Pumping Hot Air

I went to see an air source heat pump installed in a loft today. I was invited to view the installation by the manufacturer because I expressed doubts about the viability of a heat pump in a loft.

I like to think I approach things with an open mind, but don't we all. Seeing the unit in action confirmed my worst suspicions but what I really wanted to witness was the noise level. It was actually quieter than I thought it would be but there was still a low hum vibration transmitted through the roof timbers across the top floor of the house and the huge fan moving air through the enclosed space of the loft was also clearly audible. The unit was less than a year old and it's in the nature of all appliances with a fan to get noisier. How many hotel air conditioning units have kept you awake at night? Or is it just me who stays in cheap hotels? The rep's reaction was to suggest that "You can turn it off at night". That is fine provided you don't need hot water first thing in the morning.

There is nothing wrong with the heat pump as such but putting it in a loft is self defeating. Apart from the noise the heat pump is cooling the loft. Again the rep, paid to defend it at all costs, told me that it was only lowering the loft temperature by 1 1/2°c. How they know this is anybodies guess because that figure must fluctuate wildly according to a number of factors such as outside temperatures and wind speed, but even if it were true it's not something to boast about. The whole point of a heat pump is to extract heat from the air so the more it grabs the better. The problem is that the colder you make the loft the greater the flow of heat from the house below. Basic physics dictates that hot will always flow to cold and the greater the temperature difference the faster the flow. "Ah yes but you can insulate it" said the rep. He is right of course and the idea would be that if the loft were cold and the house was well insulted the heat from outside the roof space would flow in to replace the heat pumped out but this loft was very badly insulated so the chances are that the temperature of the loft would be raised by heat escaping from the house. In some areas of the loft there was a 100mm of glass fibre which is inadequate and in other areas nothing at all, just the ceiling. On the plus side the heat pump captures some of the heat escaping from the house and returns it, but it takes energy to do that and it would be better to stop it escaping in the first place. Spending a fraction of that money spent on a heat pump to improve the loft insulation would have been far more cost effective and far greener.

Leaving aside the noise issue and the lack of insulation, the sole purpose of the heat pump in this particular house was to heat domestic water. Is this an economic prospect? Because the house has a gas boiler the answer has to be no. Solar panels would have been a much better bet because they use a minimal amount of electricity for the circulating pump.

The COP of the air source heat pump is around 2 at 50°c and the hot water needs to be 60°c to prevent legionellea so it still has to be topped up by the gas boiler or electricity. The heat pump is only running during the day (too noisy to run at night) so there is no off peak electricity to take advantage of, and a COP of 2 with peak rate electricity is no better than a gas boiler.

OK you could bring the cylinder up to 50°c during the day and top it up with off peak at night but with most tariffs you need to use a fair amount of off peak to compensate for higher day time rates. The whole thing is a minefield.

What concerns me most is that this sort of ill thought out installation (which don't forget the company put up as a show piece) will harm the cause of heat pumps. This is over-selling the technology. There are plenty of other places the heat pump could have been placed around the property which would have been better and there are plenty of jobs it can usefully do but it needs to be fed into a large reservoir of water in order to stop it switching on and off all the time. Nothing shortens the life of a heat pump more than switching it on and off all the time. Competing with a gas boiler to provide just the domestic hot water is the least viable application.

I didn't share my views with the householder because she is a very nice girl who is looking to do the right thing for the environment. It's just a pity that she wasn't given better guidance by the salesman.


Roger Bisby
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