Hi, Rob, that doesn't sound fun! I'm really sorry to hear that. It's possible that one of our energy experts might have some insight, I will see. Best of luck with that.

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can anyone help me, I moved into a new house Dec 2012 and it has Worcester heat recovery system, it has two 120meter deep boreholes, the house just does not get up to temperature, all underfloor heating, early Jan I complained to the guy who installed it and he came and activated the imersion heater.  in 3 weeks I ran up £800 bill so it was sitched off but now we cant get 15 degc and its cold, the engineer has isolated 3 rooms that are not being used and its made no difference, he is a really nice guy and has even got worcester out but they are scratching their heads and have no idea what is wrong.  they have bleed the system and got a little air out and that made no difference, its costs about £18 a day, the hot water is good and hot help please

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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 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 – Sizing of the heat pump should now 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.

Heat emitters – 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 – 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.

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 the timed to suit the end user while providing the most efficient use of the heat pump. Again to produce DHW the heat pump needs to run at a higher temperature reducing the efficiency of the heat pump.

It is also worthwhile checking to ensure that any high electricity bills are actually attributable to the heat pump. This might seem obvious but fitting a lifestyle choice of an electric range style cooker can be overlooked as a high usage of electricity with the additional cost being allocated to the heat pump.

With the introduction of the MCS guidelines, 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. A dangerous situation and one which could dent customer perception of heat pump technology.

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Some heat pumps - IVT, for one - have a "pasteurisation" facility which runs once/week to boost the DHW temperature to 60 deg C to avoid legionella. Most people turn this off a) to save electricity b) to avoid being hit by over-heated water in the shower on the morning of the night when the process happens. 

GSHPs with weather compensation also tend to run their heat circulation pumps almost continuously, apart from when external temperature rises above, say, 16 deg C. I've thought that this could be set lower to save electricity as I can't see a disadvantage.

I could imagine that 'lumpiness' in heat pump ratings could give the opposite type of problem to that in this piece - poor efficiency due to frequent cycling of the compressor - if the next size up of system was installed.  

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