that stale air is extracted only from areas where air quality is below the required standard.

The system ventilation rate is always matched to the actual demand, reducing energy consumption by fans and reducing overventilation and heat energy consumption.

System 4: 

Continuous Mechanical Supply & Extract with Heat Recovery.

Mechanical Ventilation with Heat Recovery (MVHR) continually removes stale moist air from wet rooms, while supplying a balanced amount of outside air directly to habitable rooms.

The difference between MVHR and other ventilation systems is that with MVHR, the heat energy carried by the stale air is used to partially heat the fresh intake air through a heat exchanger.

This reduces the demand on the building’s heating system and minimises the loss of heat to the outside atmosphere while maintaining fresh clean air.

How does it work?

Stale, humid air is extracted from wet rooms at a continuous rate. This air is carried through ductwork to a central ventilation machine where it is passed through a heat exchanger element before being exhausted through the

Fresh cold air is drawn in from the outside through an intake grille. It is then passed through the heat exchanger where it is heated by the stale exhaust air. The fresh, warmed air is then supplied through sealed ductwork to the habited rooms of the house.

The supply rate and extract rate are balanced to maintain comfortable air pressure and reduce draughts.

It is important to note that the intake and exhaust air streams never mix.

Pressurised system:

The vast majority of Solar Thermal systems installed in Ireland are of the pressurised variety. The solar circuit is completely filled with a heat-transfer fluid, which is then pressurised to increase its boiling point. A circulating pump drives this fluid around the solar circuit, transferring heat from the solar collector to a storage tank. An expansion vessel is fitted to damp out changes in fluid volume caused by thermal expansion throughout the day, maintaining a steady system pressure.

Safety is provided in the form of a blow-off valve on the pump station, which, in the event of a system over pressurisation, will allow some of the heat-transfer fluid to escape to a catch tank.

Click here to view a detailed schematic of that type of system.

The Problems of Pressurisation:

Pressurisation of the solar circuit comes with its own drawbacks. The most notable drawback is Stagnation. The collector is always filled with fluid, even when the system is not actively circulating.

If the system settles into stagnation on a hot day, continued heat production in the collector will raise the temperature of the heat transfer fluid to the point where it begins to boil.

This has the potential to cause the system pressure to increase, ultimately causing the system blow-off valve to activate and dump solar fluid, leading to a loss in pressure when the system cools. Sustained high temperatures can also damage system components, while accelerating wear and tear.

Stagnation mitigation solutions such a heat dump do not entirely eliminate the risk. If the heat dump has not been correctly sized to match the power output of the solar collector then the system still has the potential to overheat. If the system should lose electrical power, there will be no pump to drive the circulation of the fluid, allowing the system to stagnate anyway.

Drainback system:

A drainback solar system consists of a circulating pump, solar panels, a drainback vessel, a controller and some form of thermal store. Nothing else is required.

Drainback functions on the same basic principal as a conventional pressurised system. A circulating pump pushes water up to a solar collector on the roof, where it absorbs heat from the collector, then carries it back down to the hot water store.

The difference between drainback, and pressurised solar systems is that, when the circulating pump shuts off, heat-transfer fluid is allowed to drain under gravity from the solar panels down into a dedicated drainback storage vessel. The solar circuit is unpressurised.

This drainback vessel is mounted such that the fill level is somewhere above the level of the circulating pump, ensuring the pump never runs dry and that no air can enter the circuit. A good gravity flow is also required between the solar panels and the drainback vessel to ensure they properly drain.

Click here to view a detailed schematic of that type of system.

No overheat protection or anti-freeze is required.

The chief advantage of Drainback solar is that, when the circulating pump is not operating, the collector is dry. In summer, there is no fluid in the collector to boil. In winter, there is nothing in the collector to freeze. Thus, the system cannot enter stagnation and cannot freeze solid.

The requirement for a heat-dump and anti-freeze is eliminated, simplifying the installation. There is no longer a need for holiday recooling functions and frost-protection modes within the solar controller. The system is also passively safe – should the pump lose power, all fluid will drain from the collector to the drainback vessel as normal.

The lack of pressure minimises the potential for leaks. Pipe corrosion and seal degradation due to acidic antifreeze is eliminated, improving system reliability and robustness.

Drainback Solar is a simplified, safe, robust and reliable solution.

Currently, Greentherm are the only company in Ireland offering Drainback solar thermal systems. Geentherm systems are based around Veridian ‘Clearline’ solar panels. These are one of the few flat-plate panels that are specifically designed for the Irish climate. All Clearline panels and flashing come a BBA agreement cert, verifyingtheir suitability for use as a roofing material.

For more information on Viridian solar panels Click Here

Underfloor Heating is a proven technology which offers attractive benefits when integrated as part of a heating solution into residential or commercial buildings. By its nature it is a radiant heating system as opposed to convection based solutions such as radiators.

Radiant systems heat the people and objects in the room as opposed to the convective ‘top down’ stratified heat of radiators. The result is evenly distributed heat, resulting in lower operating costs and greater comfort.

