12 Nov 2007

Pumping heat

Spent the weekend dispensing bon-mots and advice in Harrogate at the Homebuilding & Renovating show, one of six held throughout the UK each year. This year I have been delivering a short lecture on sustainable homebuilding and it has sparked some interesting questions and comments from the audience. However this Sunday it all got a little fiery when someone asked about the difference between air source and ground source heat pumps and whether either made sense for his building project. Rather like the output from these heat pumps, my response was just a little lukewarm.

What I specifically said was that heat pumps don’t make much sense if mains gas is available but that there should be a reasonable payback against oil. “You are doing well if you get a Coefficient of Performance of more than 3.0,” I said. I have been consistently saying this for some time now and at least one heat pump manufacturer, Kensa, seem happy to agree with me.

But up stands this man in the audience who said that heat pumps could now deliver over 6.0 — i.e. twice as much heat output for the power input. Before I could stop myself, I blurted out “That’s rubbish.” It obviously hit a nerve, because he stood up and started getting shirty with me. “What do I know about it” sort of stuff. I have no idea who he was but can only guess he was working for one of the many heat pump suppliers exhibiting at the show.

This made me go all defensive and I started quoting a couple of studies back at him that showed that heat pumps often don’t deliver what manufacturers claim. If only to prove that I do know something about it, if not exactly ranking at world expert status. This of course made matters worse and our man turns around and walks out of the seminar theatre in an act of brazen defiance.

You could have heard a pin drop. Normally, these events pass by without any rancour at all and everything is sweetness and light from start to finish. Here there was a definite feeling that someone thought I that I was being out of order and should be upbraided.

What I think this shows is that the heat pump market is maturing fast, perhaps a little too fast. By all means consider the merits of using a heat pump, but don’t get sucked in by the hype, and beware claims of extraordinary efficiencies achieved.


  1. I totally agree with you Mark - GSHP make sense, only where no mains gas is available. In my opinion, air-source has no place in sustainable building (its Air Conditioning in reverse).

    I base this opinion on - installation and running costs (higher price of electricity), and also carbon cost (as we all know, our UK electricity is very 'dirty', and even green tariff's are not 100% renewable energy.

    There seems to be a growing opinion amongst my colleagues and contacts to support this view as well. What is interesting is we are involved with two projects, on GSHP and one ASHP where we will have accurate data on energy use and cost for a year. Both projects compromised insulation to pay for a heat pump

    (just don't tell my rather angry local heat pump supplier...)

  2. Kensa, quite rightly, has always maintained that the financial justification to support the purchase of a ground source heat pump is only really compelling if the alternative is either an oil or LPG-fired boiler.
    Variations in the cost of mains gas will always affect precise calculations but claims made at the Harrogate Show by a certain supplier to suggest a 50% saving are wildly inaccurate and deserving of the guffaws generated amongst the more knowledgeable members of the audience.
    Even after the cost of an annual gas boiler service is considered, the figures simply don't add up and, to be charitable, almost certainly assume there is a significant cost to provide mains gas in the first place. If I was being cynical, I would suggest that certain suppliers are making up the figures to suit their purposes.
    What isn't clear is why these claims are still being made, particularly given Mark's previous expose. The heat pumps themselves all perform in a broadly similar fashion - the vast majority of models feature compressors manufactured by one of just two global suppliers - so the claimed performance will not vary very much at all across brands.
    That said, when you review the various claims, there is a wide discrepancy between how COP's (the efficiency measure showing energy output against energy input)are calculated. Some suppliers conveniently exclude the electrical input required to run the distribution pumps which tells you all you need to know about their willingness to tell the truth. Others use differing input and output temperatures to suggest that there machine out-performs its rivals!
    Kensa seems to be far more willing to outline the facts.
    Of course, self-builders with access to mains gas might still elect to install a heat pump if their motivation is to reduce their carbon footprint. Equally, some may feel they are 'future proofing' the value of their home but there are no conclusive studies to demonstrate any additional value beyond the anecdotal 'evidence' supplied by the less scrupulous heat pump suppliers.
    Yes, the industry is booming but care must be taken to ensure the rather tarnished reputation of the solar industry is not replicated within the groundsource heat pump sector.
    Take the time to take care and seek written assurances if anything sounds too good to be true.

  3. Unfortunately heat pumps don't appear to save CO2 when compared to A-rated gas, oil or LPG condensing boilers. There was a discussion on the AECB forum aecb.net about the real CO2 emissions of 1 kWh of electricity, which affect this comparison.

    It seems highly unlikely that a heat pump operated in winter - when the national grid is burning a lot of coal, not just nuclear and gas - could do better on CO2 than a gas boiler. Same with LPG, given that this emits 0.24 kg per kWh and electricity from coal emits 0.92. People could do something sensible, which could be to install the boiler but spend the cost saving on improved insulation and airtightness.

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  5. I can't disagree with any of the above. If you are looking to compare CO2 "costs" then you need to consider the fuel used by the electricity generator & the efficiency of transmission. If you've got mains gas, my guess is that today, the CO2 cost (& the monetary cost) will favour a modern condensing gas boiler over a GSHP. On the other hand, a gas boiler will only ever use gas, so the cost can only deteriorate as the boiler efficiency declines with age & the cost of gas increases. I'd like to think that the impact (& running cost) of a GSHP will improve over time as the technology of production & distribution of electricity improves. At least, that's what I keep telling myself (I've bought a GSHP, but then the choice for me was that, LPG or oil)