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Last forum (17th June 2014) Thermal comfort in older houses
Download a condensed version of this FAQ as an A4 PDF here
A heat pump works like a refrigerator, only instead of cooling the inside and dumping waste heat outside, it is used to heat a house by cooling the outside (either the air or the ground or sometimes water) and dumping the heat inside the building. The great thing is you get more heat energy into your house than the energy you use to run the pump: typically 2-3 times more.
There is a third sort of heat pump called an exhaust air heat pump. This takes in air from inside your house rather than from outside. It may seem bizarre to be using air which is already warmed up but it is much more efficient to do this than to let the warm air escape through draughts taking its heat with it. The EAHP system is a bit like MVHR (mechanical ventilation with heat recovery). However, whereas heat recovery will cool the exhaust air to close to outside temperature, the heat pump will cool it considerably further (using some extra energy to do this but not as much as the extra heat energy extracted).
The super efficiency of heat pumps does however have some limitations and in particular they aren't very good at generating high temperatures. You can expect only up to 50C from a ground source heat pump or EAHP and just 45C from an air source heat pump with reasonable efficiency. For comparison, a gas central heating system will usually run the radiators at around 75C.
To use this lower temperature heat effectively, you can either use underfloor heating, skirting board heating, or warm air heating, or forced air radiators such as ecovector. As well as operating at lower temperatures these systems are usually more efficient too because they heat from below. With conventional radiators relying on natural convection the warm air goes straight up to the ceiling but floor level heating warms your feet first.
You can probably get away with water at 45C most of the time (just mix in less cold in your bath) but you need to heat your hot water cylinder up to at least 60C once a week to avoid the danger of legionaires disease. You can do this with the heat pump or, more efficiently, with a backup immersion heater.
However, if you are using warm air heating then you will need a separate heating system for your hot water.
You can combine a heat pump system with a solar hot water panel as well: see our solar thermal FAQ for more information on this.
It will almost certainly save energy overall but not necessarily carbon emissions, at least at the moment. With the current UK power station mix, generating each unit of electrical energy produces 1.8 times the carbon emissions as a unit from heating oil, or 2.6 times as much as a unit of gas (See Digest of UK Energy Statistics). So most heat pump systems will have lower carbon emissions than an oil fuelled heating system but they are unlikely to be be much better than a gas system at the moment. Hopefully, this will change as our electricity grid system uses more renewables and becomes less carbon intensive.
Similarly, whether or not you will save money on your bills depends both on the heat pump efficiency and the relative price of your alternative fuel, which you can also expect to change over time. With peak oil and peak gas approaching one would have thought electricity would be relatively less expensive in the medium term, but this is not guaranteed. Right now, heat pumps are usually cheaper than oil or electric storage heaters but not necessarily cheaper than mains gas.
If your house has a high thermal mass you may be able to run the system only at night and use an economy 7 tariff.
However, whether or not you save money on your bills you could soon earn money through the renewable heat incentive (RHI).
The Renewable Heat Incentive (RHI) is a government scheme to encourage people to invest in renewable heat technologies such as solar panels for hot water, heat pumps and biomass boilers. It is like the Feed-in Tariff Scheme (FIT) in that it gives you an income based on the amount of energy you generate, and hence a guaranteed income independent of fuel prices. The RHI currently only applies to non-domestic installations. Households will be added later: currently the timetable is for summer 2013.
Currently the RHI only covers ground source and water source heat pumps, not air source. They may be included at a later date.
You don't have to delay your installation in order to qualify for the RHI. Any heat pump systems installed (by an MCS accredited installer) since 15th July 2009, when the proposal was announced, will qualify as if they were installed on the start date of the scheme. However, we don't know yet exactly what the conditions and rates will be.
According to the proposal, rather than measure the amount of heat you have generated, your property will be assessed as to its heating needs and what the heat pump will supply. The assessment is based on the average heat load of a property of the same type in which reasonable energy efficiency measures have been taken. This means that you won't be paid extra for not making your house more efficient in other ways.
The current tariff is 4.3p/kWh for ground source heat pumps for 20 years. In the case of the FIT the payments are tax free but there is no mention of this for the RHI.
In the meantime, your house may qualify for a Renewable Heat Premium Payment grant towards the installation of a heat pump. To qualify you need to use oil, compressed gas or other heating - not mains gas. The grants are £850 for an ASHP and £1250 for a GSHP.