In new build construction of residential units wet screed systems are the preferred method of installation on the ground floor. Upper floors can pose some design issues, namely structural requirements in timber frame construction or joisted upper floors require significant structural enhancement to allow a screed to be poured.

The potential alternative solutions to install underfloor heating in areas of light build construction are as follows:

                                      Screeded solution

As introduced above, the building structure is designed to facilitate the application and mass of screed, and in addition to an insulation layer either through the use of precast flooring, structural joists etc.

Modular composite wooden flooring boards

These systems are factory manufactured chipboard or ply panels with integrated under floor piping. The panels are typically placed on top of the joists in place of the floor boards, and typically laid in back to back arrays. Piping protrudes from one end of each flooring panel. The panels are piped in series within the floor joists with a main flow and return piping running along the end of the panel. The finished floor is laid on top of this product.

Piping and aluminium diffuser tray

In this configuration battens are applied a set distance below the top of the joist and plywood is used to form a carrier layer between the joists. Insulation is manually cut and inserted between the joists on the carrier plywood which fills the void almost to the top of the joist.

The insulation is routed continuously along the centre line. The aluminium diffusion tray is laid parallel to the joist span on top of the insulation. The routed channel is designed to accommodate the u-channel in the centre of the aluminium tray. This u-channel in the diffusion plate provides a recess to take the underfloor heating pipe. At both ends the joist is routed to form a notch to allow the pipe to turn 180 degrees.

EPS panels with aluminium diffusion layer

This solution is available in configurations as low as 13mm deep. The EPS panels contain a series of factory cut groves which run along the linear length of the panel to hold the pipe. The complete panel including the grove has a factory applied aluminium foil diffusion layer. The layer ensures an even distribution of heat and prevents heat deflection the zones below.

The floor joists are covered in plywood or a similar substrate and the EPS panels are bonded to the ply substrate layer. The pipe is then laid into the panel groves. Several manufacturer approved tile adhesives can be applied directly to the product and tiles can be applied directly on top. To facilitate the use of a soft floor finish eg carpet or cork a 6mm floating plywood floor can be laid on top of the EPS panels. Wooden floors can be laid directly onto the panels as a floating floor.

Contact us to find out which system is most suitable for your project. 

More and more these days, Solar Panels or other renewable energies such as Heat Pumps are being combined with solid-fuel and back-boiler systems. Either as a new build, or as part of a retrofit. It wouldn’t be unusual to have a system combining solar, an oil boiler and a solid fuel stove into the one tank – one like in the diagram below:

Both the solar and the boiler circuit are pumped circuits – water is pushed around them by a circulating pump. The stove circuit is different; there’s no pump. For safety reasons it uses a gravity flow to heat the tank. The temperature difference between a hot stove and a cooler tank creates a natural circulation of water between the two. This is sometimes called a Thermosiphon.

This, however, can cause some unusual problems.

The Cold Stove Problem.

On good days the solar panels will heat the bottom of the tank, warming the water inside. On other occasions, this water may be heated by the boiler. Especially if it is warm out, the stove will likely not be lit – so will stay cold. In the diagram above, its outlet temperature is a cold 20°C.

The temperature difference between the heated water in the tank and the unlit and still cold stove will be high enough that circulation between the two begins. This is the same Thermosiphon effect, but operating in the reverse direction. The cold stove will begin to leach heat out of the tank, cooling it down. This heat is lost either up the flue, or into the air in the room where the stove is located

If this heat came from your solar panels, then you will have to make up for the loss with an immersion heater or by calling in the oil boiler. If it came from your boiler alone, then you will have to run your boiler for longer.

Either way this is costing you money. And nobody likes the surprise that comes from turning on what was expected to be a nice-hot shower, only to be met by a rush of lukewarm at-best water.

A Solution?

It seems like the quick and easy solution is to simply swap the solar and stove heating connections at the tank. This will mean that the stove is now heating the entire tank from the bottom. The solar will now only heat the top half of the tank. The modification would look like this:

The bottom of the tank will remain cool unless the stove is lit, meaning there will be no temperature difference to start the Thermosiphon. This solves the Thermosiphon problem, but introduced a new one.

This approach effectively halves the amount of hot water available from the solar panels. Whatever would’ve been lost to the Thermosiphon, is never received in the first place. Instead of losing hot water, now you never have it. Also, if this is a modification to an existing system, you would have to factor in the cost of hiring a plumber to do the work, or the time to do it yourself. Three to Five hours would be a good minimum for a skilled plumber.

Clearly, not an ideal solution. It solved one problem, but in a way creates another one in the process.

The Greentherm Way.

At Greentherm, we’re an engineering company first and foremost. We’re all engineers here. What this means is, we’re always looking for the best solution to a problem. Both in terms of cost-effectiveness, and long-term efficiency.

