Time to find out about heat pumps…

Jenny Love, UCL Energy Institute

I’ve been spending the last 3 months working on domestic heat pumps –  major players in the debate about future heating for the UK –  but whilst trying to converse with my family about this over Christmas, it became clear that they had no idea what I was talking about.  So I’ve decided it’s about time to make heat pumps known. This article is based around some of the questions you might ask if you’ve never heard of a heat pump and don’t have an unhealthy obsession with thermodynamics. It’s always good to be able to impress your friends or potential mate by pointing out a heat pump when you see one:

air source heat pump

(source of photo: http://gogreenheatsolutions.co.za/category/project-type/domestic-heat-pumps)

1)      What the heck is a heat pump?

Normally in Britain you heat your house by switching on your boiler, which burns gas, heats water, and sends it to your radiators. The source of heat is the chemical energy in natural gas. Now, gas may or may not be around to stay – if it is it will only get more expensive – so there is talk about whether electricity will replace gas for heating homes. Electricity is normally made in a power station from heat, a process which loses a lot of energy, so putting the effort in to make electricity only to transport it along a wire and turn it back into heat for your house is quite wasteful. But a better thing you can do with electricity is move heat which already exists – I don’t just mean from one physical location (like the ground) to another (your house) – I also mean from one temperature (cool ground temperature) to another (warm living room temperature).

A heat pump uses electricity to move heat from a cool place to a warm one. When I first heard about this I thought it must be some kind of miracle, but then someone pointed out that that’s how a fridge works (electricity is used to move cold heat from inside the fridge to the warmer exterior, keeping your fridge cool).

Why would you do this – well, if you do the physics, you can work out that putting in a bit of electricity can move a lot of heat (about one unit of electricity can put 3 units of heat in your house), meaning that the energy use from heating your house with a heat pump should be much lower than that from using a normal boiler.

Now, you may not have heard of one of these devices, but David Mackay, the chief scientific advisor at the Department of Energy and Climate Change, recently gave a lecture (at this event: http://www.lolo.ac.uk/newsandevents/page/id/53) in which he foresaw 20 million of them installed in homes by 2050 (almost one in every property). So maybe it’s time to find out more about them…

2)     In what ways is using a heat pump different from how I normally heat my home?

Heat pumps  work best when the temperature difference between the heat’s original location (e.g. the ground) and its end location (e.g. the living room) is low. This normally translates to the temperature of the thing in the room which is going to give you heat (called the ‘emitter’ – e.g. a radiator) not being as high as we’re used to. Here are a few consequences of this that you might find strange:

a)      Heat pumps work best with underfloor heating instead of radiators.

If the temperature of the emitter is low (e.g. 30 degrees C), you need a high area of emitter to give out enough heat. For example, the area of the floor of the room. It would be silly to have a radiator this big so underfloor heating is often used.

b)      They work best if you have them on all the time.

If the temperature of the heat delivered to the space is low, you can’t get enough heat out if you just have the system on for a bit in the morning and a bit in the evening. Having the heat pump on all the time is something that we’re not used to but is necessary for the right amount of heat, delivered at a low temperature.

c)       They are very easy to operate sub-optimally

Take a normal condensing boiler. Its efficiency (see below for a definition) is probably around 83% whether it is on for a long time, a short time, whatever the temperature settings on it, however it is installed in a property. Boilers are relatively robust against variation in operating conditions. Heat pumps, however, are a very different kettle of fish. If any part of it (e.g. the hole in the ground, the compressor, etc..) is too small, it can’t provide enough heat and a backup electric (i.e. wasteful) heater comes on. If it is too large, firstly it might suck too much heat out of the ground and freeze it; secondly it might switch off and on quite a lot – if it does this more  than once every six minutes (https://www.decc.gov.uk/assets/decc/11/meeting-energy-demand/microgeneration/7389-effects-cycling-heat-pump-performance.pdf) then this is detrimental to its performance. There are lots of things to set correctly: pump speeds, temperatures in the system such as water flowing around to the emitters, the way it ramps down when the weather outside it warmer (called ‘weather compensation’) and plenty more. The thing is, you probably won’t know whether it is working optimally or not. I would like to see a heat pump which monitors itself as a whole system and tells you that kind of thing.

