How to turn a lovley old house with solid walls into a warm efficient home






I was very keen to make my house efficient and was interested in several new technologies. Before I dived in head first or did too much work to the house I thought I should do some basic research. Below, I detail my initial thoughts and research regarding Air Source Heat Pumps. This is not intended to be didactic or definitive, but I hope for those of you considering an Air Source Heat pump it will be of interest and a helpful contribution to your own research.

I wondered if a heat pump live up all of the hype pedalled by the companies trying to sell them? Well, I found that there is performance data available. DECC monitored a series of UK based Ground and Air source heat pumps in the ‘real world’ some years back. Unfortunately, the results do not make great reading. Pumps advertised as having a COP of 3 or more, on average, performed in the real world offering around 1.8 (many less!). For many that may seem reason enough to give them a wide birth. However, DECC’s conclusions were that the underperformance was often because of poor or inappropriate installation. DECC themselves suggested the results were inconclusive and that further research is needed, so I thought I would dig around and draw my own conclusions.

Examination of product manufacturers ASHP performance data shows pretty clearly that there is a fairly small envelope of conditions where high efficiency is achieved. The two critical factors to good performance are temperature of the input air and temperature of the output water. The narrower the margin between these two temperatures the higher the efficiency of the pump, but as that differential broadens efficiency falls off pretty quickly - it's not a linear progression. My conclusion from the data was that ASHPs do work and can work very efficiently, but only under certain conditions, outside of which their efficiency becomes somewhat questionable.

The attractive COP figures of 4 or more that manufacturers like to entice us with are generally quoted when there is an air temperature of +7c, so I looked for data to see what the average temperatures are during a British winter. Taking Met Office data between 1911 and 2013 the average winter temperature was +4.7c Warmer than I suspected. This is still in a favourable range, so a heat pump should be able to achieve a decent COP at these average temperatures. Part of our heating season falls in the late Autumn and early Spring months when temperatures are more likely to be +7 or above, so a pump will do better still. I can’t control the outside air temp but was encouraged that maybe winters aren’t as cold as I thought and a pump could still cope well.

The other demand of an efficient system, often overlooked, is to ensure output water temperature can be kept low whilst still heating the house sufficiently. Most manufacturers’ data is quoted assuming a water temperature of +35c (A traditional Gas or Oil boiler may well be supplying water to radiators at around +70c so +35 is quite a drop). The key, therefore, is to have a heating distribution system capable of delivering sufficient heat to the home even with low temp water. The design of the distribution system and, crucially, the overall thermal efficiency of the home will each play a part in ensuring this is feasible.

Many (most) sales people I spoke to did a ‘finger in the air’ assessment of my home’s suitability for a pump – often merely one or two questions over the phone, but because I believe a small difference in performance of a heating system or a house’s insulation levels can make quite a substantial difference to it’s heating demand, I tend to think such rough guides are insufficient. It is not particularly difficult to work out heat losses room by room using free online calculation software. This will not only give you an overall figure for the home’s heating demand but will also help you work out exactly what size radiators or UFH output you need per room. For those not keen to go to such lengths themselves I would recommend finding an ASHP company willing to do and supply you with such detailed calculations, and not just guess. I have to say that on this score I was impressed by Husky Heat Pumps. They have their own software to make exactly these calculations and advise you accordingly.

It is often said that a heat pump can not heat old, less well insulated properties and that they are suitable only for new builds. Personally I do not think this is quite true. The two factors that need careful consideration are that the heat pump is 1) Capable of delivering sufficient heat to meet the house's demand, and 2) That the distribution system (UFH or radiators) can keep the house warm using the low water temperatures needed to allow the pump to run efficiently.


True, an inefficient property will use a lot of electricity to run the pump(s) but it would also demand a lot of oil to be burned too. Provided the pump system is designed correctly there is no reason you should not still see savings.

None the less, the bottom line for those of you, like me, with an old house is that it will undoubtedly be advantageous to make some improvements to your home’s insulation levels before installing a heat pump. Chances are that it will also be necessary to buy larger radiators or if, like me, you are undertaking major works, install underfloor heating.

I concluded that the sensible approach was not to hand money over to eager heat pump salesmen, but to spend my money on improving my home’s efficiency. I figured that if I improved the home’s efficiency then whatever I later used to heat my home, be it an oil boiler or a heat pump, I would need less of it so I could not lose. Once I had improved the home as best as I feasibly could I would then do the calculations to see if I thought a heat pump could the heat it efficiently. I was undertaking major extension and building works on my home anyway, so for me it was the perfect time to consider insulation levels, draught proofing and UFH installation. All those articles are available via the main menu.

© Christopher Thompson