A customer came to us with a Thermosiphon problem on their system. This was the solution we offered:

A Solid Fuel Loading Valve

A Solid Fuel Loading Valve is a device which is designed to automatically regulate the flow of water coming out of a solid-fuel boiler or stove. The purpose of the loading valve is to allow the stove to heat up quicker and burn hotter, reducing coking of the flue and minimising internal corrosion. It does this by diverting the flow of water coming from the stove back towards the stove in a short loop, bypassing the rest of the heating system.

This diagram shows how the valve operates. When cold, the valve switches to divert, recirculating water back to the stove rather than through the tank:

When it warms up, the valve switches open, allowing water to flow to the tank, as seen below.

But what relevance is this to our Thermosiphon problem?

When the solid fuel stove in the above system is cold, the Loading Valve will be closed. It will effectively isolate the stove in its own short loop until the stove heats up. This means that cold water in the stove cannot flow towards the tank. This also means that warm water from the tank cannot flow back to a cold stove.

Therefore, the modification we ultimately made to the original system above, looked like the following:

If the stove is cold, the valve is switched to divert. When the stove is lit the valve will open once the water has reached a high enough temperature, allowing heat to move up from the stove to the tank. If the stove cools down, the valve closes, preventing circulation through the tank.

It’s that simple.

In a nutshell

Fitting a solid fuel loading valve to the system eliminated heat losses from a Thermosiphon. The efficiency of the system was improved. The Loading Valve also improved the combustion efficiency and lifespan of the stove by fulfilling its original function.

Physically adding the valve and divert loop took a competent plumber two hours. Including the cost of materials, this solution not only worked out cheaper for the customer to implement, but left her with a much more efficient system to boot.

Most importantly, this solution is safe*. The Solid Fuel Loading Valve is fully automatic. It isn’t reliant on electrical power or any external control unit- it’s operated solely by the heat in the water. This valve won’t be left stuck in position by a power failure, or quietly forgotten when the homeowner goes to sleep.

Problem?

Contact Us now to see how we can provide similar solutions to your heating and renewable energy problems.

If you have the same problem with a system loosing heat through a cold gravity circuit purchase Solid Fuel Loading Valves from us here, and implement the above solution yourself.

*Provided the System has been installed in accordance with regulations and industry best-practices.

Multi-Energy Jaspi 500L Thermal Store.

Only €1175!^*

Tanks, built like a Tank.

• Heavy Duty Construction

Jaspi Thermdal Stores are manufactured from thick sheets of high-quality steel, guaranteeing a long, leak-free service life under demanding conditions.

• Suitable for Heat Pump, Solid Fuel and Solar

Flexible tappings allow for the combination of any domestic heat sources for maximum efficiency and reliability. An internal baffle ensures that low temperature heat sources in the tank bottom will always have work to do.

• Domestic Hot water and Central Heating storage in one tank

A pair of indirect domestic hot water coils instantaneously create hot water only as it is needed, for maximum system efficiency.

• Leak Detecting Tappings

All tappings complete with Leak-Detect couplings, making any problem with sealing on installation clearly visible, verifying the integrity of the installation.

• Space Efficient Design

Designed specifically to fit through the average household door and maximise storage volume using the minimum amount of space.

• Insulation Ensures Efficiency

Designed with frigid Finland winters in mind. Each Store comes with high-density Polyurethane foam insulation, baked on for outstanding efficiency and thermal performance.

• Experienced engineering support.

We know our products and understand the customer’s needs. We have the in-house know-how to do what it takes to ensure your specifications are met.

• Special Introductory Pricing available Until December 31^st

Click Here to visit our online store and order.

Peace of Mind is Priceless.

There is nothing more expensive than a cheap thermal store. What’s saved in the purchase price is quickly lost to recalls, repairs and replacements. No matter your application, your Jaspi Hybrid thermal store can be trusted to give decades of hassle-free service.

Call us now, at 01 5314781 for no-obligation sales and technical inquiries. For more detailed information on the Jaspi range, send a message to info@greentherm.ie.  Visit our website at www.greentherm.ie for more information on other products we offer.

-The Greentherm Team

*Until June 30th 2017. Terms and conditions apply

Will it Fit? Cylinder Sizing and You

In our first post, we compared two different heat-sources, a heat-pump, and a condensing boiler. We took a look at the different performance characteristics, capabilities and expected running costs of each

In our second, we saw how the choice of heat-source could affect the temperatures we store water at, and what affect changing the storage temperature had on the amount of water we needed to store to get the same performance from the system.

Here in our final post, we will have a look at one of the most important components in a Greentherm central heating system; the buffer tank, Thermal Store or Combi-Cylinder.

We’ve already seen that to efficiently meet the hot water demands of a  modern home, the cylinder has to be far larger than the traditional copper tank tucked away in the hot-press.

The actual amount of storage required is calculated by us here at Greentherm as part of our design process. Based on the plans supplied to us by your architect, we calculate the heat load and hot water demands for your home and match them with a heat source capable of delivering the required performance. We then size the buffer tank based on the most efficient operating point of this source and the demands we have calculated.

This is a fairly involved process the details of which go far beyond this post, especially when modern regulations such as Energy Performance and Carbon Performance have to be accounted for.

