Unlike fossil fuel boilers, heat pumps are unforgiving when it comes to sizing. With a fossil fuel boiler increasing the size to add a safety factor is generally acceptable. The cost difference between a 20kW or 30kW boiler is not large. However, increasing the size of a ground source heat pump can have a marked effect on the cost and cause issues with cycling of the heat pump. Fitting a smaller heat pump also has an effect as the heat pump runs harder, relies on in-built expensive immersion heaters and ultimately could cause the ground to freeze.
Some key points to remember when looking at any heat pump ground source or air source are:-
Sizing a heat pump
Sizing of the heat pump should follow MCS guidelines. This means the heat pump needs to be sized for 100% of the heating load without the use of in-built immersion heaters. Immersion heaters are allowed but only when the heat pump needs to go outside of its design conditions. The more immersion heaters are used the higher the running costs as you are paying for direct electricity. Heat loss calculations should be completed to provide information on the heat pump peak heat load and annual load which will be used to size the ground arrays.
Sizing heat emitters for a heat pump
Heat emitters should be sized for the lowest possible flow temperatures. Any heat pump works more efficiently the lower the required outlet temperature. For example underfloor in screed generally requires a flow temperature of around 35C, where as radiators require 45-50C. Even this 10 to 15C rise in outlet temperature can result in a 25% drop in efficiency. Remember if underfloor is used, a higher flow temperature might still be required if wooden floors or thick carpets/rugs are placed over the underfloor pipes.
Insulation with a heat pump
If the insulation of the property is not as expected then the sizing of the heat pump calculated will be wrong. This might mean that the heat pump (in order to keep the building warm) has to run at a higher flow temperature or run the internal immersion heaters which as mentioned above reduces efficiency. In extreme cases the heat pump at maximum output might not be able to produce enough heat to actually keep the building warm.
Heat pump control strategy and installation
The control strategy and installation needs to be completed to maximise the use of the heat pump and minimise the use of any immersion heaters. DHW production needs to be timed to suit the end user while providing the most efficient use of the heat pump. Again to produce DHW (domestic hot water) the heat pump needs to run at a higher temperature reducing the efficiency of the heat pump.
With the introduction of the MCS guidelines for heat pump installations, issues with sizing and installation should now be avoided, leading to installations having the best possible running costs. However with the Government dragging their heels regarding the RHI and the additional costs of complying with the MCS guidelines we are now seeing the development of a two-tier market – MCS approved installations and non-MCS approved installations. This is a dangerous situation and one which could dent customer perception of heat pump technology.