COP means coefficient of performance and it indicates how much heat you get out of your pump for every unit of electricity you put in. The COP varies with the temperature difference and is often quoted based on a 7C ambient temperature. The overall system efficiency reported in the EST report takes into account the electricity you need to drive your central heating system and backup systems as well. In most cases the overall efficiency will be a little lower than the average COP e.g. if the COP is 2.3 the efficiency could be 2.2.
The Energy Savings Trust (EST) conducted a field trial recently of 83 systems of various types. They found excellently performing systems with efficiency > 3.0 of both types but the average was around 2.3 for ground source heat pumps and 2.1 for air source. Nearly half the air source heat pumps and two thirds of the ground source heat pump systems had efficiencies > 2.2, which means they were as good or better on carbon emissions than a gas condensing boiler with 90% efficiency.
Of the ones that did perform poorly, sometimes this was because the heat pump was too small or too large and some were poorly configured. Often the householders, and sometimes even the installers, did not understand the controls very well. It is important to choose an installer who is experienced and make sure that they explain to you how the system works.
The most important factor in heat pump performance is the target temperature. The higher the target, the harder the heat pump must work to 'move' heat 'up' a steeper temperature gradient. It is the same with your fridge/freezer - the freezer has to work harder than the fridge to achieve a bigger temperature difference. For example, suppose you get water in from the mains at about 10C. Heating that to 70C takes twice as much energy as heating it to 40C anyway, but also the pump has to work harder to move the heat up 60C rather than 30C so overall the system needs at least 3 times as much energy.
The outside temperature also matters, and air source heat pumps will perform worse when it gets very cold outside and use increasing amounts of energy on automatic defrost as well as fighting against a larger temperature gradient. However, they should work, even down to -15°C, which I can't remember ever experiencing here in Cambridge.
Ground source heat pumps are less affected by the weather because the ground temperature does not vary so much, especially for the vertical loop systems which can go as deep as 80m.
Some heat pumps can be run in reverse in summer for cooling, so yes. But you will need to make sure you buy a unit which has this capability and it may not be as efficient as a one-way system.
You don't run your fridge with the door open and it is the same with your house: for a heat pump to work well the house needs to be draught proofed and insulated first. For an EAHP the draught proofing is not so critical as warm air in the house is drawn through the pump rather than escaping to the outside through draughts.
Also, heat pumps aren't good at generating heat in fits and starts, they like to work steadily most the time. So it is best if you don't let the house get cold and then try to heat it rapidly in the evening when you get home. Ideally it should have a high thermal mass so it doesn't cool down rapidly even when the heating is off.
The heating system is also critical. Normal gas central heating systems run water through the radiators at about 75C. The hotter they are the faster they warm the room. However, you don't want your heat pump to have to generate more than 45C-50C at the most and ordinary radiators will not be very efficient at this temperature. You can make them more effective by having larger radiators (more surface) or by incorporating a fan to force the air past as in ecovector radiators. Alternatively, you can use an underflooor heating system or warm air ducts which are more effective at lower temperatures.
Finally, you need to have a heat source for your heat pump. For a ground source heat pump you need either suitable ground (and access) to bore deep holes, totalling at least 100m, or space to lay the same length but in shallow trenches, 1-2m deep. Deep bore holes cost more to install and whether or not you can use them depends on the geology around your house. However, they give you a more efficient heat pump because the temperature does not change with the seasons at the deeper levels. The actual holes are only about 100-150mm wide and the drilling equipment is not so large as you might think, see pictures below.
If you opt for shallow trenches, you can use a zigzag pattern to make good use of your space. It is common now to use a 'slinky' arrangement with overlapping coils about 1m wide. A 4kW GSHP with a slinky-type ground-array needs a 1x50m trench, which must ALWAYS be at least 5m from the footings of a building. A 6kW GSHP needs 2*50m trenches, which must be kept away from footings and 5m from each other, centre to centre. If the GSHP is providing DHW as well, a longer or additional slinky will be needed.
For an air source heat pump you need somewhere to install the external unit well ventilated and exposed to the elements, such as an outside wall or flat roof with around 4m2 space around it. The unit itself can be quite small e.g. 1m x 0.5m x 1m high but it depends on how much heat you need. Also it needs to be reasonably close to the house - not more than say 15m.