For argument’s sake let’s assume that, based upon the plans and specifications we’ve received from your architect, we’ve calculated you need a 1000 Litre Thermal Storage tank to meet your new home’s hot-water and heating requirements.  How much space would that take up?

It certainly won’t fit in with the towels, so somewhere else must be found.

We recommend that a dedicated plant-room be set aside for your hot water storage and heating systems, both for ease of maintenance, ease of access and for your convenience.

This dedicated plant room will have to be large enough to accommodate the Thermal Store and its plumbing. So, how large will it have to be to fit our specified Store inside?

A simple way to estimate your cylinder dimensions

The Volume we require to store 1000 Litres of water is, conveniently, 1m³. A cube-shaped tank, 1 Metre by 1 Metre by 1 Metre will be large enough to hold 1000 Litres of water. Unfortunately for us it’s not that simple; the majority of Thermal Stores are cylindrical.

The basic geometrical formula for the volume of a cylinder is πR²H.

π is a constant that relates the circumference of a circle to its radius length. It can be taken for our purposes as having a value of 3.14

H, or the Height of the cylinder will generally be limited to the ceiling height of your home, less approximately 200mm for headspace to simplify installation.

R, is the radius of the cylinder – or the distance from the centre of the cylinder to its outer wall. It is important that the depth of insulation not be included in this.

To know the space required for the cylinder in the room, we re-arrange the above formula to give:

R = √(V/πH)

Assuming we have enough room to fit a 1.8 metre tall cylinder in the room

For a 1000L tank, R = √(1/3.14×1.8),

This calculates out to a cylinder radius of approximately 43cm.

This gives a total vessel diameter of nearly 90cm. On top of this, most cylinders will have an insulation blanket that also needs to be accounted for. An extra 5cm on each side of the cylinder for insulation means a 1000 Litre cylinder will be at least a metre wide when finished.

This is too wide to fit through most doors unless you opt for a tank such as the Jaspi Oval tank which has been specifically designed to fit through a standard doorway.

It will be necessary to allow for sufficient clearance around the sides of the cylinder for proper plumbing access. Depending on the works required, at minimum this could be another 30-50cm, meaning a 1000 Litre cylinder could require up to 1.3M of space in a room to comfortably install and plumb.

This will, of course, require some special arrangements to install, to ensure the cylinder will fit.

These special arrangements will need to be discussed with your architect in the design and specification phase.

The normal capacity we would usually install would be normally be in the range of 500 to 800 for a buffer tank, and typically up to 900 L for a combi-cylinder – so 1000 Litres is a little on the large side, but still within the realms of possibility.

That brings us to the end of this short series of postings, we hope you’ve found them informative. Contact us if you have any queries or questions on what we’ve gone through in this series, we’ll be more than happy to answer them.

In our previous post, we looked at a quick comparison between a Heat Pump, and a Boiler. We saw that a boiler was capable of providing heat very quickly, heating a cylinder up to temperature faster than a heat pump. At the same time we saw that using the Boiler to heat water was more expensive than using a Heat Pump, nearly three times the cost if Night-Rate electricity is  used.

The point was also made that a heat pump would be more efficient operating at 35°C, rather than 55°C. So why not store water at 35°C, rather than 50 or 60°C?

The Advantage of lower storage temperatures:

Storing water at lower temperatures brings a number of efficiency benefits. This is especially important with a heat pump because heat-pumps get less efficient, the higher the required water temperature gets

A hidden advantage comes in the form of heat loss. A fundamental principal of physics is that energy will always try to move from the hottest point in a system to coldest, and that the rate of heat loss will be proportional to the temperature difference between the two. What this means is that, the hotter your water storage cylinder, the more heat energy will be lost from it.

In older homes, the reason the Hot Press was Hot was poorly lagged hot water cylinders acting as heaters. In newer homes with insulated cylinders, heat loss from standing water will make a significant impact, especially if hot water is left standing for long periods of time.

Losses within distribution pipework are also reduced by operating at 35°.

What can low temperature water actually do?

Greentherm Underfloor Heating systems are specifically designed to operate efficiently at temperatures of 35°C.

The heat output of a system is also directly proportional to the surface area of the heat emitter and its temperature. The larger the area and the hotter it is, the more heat energy it will transfer.

To heat a room a certain amount of heat is required to maintain a comfortable temperature. The amount required depends on the set temperature of the room, the quality of the room’s insulation, the external temperature and how well ventilated the room is. The resulting heat input called the Heat Load, and performing a calculation of the expected heat load on a room is part of the process we go through at Greentherm when specifying a heating system for your home.

35°C would be too low a temperature for a conventional radiator system to meet the required Heat Load of most rooms. There would not be a large enough surface area to transfer energy fast enough.

A low temperature over a large floor area is more than capable of quickly and efficiently heating a room up to temperature. Other technologies, such as low-temperature radiators, or fan-coil radiators enable the use of low-temperature heating system water. However, the use of each of these creates new specific design considerations.

So, how much water do I need to store?

