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.
|Hot flow (L/min)||
|Per shower (L)||
|Cost of Energy(€)||
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.