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Figure 1: ASHP Monobloc
A heat pump is an alternative to a gas boiler for providing domestic heating and hot water. Most heat pumps being installed in homes are 'Air Source Heat Pumps' (ASHPs) meaning they extract heat from the air. There are also ground source heat pumps, which take the heat from the sub-soil using buried pipes. These are much more expensive and not are so often installed in UK homes. This article solely relates to air source heat pumps.
The heat pump works like a refrigerator in reverse. A fridge extracts heat from its inside making it colder and dissipates this heat from fins on its back. An air source heat pump extracts heat from the outside air and uses this heat to warm the house and heat water. Even if the air is very cold (down to negative temperatures) the heat pump can take heat out of it.
Figure 2
A Heat Pump Domestic Heating System
A Heat Pump Domestic Heating System
Figure 2 shows a simplified diagram of an ASHP system. Most of the heat pump components are housed in a box (about 1.3 x 0.9 x 0.5m) called a monobloc (shown in Figure 1). This sits outside the house in a place where good airflow is ensured, so air can be drawn through by a fan, and the heat pulled out of it.
Figure 3: Tank assembly made from individual components
In addition to the outtdoor monobloc unit there needs to be indoor space for a hydraulic sub-system made up of a hot water tank, pump, diverter valve, expansion vessel, controller and associated components. Insulated pipework carries hot water from the monobloc into the house to a diverter, governed by an electronic controller. This switches the water between radiators and the hot water storage tank, to keep the house and hot water at the required temperatures. Hot water taps, showers, etc., are fed with hot water from this tank.
Figure 3 shows the indoor hydraulic sub-system built from individual components in an interior alcove or cuboard. The floor space taken up is about 850x850mm. The house might already have a cupboard for a hot water cylinder, and if so this can be used.
Alternatively, the Figure 4 shows an all-in-one indoor unit containing the hot water tank and the other components in a cabinet about the same size as a large fridge-freezer, say about 600x650x1800mm. Most major heat pump manufacturers can supply these all-in-one units.
Figure 4: All-in-One Tank Unit
Heat Pump Q&A
Heat Pump Q&A
Am I being forced to get rid of my gas boiler? The Future Homes standard (being phased in) specifies that new homes will have low carbon heating such as heat pumps. But there is no proposed date for banning new or existing gas boilers in existing houses.
Why might I want a heat pump? A heat pump delivers gentle heat all the time, keeping the house at a constant temperature (although cooler rather than cold at night). Some people find this more comfortable than a gas boiler regime which gives a cold house through the day, then a blast of heat to warm it up in the evening. Heat pumps are entirely powered by electricity and can be 'greener' than gas heating if fed from a low carbon source of power.
What about cost? If a heat pump is run from full price electricity, running costs should be about the same as gas. But it is important to get on a heat pump tariff, where the average cost of electricity is much lower, and running costs with the heat pump will undercut those of gas. Costs of installation can vary widely depending on the house and the installer. It is important to take advice, shop around, and get several quotations. Currently, government offers a grant which can go a long way to offset installation costs.
Does my house need to be super insulated? Insulation is the first thing you should look at no matter what heating source you use. Usually, if a house has good loft insulation, cavity wall insulation, and double glazing, it will be suitable for a heat pump. If you have poorer insulation standards, a properly designed heat pump system will still work, but running costs would be higher, as they would with gas.
Do I have to have underfloor heating or new radiators? Heat pumps run at a low temperature which works well with underfloor heating. Radiators are fine too, but some might need to be changed for bigger ones to suit the lower temperatures.
Are they noisy? Modern heat pumps are quiet, producing a steady fan sound similar to a fridge or air conditioner. The internal components are no more noisy than a normal central heating system. If properly installed a heat pump is unlikely to be any more intrusive than the sound of a gas boiler. An assessment to ensure compliance with noise standards forms parts of the design process.
How much space do I need? You need outside space for the monobloc, and room around it for good air circulation. There will need to be space inside either for a tank and associated coponents or an all-in-one integrated unit. Approximate dimensions of these are given in the article.
A heat pump seems bigger, and more expensive than a gas boiler. The cost will come down as more people use heat pumps and the economies of scale kick in. As technology is advancing the space required is reducing. As part of an integrated energy system a heat pump could well save you money in the long term.
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