With high-temperature water, you will generally need a smaller hot water storage cylinder than with low temperature water. The usual strategy is to blend in an amount of cold water to reduce high-temperature water to a comfortable and safe temperature. This can be achieved through the use of a thermostatic mixing valve. The most common example of this would be a shower.

Showers will generally use upwards of 10 litres of water every minute. Some rain-head showers have consumptions that exceed 18 Litres per minute. In the table below is a quick comparison of the hot water requirements for a 10 minute shower, supplying 18 Litres per minute of water at a comfortable set temperature of 38°C. Two different supply temperatures are shown; for a heat-pump and for a boiler, using the maximum storage temperatures from our previous post. For the boiler, that will be 60°C. For the Heat Pump, 50°C without using the immersion heater.

From here on it should be clear that to work out the required water consumption, you should multiply the required hot water per shower, by the amount of showers to be taken. The addition of teenagers to the household could more than double the required amount per-shower. The addition of Zypho waste-water heat recovery units can reduce hot water requirements by up to a third, as detailed in a previous post.

For a four-person household with a boiler system we would need a minimum of 400 Litres of hot water storage to keep from running out of hot water. With a heat pump system, we would need at least 500 Litres. The addition of a solar coil to a storage tank can take up an additional 100 Litres of capacity that is not accessible to either boiler or heat-pump – in which case this capacity will have to be added to the figures above.

We can therefore see that a 4-person household fitted with rainhead showers, a storage tank capacity of up to 600 Litres could easily be required when a heat pump is fitted, and when it is operating at 50°C.

Once this water has gone, you’re relying on the ability of your heating system to recover the temperature of your domestic hot water. As we’ve seen previously, this can take some time.

And what about my heating system?

The design of your heating system can also affect the amount of water you need to store.  For modern designs of heating system your hot water cylinder acts as a thermal store of heat energy, similar to how a battery stores electricity. It will be providing both your hot water and the heat for your central heating system. How much water you will need to store depends not only on your domestic hot water requirements, but also the heat load on your central heating system, whether you have underfloor heating, radiators or some combination of bother, and what sources of heat you have installed.

This is a calculation we perform at Greentherm as part of the design and specification of your heating system. We ensure that your system will give you enough hot water to comfortably meet your daily needs, while still providing heat to your home, without the high fuel bills.

Now that we know how much water we need to store, how much space do we need to set aside to store it? That will be the subject of our next post.

The first in an upcoming series of short articles discussing the basics of design for your heating system, a comparison between boilers and heat pumps. What can each heat-source actually achieve?

We’ve already discussed the technical and physical side of heat pumps before. In summary, the higher the output temperature required, the less efficient the Heat Pump becomes. The Coefficient of Performance is a measure of the Heat Pump’s efficiency, the higher the better. The Hitachi Yutaki series of Heat-Pumps offered by Greentherm offer an approximate Coefficient of Performance of between 2 and 4 under normal operating conditions.

For the purposes of this article, we shall consider a standard gas-fired condensing boiler with a good efficiency rating of 90%. This means that 90% of the energy in the burned fuel is delivered as useable heat, with the other 10% lost.  Other boilers are reasonably similar, though with different efficiencies. Solid Fuel Boilers have certain specialised requirements that are beyond the scope of this article for the time being.

In the table below, Flow temperature is the maximum output water temperature from the Heat Pump or Boiler. Return temperature is the temperature of water returning to the Heat Pump. The maximum storage temperature will be somewhere between these values – less a small amount for losses in the cylinder heat-exchanger.

To demonstrate the effect this on system performance we will assume that both Heat Pump and Boiler are each connected to a 300 Litre domestic hot water cylinder with an ideally sized coil. The water in the cylinder is starting at a cold temperature of 10°C and needs to be raised to 50°C.

The time taken to heat the cylinder to temperature can be estimated and the approximate cost to heat the cylinder is then calculated using data available on the SEAI website here:*

The higher output temperatures, and greater delivered energy from the boiler heat the water much faster. Therefore, hot water will be available sooner from a boiler system from cold. A Heat Pump will take longer to deliver the same amount of heat. However, the cost of that energy from the heat-pump is a good deal less.  If night-rate electricity are used, the Heat Pump costs are halved. There is, however, no night rate available for gas.

Because it operates at a lower flow temperature the Heat Pump requires a larger heat-exchanger area within the cylinder to give good performance. Because of its higher flow temperatures, a boiler requires less coil area to achieve the same level of performance. This smaller coil is less efficient at transferring heat, requiring a larger temperature difference to be effective, meaning more heat can be lost within the system on the return to the boiler.

It must be remembered also that the above Heat Pump is operating in its least-efficient regime. It’s being asked to operate at its maximum rated heat output, a point at which the effective coefficient of performance will be closer to the minimum. To get to temperatures beyond this, an electric immersion heater or alternative heat-source would be required.