If the heating system has been configured correctly you should be fine with a GSHP because even in cold weather the ground stays relatively warm. Similarly an ASHP system sized correctly should be sufficient though it will be less efficient when the outside temperature is very cold. It will have to run the fans faster and process more air and the COP will be down - maybe from 3.5 at 10 C to 2.0 at -5 C. However, an EAHP system may need topping up because even though its incoming air is (or at least should be) at a constant temperature it can't increase capacity by increasing air flow -- if it did you would quickly notice more cold air being drawn in from outside.
If you have an EAHP system you could supplement it with an additional heat source, for example a wood stove, for when it is very cold. The EAHP will redistribute this heat all around the house. If you top up an ASHP system in a similar way, you will only heat the area around the stove.
In the case of air source heat pumps and shallow ground source heat pumps you are using solar energy. The sun warms the air and the soil surface. For deep bore ground source heat pumps at least some of the energy comes from the heat in the rock below us - from the earth cooling down and from radioactive decay in the crust.
An EAHP system is mostly inside and only has an outlet for the cold air. It is normally built into your hot water tank. It will need some internal ducting, to take in warm air from suitable inlet points to the main unit (not nearly so much as with MVHR which needs an inlet or outlet in every room).
When it is in the ground, the heat pump is invisible. Here are some pictures from drilling boreholes, and links to the EST website picture of a shallow trench with slinky.
All heat pumps will be frost resistant and the circulating fluids contain anti-freeze. Even air source heat pumps should be able to operate down to air temperatures of -15&dec;C, although they will not be very efficient in those conditions. When frost builds up on the external heat exchanger up it will automatically enter a defrost cycle.
We haven't found any literature on this but from our rough calculations we would say no, provided the array has been sized appropriately. Suppose you have 3x30m trenches in your garden, 5m apart, laid 1.5m deep.
The heat energy comes from the sun: even in winter you should get an average of around 0.6 kWh/m2 (PVWATTS) so on 3x5x30m that will be 270 kWh/day, ignoring shading. One would hope that your heating needs will be less than this! Of course some days will be very overcast so you will get less but the soil will act as a buffer. The volume above the array is 1.5x3x5x30 = 675m³ and this has a heat capacity of 100-270 kWh/C depending on the type of your soil (600-1400 kJ/m³/K from Lyndon State College Atmospheric Sciences) so for every 100kWh you draw out of the soil you will lower the temperature by less than 1°C.
Heat pump systems need very little maintenance usually but you should have an annual check to ensure all is OK.
Ground source heat pumps should have a design life of around 25 years. Air source heat pumps may not last quite so long as they have more moving components. You can expect a ten year warranty.
A ground source heat pump will cost around £7,000 - £13,000 to buy and install. An air source heat pump is not likely to cost more than £10,000 (according to the EST). An EAHP can cost from £1,000 to 4,400, but bear in mind that this includes a hot water cylinder. Installation will be in addition, but is almost as straightforward as for an immersion heater system.
If you need to make changes to your radiator system that will be extra. Also, if the heat pump is providing you with hot water you will almost certainly need a new hot water cylinder which has a large surface of heating coil. This allows for the lower temperature in the heating circuit coming from the heat pump.
Ground source heat pumps do not need planning permission unless your home is a listed building or in a conservation area. Exhaust air heat pumps do not need planning permission either but normal air source heat pumps do. Also heat pumps will have to comply with building regulations but your supplier will probably be qualified to self certify their work.
You must use a supplier certified with the MCS certification scheme in order to qualify for the RHI. You should also compare quotes and ask for reference sites, as with any supplier, and check out the details of the warranty - what is and is not covered. You should have a minimum one year warranty on the installation.
The heat pump supplier may or may not provide the heating system as well. If they do not, you need to make sure that the two systems are designed to work together. Before choosing the heat pump you must know the target temperature for circulating air or water in the heating system and the overall heating demand.
You should ask them to estimate the system efficiency you can expect and ask them how they calculated the value they give you. This needs to be based on the heating system needs for your particular house, not just the average given in a product brochure.
Your heat pump supplier may not be able to dig trenches or drill bore holes themselves in which case they should at least project manage that part of the work, unless you really want to of course.
We don't know of very many installations near here. If you would like to recommend someone please do tell us. Here are some local suppliers we have talked to:
You can find general information about heat pumps at the
From suppliers (but not necessarily entirely objective) - as well as the local suppliers listed above here are ones we know of, do tell us if these guides are helpful or if you know of better ones.