A heat pump is a heating system that extracts heat from the things around it.  A normal refrigerator extracts heat from the food inside it.  That heat will come out of the back of the fridge somewhere.  Heat pumps extract heat from the air (air source heat pump) or the ground (ground source heat pump.)  They work in the same way as a refrigerator and the heat is sent into the central heating system.  The heat pump is effectively an electric heater  that is much more efficient than an ordinary electric heater would be.  In a typical hot water central heating system, an air source heat pump produces 2.8 times as much heat as you would get from an ordinary electric immersion heater.  2.8 isn’t the maximum limit and some modern companies will quote a heating COP (efficiency) of 5 or 6.

Which is the best type?  Ground source is better than air source.  The problem with a simple ground source system is that it is useless unless you live in a field.  The pipes aren’t all that deep down and you need to keep them away from trees.  You could cover a quarter of a square mile with these pipes.  These systems are used – they are very good in a rural area that doesn’t have a gas supply, in which case a large field near the house might actually be available.  If you have a lake near you, that can also be used by a water source heat pump.  A more complicated ground source system has been used in cities which can fit all the pipes under a manhole cover.  This is called vertical bore.  It has the disadvantage that you need to bore a hole a long way down for the pipes – up to 400 feet for a customer I had in London who wanted to heat a block of flats.  A good idea technically but obviously very expensive.  A more common system is an air source heat pump.  This is the propellor in a box that sits on the side of some modern buildings.  The efficiency is lower than a ground source heat pump, but it can still achieve 2.8 in a standard radiator system.  This is more convenient and can be used anywhere as long as you have room to put the box on the side of the building (it’s about one metre square) and have three metres of space in front of it to allow it to draw plenty of air.  Heat pumps work better with low temperature underfloor heating but some can still drive conventional radiators.  Air to air heat pumps with ceiling cassettes are very efficient but more usual in an office than they are in a home.  It all has to do with the temperature difference between the stuff going into the heat pump and the stuff coming out.  Conventional radiators are very hot things.  Low temperature underfloor heating and air cassettes run colder so they are more efficient when used with heat pumps.

What do I think of heat pumps?  They sound like miracle free energy machines but the reality is somewhat different.  The electricity they run on is generated in coal fired power stations so their green credentials aren’t all that good.  If the heat pump has an efficiency of 2.8 and the power station has an efficiency of 40% you’re only getting back what you put in – it’s like a 100% efficient coal fired heater.  Bear in mind that some modern gas boilers are almost 100% efficient anyway.  The CO2 emissions are round about the same for gas and heat pump and electricity is a more expensive fuel than gas, so they wouldn’t save you any money.  I would certainly use them in a rural area.  It is almost impossible to get a house to pass the SAP test with a conventional oil boiler.  The heat pump is particularly good if you have no gas supply.  An air source would be as good as a gas boiler, a ground source heat pump even better if you have a lot of empty land with no trees near the house.  In a SAP rating you get a higher Target Emission Rate (TER) for having the heat pump as it is an electric heater instead of a gas heater.  The house has to have CO2 emissions below the TER in order to pass the SAP test so the heat pump can make the job easier.


What is a Heat Pump?