In truth, how often is hot water required at 50°C? For most washing water will be used at more comfortable temperatures below 30°C, so this store of hot water will likely be blended down with some cold water to achieve a desired comfortable temperature. The Heat Pump will operate far more efficiently if we allow it to work at a cooler temperature – say, 35°C.

Is there a way to meet our hot water demands using cooler water? This leads nicely into what will be the next post in this series:

Water Storage Temperatures and Why.

*As of September 2014. Energy Prices may vary.

Greentherm supply Solar P.V. and Solar Thermal Systems to the Irish Market. Solar Thermal has been discussed before, but what about Solar P.V.? It seems like magic, that a simple flat sheet of silicon can create electricity from the sun. So how does it happen?

A Semiconductor.

The majority of Solar P.V. Panels on the market today are manufactured from various forms of Silicon. This is the same basic Silicon that is used to manufacture computer chips – in fact, many of the early production processes are shared between the two. Silicon is what’s known as a semiconductor. It’s not quite an insulator like plastic. It’s not a conductor  like copper  either. It’s crystal structure gives it unique properties. When exposed to an external source of energy – such as heat – it’s ability to carry current – it’s conductivity – increases. Also, the conductivity and electrical characteristics can be precisely controlled through the use of special manufacturing techniques such as doping

Doping.

By adding other materials to the Silicon crystal, it’s current carrying characteristics can be precisely changed, making it more susceptible to current flow. Positively Doped (P-type) Silicon has been altered so that it has a slight positive charge. Negatively Doped (N-type) silicon has been doped to have a slight negative charge.

When P-type and N-type silicon are combined together, they form what is known as a P-N Junction. A natural barrier forms between the pair that will only allow current to pass in one direction, and only when a certain switch-on voltage has been reached. Think of it like a dam in a river. Only when the river level rises above the level of the dam wall will water be able to flow over . Also, a dam blocks water from flowing back up the river – in the event of a tidal flood or similar event. A P-N Junction connected in reverse will block current flowing backwards.

The P-N Junction forms the core of most modern electronics, the most basic use being in a Diode – a device normally used to limit the flow of electric current to one direction.

Strange Semiconductors

Semiconductors don’t behave like normal electrical devices. They have a number of strange characteristics. Once a diode has ‘switched on’, for example, it’s effective resistance drops to almost nothing. This means that – even for small increases in voltage – a rapid increase in current will be experienced. This switching function makes them very useful for modern electronics hardware – such as computer processors.

The ‘switch’ however, doesn’t necessarily have to be the voltage across the junction. It can come from any source. A Photodiode is a diode that is switched on by light-energy for example.

But, how do we use this to generate electricity?

The Photoelectric Effect

Light is a form of energy. When light strikes an object, it has the potential to knock electrons loose within that object, generating a small voltage. This is the photoelectric effect. Some substances are more prone to it than others. However, due to the nature of most substances, these voltages tend to rapidly dissapate unless some sort of barrier exists between charges.

Like a p-n junction.

The p-n junction in a solar cell prevents these charges from dissipating with in the cell, allowing them to build up. This creates a voltage across both terminals of the cell. This voltage is limited by the characteristics of the solar cell – including its size and the quality of the p-n junction seperating the charges. The lower the quality, the easier it is for charge to leak back across. Eventually, this leakage matches the rate of charge production, and creates the Open Circuit Voltage of the panel.

If these terminals were connected with a piece of cable however, current would begin to flow.

A Current Source

Under ideal circumstances, a Solar Cell functions as what’s known as a Current Source. This means that, when connected to any load it will attempt to push a fixed current through that load – say 3 Amps. It will increase the voltage to whatever is required to push that amount of current through the circuit. The amount of current the cell attempts to deliver is directly related to the amount of solar energy falling on it.

The more sun, the higher the amount of current that will be delivered. It’s that simple.

There is, however, a limit. Remember above, the Open Circuit Voltage?

If the resistance to current flow through the load is high enough, then the voltage required to drive current through the load will get so high that more and more of the charge will begin to dissipate within the panel. The voltage will slowly increase to the Open Circuit Voltage, while the delivered current will drop off rapidly. A side effect of this is that the dissipation of energy through the panel will cause it to begin to heat up.

This heating reduces the performance of the panel somewhat by making it easier for charge to leak across, limiting voltage and current further.

The Power of the Panel

The I/V characteristic Curve for the Viridian Clearline range of Solar Panels  Greentherm offers, is shown below. It shows curves for the 250W, 300W and 500W PV panel models. It can be seen that the maximum operating current off all three models of panel remain the same – the only change is the maximum voltage this current is supplied at. It’s also clear that the open circuit voltage of the 500W panel, is double that of the 250W panel. This is due to how the panels are manufactured – from a whole bank of individual solar cells connected together in series, such that their open circuit voltages are added to each other.

Effectively, these lines represent the operating power of the panel, for various loads. The delivered power through a load can be easily calculated from these graphs for any attached resistive load.  The Maximum Power Point of the panel – where it is operating at its most efficient – is achieved when the voltage and current multiple (VxI) is at its maximum. This happens right as the current begins to decrease, and is marked on the graph above with a red line. These lines represent the Maximum Power Voltage and Current on the datasheet. This point, is where the panel achieves it’s rated power output, or Watts-peak (kWp, or Wp).

