Sustainable heating solutions
Heat pumps are a more efficient and cheaper way to heat your home compared to an electric, gas or LPG heater. 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 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.
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 timers and thermostats.
It is important to get expert advice when choosing and installing a heat pump.
A ground source heat pump maintains its performance at all outdoor temperatures. 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 metres 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.
It can be cheaper to burn firewood in an efficient wood burner than to pay for electric or gas heating. This is certainly true if you have access to free firewood. Furthermore, wood is a renewable source of fuel, whereas gas isn’t. Assuming that it is regrown it is also carbon-neutral.
The quality and condition of the wood you use are important. It should be well dried and stored under cover. If the wood is wet some of the energy from the fire is needed to dry it which makes the fire less efficient. Where possible, use wood from plantation forests rather than native types such as Manuka.
When choosing a wood burner, consider the size of the area you want to heat, the thermal needs of your home and its layout. A draughty, uninsulated a house needs a bigger burner which will use more wood.
A wood burner can be inserted into a fireplace to replace an open fire. However this lets less heat into the room than a freestanding model, as heat is lost up the chimney. Wood burner installation needs a building consent from your local council.
If the layout of your house and its hot water system are suitable, wood burners can be equipped with a wetback system to heat household water.
Inefficient wood burners are very polluting. To address this problem, as of 1 September 2005 all wood burners installed on properties less than 2 hectares must have a discharge of less than 1.5 grams of particles for each kilogram of dry wood burnt, and a thermal efficiency at least 65%.
Wood pellet stoves
A wood pellet stove burns pellets made from sawdust and ground wood chips – waste products from the forest industry.
The pellets are stored in a hopper bin within the unit and are automatically fed into the fire chamber with an auger. There is a controller on the pellet stove which controls the speed at which the auger moves the pellets into the fire chamber, and hence the temperature of your house.
The products of combustion are vented to the outside through an exterior wall – a chimney installation is not required. Combustion is very efficient so wood pellet stoves release few particulate emissions.
Some units can be equipped with a wet back that will heat your water as well.
The auger that transports the pellets into the fire chamber, and the fans in the heater that circulate the heated air through your house, both require electricity. Therefore heating your home during a power outage could be a problem.
The size of stove you will need and the amount of fuel you will need is proportional to the size and the thermal performance (insulation efficiency) of your home. A 20 kg bag of pellets will heat 150 sq m of space for approximately 24 hours. Wood pellets are priced at about 38 cents per kg.