Articles
Heat Pumps: A Layman's Guide
Jan
2010
There are 3 different types of heat pumps – air source, ground source and water source. They all use the same compression/expansion technology, identical in all but size to that used in domestic refrigerators. They are called heat pumps because they are intended to move heat from one place to another. They do not, or should not, generate heat. Heat pumps work by collecting heat from air, water or the ground and, in effect, magnifying the heat and transferring it to water or air.
Because the technology “magnifies” the heat, the less magnification the more efficient they are. That is, the closer the source temperature and the output temperature, the more efficient the heat pump. The ground below 1m deep remains at a fairly constant 70C to 120C through out the year. We want a temperature of 210C in the house, a temperature difference of 140C to 90C. External air, on the other hand, can vary enormously in temperature. This winter has seen temperatures of -200C giving a potential temperature difference of 410C.
Heat pump manufacturers of heat pumps advertise efficiency as a Coefficient Of Performance (COP) – typically 3, 4 or 5. This means that for every 1kW of electricity you put in, you will get 3, 4 or 5 kW’s of heat out. It is calculated based on optimum input and output temperatures. The output temperature is the critical factor and will generally be between 300 and 400C. This is to minimise the temperature difference and achieve a reasonable COP. 300 to 400C are good operating temperatures for underfloor heating. Radiators need temperatures over 550C and hot water needs to be stored at 650C to avoid Legionnaires disease. Heat pumps working at these temperatures will have a different COP to that advertised.
The big questions around heat pumps are:
Do they work? Yes, is the short answer. Fridges work, have done for years and heat pumps use the same technology. The first UK heat pump was installed in the Royal Festival Hall in 1952 and was still working well into the 1980’s.
Which type is best? Depends on the situation. Where the insulation levels are high and the heat load low, then the lower capital cost of air source makes sense. If the heat demand is higher then the better COP inherent in ground source is preferred. If there is a source of water available or the cost of a borehole is low, then water source will be best.
How big are they? Again it varies. In broad terms from the size of an under-counter domestic fridge to a 4-draw filing cabinet.
Do they make much noise? Noise output will vary from 40dB to 55dB (at 1m distance), where a library is reckoned to be around 30dB, a fridge about 50dB and normal conversation about 65dB.
What do they cost? A typical 8kW heat pump might cost £5K to £6K for air source and £8K to £10K for ground source. Water source prices will be dependent on the cost of boreholes but the equipment will be a similar price to ground source.
Why are they so expensive? In 2008 only 14,000 units were installed across the UK compared to 1.6 million gas boilers. Because they are sold in relatively small quantities they do not benefit from the same economies of scale. None-the-less, over their life they cost less to buy and run than an equivalent gas condensing boiler.
Why do heat pumps fail? Generally for 1 of 3 reasons:-
- The heat pump is too small to heat the house. Some suppliers are rigorous in ensuring that the heat pump matches the heat demands of the property. Being rigorous can mean a bigger machine, and a bigger price tag, than the customer expects.
- The heat pump is driving inefficient radiators. Generally this applies to retro-fits. Heat pumps can drive radiators but they need to be high efficiency, low flow temperature rads, and the COP will still be poorer than with underfloor heating.
- Not enough ground. This is the most common cause. The ground is capable of rendering up 20W of heat per square metre. That is the amount of heat that the sun is able to put back in the ground each year. For every 1kW of heat output from the heat pump the system needs 50m2 of ground. For a typical 8kW heat pump that is 400m2 of ground. Any less and gradually, over time, the temperature of the ground will fall, as will the efficiency of the heat pump.
Heat pumps are a good, efficient and convenient method of heating a modern, well insulated home. A good system, properly installed will give up to 20 years good service. The wrong system, poorly installed is a square peg in a round hole. It can be hammered home, but it still don’t fit.