Maximum Power Point Tracking (MPPT)

If a load is directly connected to the panel, it’s unlikely that the panel will be operating at it’ maximum power point. For example, if a battery to be charged were connected directly to the panel, the voltage in the system will be clamped down to the battery voltage. The maximum current delivered will always be limited by the characteristics of the panel – in the case of the above panel, approximately 8 Amps. With a 12 Volt battery connected, this would limit the maximum delivereable power to the battery to less than 96W – less than half of the rated power output.

The same principal applies to other electrical loads. Attaching a simple resistive immersion heating, or a basic chanrge controller to the PV panel will pull it outside of it’ most efficient operating regime. This also happens if an individual panel is shaded in an array, or otherwise malfunctions – the output of the array is clamped down to that shaded panel’s maximum voltage or current capability (Depending  on how they are connected together)

The get the best out of a Solar PV Panel, the load has to be matched to the Maximum Power Point of the panel.

The Maximum Power Point is not necessarily a fixed point on the graph. It will vary according to the amount of sunlight being provided to the panel. The maximum output current of the panel is directly related to the amount of sunshine received by the panel. Shading of individual panels or modules within the PV system will also effect the maximum power point.

It becomes necessary to have an electronic device – either a charge controller, or inverter for grid-tie connection, with Maximum Power Point Tracking capability. Devices with MPPT capabilities can adjust their electrical characteristics to ensure that the connected Solar Panels are always operating close to their regime of peak efficiency, regardless of what load is connected to them.

Greentherm sell charge controllers and inverters with this capability.

For more Information

Contact us for  more information on the designing, specification and installation of Solar PV systems, or to arrange a no-obligation quotation or consultation.

A common question we get asked is, where do heat pumps get their heat from? How do Heat Pumps still manage supply heat when the temperature outside has dropped below freezing?

Heat is Energy

First and foremost, Heat is energy. More specifically, heat is a measure of how fast the individual molecules that make up a substance are moving. Even in solid ice that appears still – as it’s heated the molecules of water are vibrating in place. As it melts to liquid water they gain energy enough to be able to move freely against each other. Heat them up further still and they gain enough energy to separate from each other entirely and boil, to form steam.

Zero Degrees isn’t Absolute Zero.

The Celsius scale which we are most familiar with was defined by Andre Celsius based upon the melting point of ice and boiling point of water – effectively meaning the Zero-point of the scale is arbitrarily placed. A useful analogy is the A.D. and B.C. dating system commonly used. An arbitrary year was chosen as Year 1 and everything after that year was considered positive, while everything before that year is, effectively, negative.

Year 100 B.C. is more recent than Year 200 B.C., for example. A person born in 4 B.C and who died in 33 A.D. would’ve been 39 years old when they died.

What this means is that just because something is at a temperature below 0°C, doesn’t necessarily mean it’s at ‘negative energy’. Something that’s sitting at -10°C, will still contain more heat energy than something sitting at -20°C, but both will have a positive amount of energy. One simply has less than the other.

In fact, the Celsius scale goes all the way down to -273°C – to what’s known as Absolute Zero. No temperature lower than Absolute Zero can exist – it’s a physical impossibility in this universe. At Absolute Zero there is no energy whatsoever left to be extracted. Above -273°C, there’s always some positive energy available.

Just because something is at 0°C does not mean it has no energy left to give. Even air at -20°C still has a lot of heat energy available. Because it’s still 250 degrees above Absolute Zero and so still has 250 ‘degrees’ of heat energy available to use in some way.

Just because it feels cold out doesn’t mean there isn’t heat energy available. It just means the air is cooler than you are.

The hidden heat.

All matter has three phases; Solid, Liquid and Gas. For water, this is Ice, Liquid Water and Steam. It’s well known that water, under normal atmospheric pressure, will begin to boil at 100°C. To actually turn water into steam however, requires far more energy to be given introduced – energy which goes towards actually creating steam. This additional energy is called the Latent Heat of Vapourisation.

To turn a litre of Water to Steam requires 2260kJ of energy. Or, referring to our most recent post, the equivalent of just under 0.63 kWh, or  a 1kW electric heater operating for about 40 minutes.

All this energy goes towards turning that water into steam powerful enough to move a train.

However, when Steam is condensed back down to Water, all of this Latent Heat will be released and has to be removed in order to fully condense the Steam. The Steam is said to Give Up its Latent Heat of Vapourisation. There’re two ways to condense Steam – either by cooling it to remove the Latent Heat, or by compressing it somehow to a point where the pressure is so high that it begins to condense on its own.

If 1 kilogram of Steam at 100°C and at atmospheric pressure were to be compressed up to 3 Bar (3 Atmospheres), it would begin to condense back to Water. In the process of being compressed, it will still give up its Latent Heat of vapourisation – that 0.63kWh will be forced come out again.

