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Heat pumps
Heat pumps are a more efficient and cheaper way to heat your home compared to an electric, gas or LPG heater. This is particularly true when the home is well insulated and the heat pump is installed and used properly.
Heat pumps, when used in heating mode (they can also cool your house) extract heat present in the air outside your house and move it indoors. The work to accomplish this transfer is done by a small electric motor that runs a compressor and fans.
Heat pumps have two units, one outside the house that takes the heat out of the air, and one inside the house that delivers the heat. The inside unit can be placed on the wall, floor or even ceiling.
Heat pumps can be very efficient at transferring heat. Their performance is measured in terms of their Coefficient of Performance (COP).
When used for heating a house on a mild day, a typical heat pump has a COP of 3 to 4 whereas a typical electric resistance heater has a COP of 1.0. That is, one Joule of electrical energy will cause a resistance heater to produce one joule of useful heat, whereas one Joule of electrical energy will allow a heat pump to move 3-4 Joules of heat from outside your house to inside it.
Note that the efficiency of a heat pump is related to the difference in outdoor and indoor temperatures. When you are heating your house on a very cold winter day, it takes more work to move the same amount of heat indoors as on a mild day. A heat pump's performance will approach a COP of 1.0 at around -7°C outdoor temperature.
For a heat pump to work properly it needs to be the right size for the volume of space to be heated and it must be positioned correctly. It also needs to be used properly. Heat pumps have internal timers that can be set to turn on and off automatically when heat is required. For example, they can be set to come on at a certain time each morning so that you can get out of bed to a heated house. They can also be set to a desired temperature which is then automatically and efficiently maintained.
It is important to get expert advice when choosing and installing a heat pump.
Note that a different kind of heat pump – a ground source heat pump – maintains its performance at all outdoor temperatures. Ground source heat pumps are not yet common in New Zealand, but are widely used overseas.
Ground source heat pumps – also known as “geo-exchange” or “earth energy” systems take advantage of the fact that the earth is a massive storage facility for energy that can be used for heating and cooling. The earth contantly absorbs and stores heat from the sun, keeping temperatures just below the surface relatively constant all year round. To tap this renewable energy source involves installing a network of underground tubes through which a heat absorbing fluid, such as ethanol, moves, transferring energy. These tubes can be installed horizontally underground or, where land is costly or scarce, they can extend vertically down 60 metre or more. When the air is cold the system will capture and transfer the earth’s heat which is warmer than the air. When its warm outside, the system will capture cold. A typical system will produce three units of energy for every unit of electricity used to run them.
The following website is a good source of information on ground source heat pumps:
http://www.canren.gc.ca/prod_serv/index.asp?CaId=169&PgId=1023
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