Heat Pumps FAQs


We worked with Irish partners, experts in the field with over thirty years experience: they put in their first air source heat pump thirty odd years ago and it's still working!  Try and get independent evidence that they're good - not just that they work but they're good.
Heat Pumps
Heat pumps take heat from one place – usually outside: compress it to raise the temperature and use it to heat the house, office, swimming pool etc.  They work like fridges or air-conditioners in reverse. Domestically they typically work with underfloor heating (at ~30 degrees C) but the best are good enough to work with radiators (which need hotter water ~60 degrees C).  Commercial systems can go even better, if you have a consistent requirement for heat they're worth looking at.


They can take the heat from the air outside, from the ground, under a large area or drilled straight down, or from flowing water.  They can even take heat from inside and use it to heat up water to 70degrees but that’s a special case.


They are usually used with a heat store – the equivalent of a hot water tank – to store the heat and use it later.
Why is it viable, if it’s just moving heat around? 
Well each unit of electricity used can provide up to five units of heat. The cheapest electricity can produce 1kWh of heat for less than a penny compared to 4.2p for the most efficient gas boiler. (prices vary a bit over time obviously but the ratio stays similar)   The ratio of heat to electricity is the COP, the higher the better. 


What’s the COP?

The key measure for heat pumps is the COP the Co-efficient of Power: i.e. amount of heat output for a given input of electricity. 


Under normal conditions a typical COP should be 3.0 to 3.6. The best systems reach 6.5.  Claims of COPs of 5 or over should be checked with independent sources.


So they only work when it’s warm outside?
No they can work when it’s below freezing but they are more efficient at medium temperatures.


How can they work when it’s below freezing? 
OK, bear with me here; in a one degree drop from 20 to 19 degrees above freezing there’s the same amount of heat as a one degree drop from -19 to -20 degrees below!  We normally measure temperature from the freezing point of water rather than say that of mercury or oxygen or even absolute zero.  E.g. see here for a system in Antarctica !


So the bigger the COP the better the heat pump? 
In general yes, but on all heat pumps the COP will vary with the input temperature and humidity. Salesmen will use the best COP they can, even if the system isn't going to achieve that in practice.  The salesmen will also forget to include in the COP when the system uses an immersion to defrost the blades – because it’s an expensive way to do defrost the blades but a cheap way to built heat pumps.


Is there a standard COP measure?

The headline COP should be the number in the yellow column and you can see the COP can go up by 20% or more depending on the test.


So what is the best heat pump? 
According the German Institute for Energy Systems NTB Buchs it’s domestically the Oschner 14 plus which has a COP of 4.4 (June 2012) under the standard test. See the above link.


Are all systems the same? 
No, design, size, input and output temperature, usage all have an impact on how efficient a heat pump is.  The fluid/gas used to transfer the heat also has an impact; different fluids work best at different temperatures.


Which is best? Air Source, Ground Source or Water Source? 

Or solar thermal or solar PV or a gas boiler or any other heat source?  The source is only the source of heat.  Whether you have a fancy condenser or two boreholes or a nice lawn laid over the top of lots of pipes the key issue is what heat you want, where and when, how much do you want to pay for the system and how much the running costs are?


The cheapest heat systems to buy are usually the most expensive to run.  That goes for straight electric systems as well – look up the prices of electric bar heaters.


But to answer the question, as a rule of thumb: an underground water source is the most effective because the heat is consistent and being replenished over and over.  [This is not the same as the water in the water table a couple of feet belowthe surface.]  If you are located south of the Thames, Air Source makes sense; north of Manchester/Hull Ground Source makes sense, but there are wide variations.  In London, Thames Clay is a good insulator as you’ll know if you’ve ever been down in the Tube in the summer and not a great source of heat – it doesn’t replenish the heat quickly after it’s been first taken out.


Why underfloor heating?  The most efficient way to heat an area is slowly heating it enough to make up for the heat lost through the walls, ceiling and floor *  Raising the temperature quickly is like accelerating a car – it uses up more power than just travelling at a steady speed.  So the aim should be to heat the area steadily – this does cause some friction where men like cooler temperatures and women like warmer: turning the thermostat up and down will cost!


Can they be retrofitted to radiators?
The best systems can, but do require an interim heat store – a hot water tank to you and me!  Simply heating from the outside temperature to the sixty degrees required is like accelerating a car from 0-60 as fast as you can – fun but not cost-effective.


Air Source?

These take the heat from outside and usually prime a heat store before they deliver heat to where it's required. 


Systems can be modified air conditioners; split systems (using a condenser outside and the the heat pump inside) Compact or large. They can either take a lot of heat out of a small amount of air or a little heat out of a lot of air. The best COP comes from split systems ie larger and taking a little heat out of a lot of air.


Systems that are simply modified air-conditioners in reverse work well in dry cold places but in damp cold climates (e.g. the UK and Ireland) their blades can and do freeze up with condensation.  This will eventually split and crack the blades.

Ground source?

Ground Source systems take it from underground: usually needing at least 250m of pipe.  The geology of the area is important: sand is a good conductor; clay less so.  Hard rock is usually good; porous rock less so.  But the latter can contain an underground water source.


Get an independent geology report.  Get a reliable installer – who should be responsible for the heat in your property not simply for drilling holes in the ground. Probably the biggest issue with ground source is that if it doesn’t work for whatever reason how can you tell what might have gone wrong underground?

Where don't they work?
Well they can work anywhere but realistically if you've already a decent natural gas supply then look at insulation and better controls to reduce your bills.
If you've a large poorly insulated area to heat e.g. a church or a large hall then volume heating, heat recovery and heating controls work and maybe infra-red heating.


* So insulate, insulate, insulate!