Some energy input however, is required to compress the Steam again.

This difference between the energy required to compress the Steam to the point where it condenses and the Latent Heat of Vapourisation given up is equivalent to the Coefficient of Performance for a Heat Pump. If it’s above 1.0, more Latent Heat energy is given up than energy supplied to compress the Steam.

Futhermore, as the condenser side gets hotter and hotter it gets harder and harder to condense the Steam. It has to be compressed more and more to a higher pressure before it starts condensing, meaning more energy is required by the compressor to reach those pressures. More and more steam needs to be compressed to get more energy and reach higher temperatures.

This is one of the reasons why Heat Pumps get less efficient as the temperature difference between the source temperature and the output temperature increases.

Cool Gases.

While the principal can be demonstrated with Water and Steam, in practice Water and Steam would be poor choices for use in a Heat Pump. Water doesn’t boil until it reaches a temperature of 100°C. This is far beyond any useful temperature for a domestic Heat Pump. For a Heat Pump, what’s needed is a gas which vapourises at common atmospheric temperatures.

This is called a Refrigerant Gas.

Under normal atmospheric conditions most refrigerant gasses will boil at temperatures well below 0°C. R134a is a common refrigerant gas which has a boiling point under normal atmospheric pressure of -26°C. Below this point, it’s a clear liquid that looks a lot like water.

This means that air at a temperature of -20°C still has enough heat energy within it to boil R134a under atmospheric pressure.

When R134a boils and turns to gas, it absorbs its Latent Heat of Vapourisation. When it’s compressed again and Condenses into a liquid, this heat is then given up. In a Heat Pump, the Heat of Vapourisation comes from an external source, such as the atmosphere, or a geothermal ground loop. This heat, when given up at the compressor, can then be used within a heating circuit or similar.

How refrigerant affects performance

Different refrigerant gases have different performance characteristics, which place different limits on the capabilities of various Heat Pump models. The Hitachi Yutaki series offered by Greentherm use R410a, for example, which has a lower boiling point than R134a.

A refrigerant gas like R134a that boils at -26ºc will struggle to deliver heat in temperatures below -20c – it just doesn’t boil fast enough to extract any useful energy. On the other hand, it can be relatively easy to compress and reach higher domestic temperatures.

Conversely Heat Pump filled with a gas that has a lower boiling point – say, R13 which has a boiling point of -89°C – will reliably work in even the coldest of temperatures but will require more and more effort from the compressor to condense the refrigerant and give a high enough output  temperature for domestic use.

The lower the boiling point of the refrigerant gas the better the Heat Pump can operate under cold conditions, but the less efficient it gets as the output temperature increases. A Heat Pump with a Higher Boiling point can offer higher output temperature but will struggle to deliver heat during the coldest of winters.

Each particular refrigerant gas has a range of temperatures that it offers its best efficiencies at. Below that temperature, it just doesn’t vapourise fast enough to extract a useful amount of heat. While above that temperature band the compressor is doing too much work to extract the heat from the gas, such that the efficiency of the Heat Pump starts to suffer. Different manufacturers choose different refrigerants based upon the specific performance characteristics they’re looking to achieve, the production cost of the gas and the environmental conditions it’s being designed for.  A Heat Pump designed to be efficient in Finland may use a different refrigerant than one designed for France.

Greentherm have taken a compromise solution, to give the best of both options. The Yutaki heat pumps offered by Greentherm are rated by Hitachi to operate at minimum temperatures of -20ºC -well below the record low Winter temperature in Ireland. They also have an upper temperature limit of 55ºC which is more than hot enough for domestic hot water and most heating systems. Even at these extremes of temperature – delivering 55ºC output with an atmospheric temperature of -20ºC –  the COP is still approximately 2.

 The Bottom Line.

Just because it feels cold outside, doesn’t mean there’s no energy available for your Heat Pump. Refrigerant gasses which boil at low temperatures enable the Heat Pump to extract heat from ‘cold’ sources such as a ground loop or ambient air. Allowing the refrigerant to boil absorbs heat from the source to turn it to a gas. Compressing the refrigerant back down to a liquid allows this atmospheric heat to be extracted. Compressing, however, requires energy input. The ratio between input energy to the compressor and output from the Heat Pump is the Coefficient of Performance, and is a measure of how efficient the Heat Pump is. The higher the output temperature the more work the Compressor has to do to achieve it, reducing the efficiency of the Heat Pump.

Exactly how efficient your Heat Pump will be is determined by the refrigerant gas chosen by the manufacturer and the temperature it’s operated at. Different gases have different temperature limits and performance characteristics, suitable for different installations or environments.

Greentherm offer the Hitachi Yutaki series of Heat Pumps, which offer the best performance characteristics for Irish weather conditions. The Yutaki range offers performance reliable enough to act as the sole source of heating for a home, even in the coldest of Irish Winters while still giving enough of an energy output to enable them to be used for heating and Domestic Hot Water purposes.

Contact us

Contact us now for more information, or to arrange a no-obligation consultation.