3)      Do heat pumps actually work, and how would I know?

Efficiency, or performance, of a heating technology is generally defined by heat it provides / energy you put in. This is the source of the 83% mentioned in section 2 (from this report:         https://www.decc.gov.uk/assets/decc/what%20we%20do/supporting%20consumers/sustainable%20energy%20research%20analysis/1_20090710101754_e_@@_20090622cbl026condensingboilermonitoringfinalreportjune2009.pdf). Now, to measure this, there are two kinds of test. Firstly, lab tests: those done by the manufacturer which say, “oh look, you put one unit of electricity in and move five units of heat to your house, that’s lovely”; secondly, those which are done in real houses with real occupants. The latter are known as ‘field trials’ and have been carried out in the UK – you can read about them here: http://www.heatpumps.org.uk/PdfFiles/TheEnergySavingTrust-GettingWarmerAFieldTrialOfHeatPumps.pdf

or if you are quite used to scientific reports then here: http://www.decc.gov.uk/assets/decc/11/meeting-energy-demand/microgeneration/5045-heat-pump-field-trials.pdf

The moral of the story is that, of course, heat pumps do not work in-situ as well as in the manufacturer’s lab (like anything really), but that they can work well:

-          It has been shown that the whole system is extremely important. To work well, everything about them has to be done correctly: the sizing of the pipes which are buried underground to pick up heat (the ‘ground loop’), the sizing of the actual heat pump box, the insulation of the pipework going into the house, the various temperatures in the system…

-          There is a way of measuring this overall success as opposed to that from lab conditions – it is called the seasonal performance factor. It’s quite simple really – you measure the heat delivered by your heat pump to your rooms/the hot water, and divide it by the electricity the heat pump uses. You’re looking for an answer of at least 3, really, for it to be worth it. The SPF is what the aforementioned field trials were trying to measure. That’s what you should ask the manufacturer about.

4)      When is a good time to buy a heat pump?

In my field, we have a saying: ‘Fabric first’. What we are talking about is this: when you take a building and want to make it energy efficient, the most cost-effective thing to do first is to reduce its heat loss, by insulating the building fabric, sealing up gaps, getting rid of cold bridges, etc. Then only when you have done that should you consider changing the heating system, which will cost you a bit more for the same amount of carbon savings (after that, it’s time to think about fixing solar panels onto your roof). The reason some people do it the other way round is that solar panels are more sexy than boilers, which are more sexy than insulation. (Guess what my PhD is about: insulation.)

scale of sexiness

(image sources: http://www.swenergy.co.uk/solar-thermalhttp://www.heating-solutions.biz/,  my PhD fieldwork)

But in our field we always advise making our house more airtight and better insulated before considering getting a heat pump. There’s a good reason for this: as I mentioned earlier, heat pumps work best when they’re on all the time. If you have a leaky building, then you’re constantly going to be putting heat in, which is constantly being lost to the outside. Bit of a waste.

But if your house is quite well-sealed, then NOW is a good time to buy a heat pump. If you’re quick, there’s currently a discount from the government’s Renewable Heat Premium Payment scheme:

http://www.decc.gov.uk/en/content/cms/meeting_energy/renewable_ener/premium_pay/rhpp_voucher/rhpp_voucher.aspx

And then later on the Renewable Heat Incentive’s domestic scheme will be launched: http://www.decc.gov.uk/en/content/cms/meeting_energy/renewable_ener/incentive/incentive.aspx

However, it is not yet the best time to install a heat pump in terms of CO2 emissions. If you are on normal grid electricity, and your heat pump performs well, it will cause less emissions than a condensing boiler; if it doesn’t then it may well not. This is because grid electricity is still pretty high in carbon. If you choose a renewable tariff, you can avoid this, but otherwise you may have to wait until renewable generation is a higher proportion of the UK’s energy mix.

5)      Is it possible to be a bit too excited by heat pumps?

It is, yes. The best example I’ve found of this is the following three minutes of video captured excitedly by a phone camera of a heat pump in defrost mode:

I’ll leave you with that…

One thought on “Time to find out about heat pumps…

  1. Hi

    Very informative, thanks for posting. Will try and follow your blog from now on.

    all the best

    Chris Avgherinos

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