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.

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.

1 Kilowatt-hour is 1kWh.

It doesn’t matter whether it’s a kilowatt of electrical energy through an immersion heater, or a kilowatt from a boiler. One kilowatt-hour of heat energy, no matter where it comes from, will have much the same effect.

A single kilowatt-hour of energy will raise the temperature of a 100 litre hot water cylinder by 8.6°C. Or 200 Litres by 4.3°C, or 50 Litres by 17.2ºC.

A kilowatt-hour of energy, is equivelant to a 1 kilowatt appliance being left on for 1 hour. Or, a 500W Appliance for 2 hours. Or a 10kW boiler burning for 6 minutes. Or the output of the 1000kW engine of a Bugatti Veyron travelling at full speed for just under 4 seconds. All of these equal One Kilowatt-Hour (kWh). This is also, conveniently, a single unit of electricity, for billing purposes.

Energy is Energy.

What does it replace?

The first and most important thing you need to know when working out the benefits of your heat pump install, is to work out what it replaces. Energy that comes from a Heat Pump is energy that does not have to be provided by your existing, or backup, heating systems. 1 kWh of heat from your Heat Pump is 1 kWh of energy that does not have to come from your boiler.

In other words, from fuels you do not have to burn and money you do not have to spend

How much per Kilowatt-hour?

The SEAI has produced a document which gives the effective cost per kWh from various fuel-types. It’s summarised quickly in the table below.

You must, however, factor in the efficiency of your heating technology. Modern boilers can be up to 95% efficient, meaning 95% of the energy delivered from the fuel can be actually used. For older boilers this value can be as low as 75%, meaning that for every kilowatt-hour of fuel you pay for, up to a quarter of it goes to waste. Put more succinctly, a quarter of your fuel budget is being burned for no benefit to you whatsoever. In fact, this system will have to burn 1.3kWh worth of fuel, to achieve a 1kWh output.

Or, an increase in fuel consumption of a 33%.

Your boiler or appliance should come with an efficiency rating – from this you can calculate the actual cost of the fuel being burned to achieve a 1 kWh output from this system. Assuming an efficiency rating of 80%, the effective cost per-kWh from these appliances may be as high as.

Electrical heating systems such as Immersion Heaters, on the other hand, are effectively 100% efficient. 100% of the electrical energy goes towards heating. Howver, it can also be seen above that raw electricity, on a per-kWh basis, is the most expensive energy source by far.

How much does the energy from a Heat Pump cost?

Heat Pumps are electric devices, so draw power and energy from the National Grid. How heat pumps work has been discussed by us before. The energy you receive from a Heat Pump in a multiple of the energy used to drive the compressor. This is the Heat Pump’s Coefficient of Performance, or COP. Unlike the rating on a boiler, this is not a static figure. It varies depending on the difference between the output temperature of the heat pump and the ambient temperature of the atmosphere. The higher the difference, the less efficient the Heat Pump will be.

The Hitachi Yutaki heat pumps offered by Greentherm gives an average COP of above 4 under normal condtions, when matched to a well-designed heating system.

Or, for every 1 kWh of energy the Heat Pump uses, it gives you 4 kWh worth of heat out.  This means that, with a delivered electricty cost of – on average – 18c per Unit, the effective cost per-kWh of heat received is 4.5c.

Adding this to the above table, gives the following comparison:

Clearly then, the Yutaki heat-pump offers the best cost-per-kWh of any heat source.

1 kWh of energy from a heat pump is almost half the price of 1 kWh from Gas.

The Bottom Line.

1000kWh of heat from a gas boiler will cost you €82.

1000kWh of heat from a kerosene boiler will cost you €119.

1000kWh of heat from a heat pump will cost you €45.

The Average Irish home can require up to 30,000kWh of energy to heat, annually.

For more information.

Contact us. This is only a quick, generic overview. The exact results depend on many other factors – such as your lifestyle, specific heating system design characteristics and heat-loads of the building. We take these factors into account when we design our heating systems to ensure that you receive the best value for money, while still achieving low runnings cost and guaranteed, hassle-free comfort all year round.

Irish conditions are unique, to put it mildy. If you don’t like the weather, wait five minutes and it’ll change. To meet these changes, you need a heating system capable of rapid response and adaptation. A Hitachi Yutaki heat pump from Greentherm is ideal for weathering Irelands weather. Greentherm supply both Air-To-Water and Geothermal heat pumps, as requested.

Can I heat my home with a Heat Pump alone?

We’ve completed many projects in homes where the sole source of heat is a single Hitachi Yutaki Heat Pump. These have continued to operate through the coldest of winters without complaint, providing both central heating and domestic hot water.  This requires an appropriatel designed central heating system, suited to heat-pump operation.

What temperatures can be achieved?

Hitachi Yutaki heat-pumps can supply water at up to 55°C. Under normal circumstances we design our systems to optimise heat-pump operating temperatures. Heat pump efficiency decreases as the output temperature increases. Our heating designs focus on achieving low operating temperatures for your heating system without sacrificing your comfort.

What’s the minimum air temperature a heat-pump can operate at?

Heat-pumps suppled by Greentherm are rated to operate at -20°C. This is comfortably within the margins of even the worst Irish winter.  Geothermal heatpumps are unaffected by air-temperatures as they extract ambient heat from the soil.

Do I need a Buffer Tank?

Greentherm always recommend installing a buffer tank alongside a new heat-pump. A well-sized buffer tank will act as a thermal ‘battery’, which can be charged up on night-rate electricity to a higher temperature, with heat being drawn off throughout the day. A buffer tank will also ensure there is no disruption to your hot water supply when the heat-pump switches to a defrost cycle. Finally, a buffer tank eliminates short-cycling of the heat pump compressor, reducing wear and tear. Heat-pumps are an ideal match for a Greentherm Tank-in-Tank system.

What is a defrost cycle on a heat-pump?

Air-to-Water heat-pumps by design tend to accumulate ice. This is a natural part of their function. Ice acts as an insulator, stopping the flow of atmospheric heat to the heat-pump. Therefore, the heat pump must switch to a defrost cycle to clear this ice. The heat pump effectively operates in reverse, drawing a small amount of heat from the buffer store to melt the ice. Geothermal systems do not require defrost cycling.

Can I retrofit a Heat Pump to my existing home?

Heat pumps operate at their best efficiency at lower temperatures and should ideally be paired with heating technologies that work at those temperatures. Conventional radiators require higher temperatures to give their best performance. Ideally, these would be replaced with low temperature radiators, with fan-coil radiators, or underfloor heating being an ideal companion. If conventional radiators are to be used, a bi-valent system with a backup boiler will likely give the best operating economics.

Can Heat Pumps be used with Underfloor Heating?

Heat Pumps and Underfloor Heating make ideal companions. Underfloor heating operates at lower temperature than conventional radiator heating systems, allowing the heat pump to operate in its most efficient regime. Greentherm supply both Heat Pumps, and Underfloor Heating Systems, and can advise on the installation of combined systems.

Why Should I go with Greentherm for Heat Pumps?

First and foremost, we at Greentherm are an Engineering company. Which means we don’t just supply generic one-size-fits-all products, we offer genuine solutions, tailored to meet your budget and your unique requirements. We will consult with you to discuss your needs, expectations and budget, and work with you to craft a solution that will fit all three.

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

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