Specialists in Low Energy and Heat Pump Heating Technology

Water Temperature

Now you know the size of system that you need its time to decide on what type of system you need and the water flow and return temperatures to use. This is important as a heat pump performs better with lower temperature water and at higher temperatures both performance and efficiency suffer.. A heat pump cannot produce water higher than 55C (65C with a HT High Temperature model) and you will not qualify for the RHI with a flow temperature higher than 52C. But this affects your choice of heat emitter because these perform better at high temperatures.

Types of heat emitter

You can use conventional radiators and towel rails, low temperature fan assisted radiators, fan coils and under floor heating with a heat pump system. Typically a radiator system will need water flow temperatures between 45C and 50C, low temperature radiators or fan coils between 35C and 45C and under floor heating at 35C.

Retaining your old radiators?

If you are intending upon retaining your existing radiators then this part is already done for you. You simply need to use the figure for heat loss in W/m2 in the initial sizing chart to work out the individual room requirement and then calculate what temperature water is needed for that radiator to give the heat required by that room.

Bear in mind that a heat pump does not produce water as hot as a boiler with a maximum flow temperature of 55C (65C with a HT model) and, crucially, the efficiency and performance of the heat pump decreases at hotter temperatures. For your heat pump to work at its best then you want the flow temperature to be as low as possible. You will not be eligible for the RHI using flow temperatures of 53C and above.

Rated Performance of your radiator

Every radiator has a 'rated' performance using 50C TD or rather with flow at 75C, return at 65C, average radiator temperature at 70C and air at 20C (so the Temperature Difference or TD between the average radiator temperature is 70-20 or 50C TD.

Performance with a condensing boiler or HT heat pump using high temperature radiators

But if you are using a condensing boiler or HT type heat pump then you need to use water at 60C flow and 50C return in order for the boiler or heat pump to operate correctly. This is a TD of 35C and will require a correction factor of 63% to be applied to the radiator 'rated performance'. A radiator rated at 1.0 KW will only give 0.63 KW using a condensing boiler or HT heat pump. At these temperatures your heat pump will not qualify for the RHI. Typically the COP (Coefficient Of Performance - the method of measuring a heat pumps efficiency - the higher the better) of a 9KW HT system is around 2.6 which is pretty low.

Performance with a heat pump using standard radiators

Using a heat pump with a flow temperature of 50C, return at 45C and air at 20C gives a TD of 27.5C and this requires a correction factor of 46%. A radiator rated at 1.0 KW will only give 0.46KW of heat under these conditions. Typically the COP of a 9KW heat pump at these conditions is a much more respectable 3.26.

Performance of a heat pump with oversized radiators

If you are lucky then your radiators will be big enough to use water at 45C flow and 40C return. This has a correction factor of 35% and so a radiator rated at 1.0 KW will give 0.35 KW. Typically a 9KW heat pump will have a COP of 3.66 using 45C flow water.

Typical radiator performance figures

Figures for a 1m wide Stelrad Compact radiator of various sizes and types are shown below.

The depth of these is 78mm for a P1, 94mm for a K1, 112mm for a P+ and 135mm for a K2

Figures for a 450mm high 1000mm wide Stelrad Compact

 Model Rated TD50C Cond Boiler TD35CStd Rads TD27.5C LT Rads TD22.5C 
 P1 0.47KW 0.30KW0.22KW 0.16KW 
 K1 0.76KW 0.48KW0.35KW 0.27KW 
 P+ 1.06KW 0.67KW0.49KW 0.37KW 
 K2 1.37KW 0.86KW0.63KW 0.48KW 

Figures for a 600mm high 1000mm wide Stelrad Compact

 Model Rated TD50C Cond Boiler TD35CStd Rads TD27.5C LT Rads TD22.5C 
 P1 0.62KW 0.39KW0.29KW 0.22KW 
 K1 0.98KW 0.62KW0.45KW 0.34KW 
 P+ 1.73KW 1.09KW0.80KW 0.61KW 
 K2 2.39KW 1.51KW1.10KW 0.84KW 


Obviously for radiators slimmer than or wider than 1m can be calculated by multiplying the above figures by their actual size in metres in order to get an approximate capacity.

You can use this table to calculate at what flow and return temperature you need to operate your heat pump at to give you the heat that you need in your room.

Example 1

You have a living room of 3m by 4m or 12m2 and you have a modern building at 0.05 KW/m2 so need 0.6KW. This has a double convector radiator type K2 that is 600mm high and 800mm wide. Using the figures for a 1m wide K" and reducing them by 80% for the narrower width then we see that the radiator will still produce 0.67KW at a TD of 22.5C. This will perform correctly with a water flow temperature of 45C and so will work well with your heat pump.

Example 2

You have a bedroom of 10 m2 in an old 1970s house and at 0.1KW/m2 this needs 1.0 KW. It has a 600mm high 1m wide twin convector P+ radiator. From the chart we can see that while this is adequate using a TD of 35C, it is not large enough to operate with TD's any lower. This means that you must make a choice.

  • You can fit double glazing and maybe consider extra insulation to bring the heat loss down
  • You can change the radiator for a larger radiator or maybe a higher efficiency K2 model
  • You can change the radiator for a modern low temperature fan assisted one
  • You can leave the radiator as it is but use a HT model heat pump.

If you do any of the three first options then this will reduce the flow temperature and so increase efficiency. If you choose the last option then you do have the option of installing the heat pump now but then in the future improve the building or change the radiator to reduce the flow temperature and increase efficiency.

Once you have examined each room

You will now know for each room what flow temperature you need. It may be that some have radiators that will cope at 27.5C TD and some at 22.5C TD and you need to choose whether to operate the heat pump at the higher temperature or replace those less well performing radiators to use a lower flow temperature. Or you may choose to replace the radiators anyway.

Changing the radiators anyway?

If you decide to change the radiators then you can decide upon what flow temperature that you want to use from the start. Using modern fan assisted low temperature radiators  or under floor heating means that you can operate the system using water flow temperatures as low as 35C. Typically the COP of a 9KW heat pump at 35C flow is 4.84.

Now you know the flow temperature to use then click here to select the heat pump