1. Efficiency. Due to the higher temperature required to generate hot water, the output temperature has to be as hot as possible. With all heat pumps the higher the outlet temperature the lower the efficiency (for every one degree raise in outlet temperature the efficiency drops by at least 1%). Therefore at 55C the heat pump is 20% less efficient than when it is producing space heating for an underfloor system.

2. Domestic hot water temperature. The maximum temperature of the hot water depends on a number of factors: the refrigerant used in the heat pump, the coil in the DHW tank, the usage, etc. 

i. Changing the refrigerant can allow the heat pump to operate at higher temperatures, generally up to 65C. However, it is important to remember that operating the heat pump at these higher temperatures does still result in a drop in efficiency and also a drop in output power (this is due to the higher temperature as well as the physical properties of the higher temperature rated refrigerant).

Using R407C refrigerant the maximum temperature generated in the DHW cylinder is generally 50-55C. With a higher temperature unit you might get an additional 5 degrees. Due to the lower temperatures generally, you'll probably need a larger cylinder than one linked to a fossil boiler, simply because less cold water will be used to mix the hot water down to an acceptable temperature at point of usage.

ii. DHW cylinder coil size – for a heat pump the size of the coil is vitally important. Due to the lower temperature (compared with a traditional fossil fuel boiler) from the heat pump, the surface area has to be larger to transfer the required heat into the water. This has to be a true surface area and not just fins added to the coil.

The tank has to be specially designed for a heat pump as opposed to an off the shelf standard unit. If the coil is too small the DHW will simply not reach the required temperature.

3. Protection from Legionella.  While it is only advised in domestic applications that the water is bought up to above 60C to destroy any legionella, it is strongly advised that an independent immersion heater is linked to a timeclock and this is done at least once a week. This process should be timed after the heat pump has raised the temperature of the tank as high as it can. This then means that the lowest amount of direct electricity is used for this process.

4. The heat pump can only produce space heating or domestic hot water, generally not both together (there are a few which use the superheat from one of the internal heat exchangers, however in summer you need to generate space heating to produce DHW, the last thing you want in a hot summer!). So you need to be aware that in a cold winter (like the last few years) if the heat pump is outside of, or at, its design condition, it may be operating for 24hours a day just to keep the property warm.

As DHW is usually a priority it will mean that while the heat pump is producing DHW it is not heating the building and this could mean the building becomes cold.

An alternative method for producing DHW for these short periods might need to be relied upon such as an immersion heater. This also needs to be a consideration if off peak electricity tariffs are being used as time generating DHW is at the expense of time heating the building, so under certain conditions expensive peak electricity might be required to keep the building warm if the heat pump is the only means of generating space or DHW.

5. Ground array sizing.  Due to DHW being an all year round demand and additional to the space heating, it makes perfect sense that the amount of ground array buried needs to be increased. If it is not then the system runs the chance of exhausting the energy in the ground, maybe not in the first year, but the second or third.

I’ve only touched upon some of the main considerations of producing DHW using a heat pump in the above article, there are many more. However, it can be done and result in savings to the end client. DHW is always very much down to client’s perceptions and requirements, a good installer/supplier should always take the above points into consideration.

Photo: doug88888

About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.

If you have a question about anything in the above blog, please ask it in the comments section below.