Air Source Heat Pumps (ASHPs)
Many of the wall-mounted air-conditioners that you see outside offices and shops are reversible, i.e. can operate in heating-mode as well as cooling. These are often optimised for cooling mode, so their energy efficiency in heating-mode can be inferior. Systems specifically designed for heating have been with us for some time. These are usually more energy-efficient, but the build-quality and efficiency will vary greatly from model to model; In many ways – you only get what you pay for.
The more common Air Source Heat Pump (ASHP) heats water, and is often optimised for house heating. The unit usually sits outside you building, and contains an electrically powered refrigeration mechanism with a finned heat exchanger and a large fan. The heat (extracted from the outside air) is transferred to water. This heated water is piped to radiators or underfloor heating inside the house. It will probably heat a domestic hot water cylinder too (DHW).
ASHPs are becoming quite popular. The Japanese-type design has become very sophisticated and energy efficient. They are relatively easy to install but like all heat pumps, great care must be taken with the design to ensure that running costs are low.
The outside air is however not the ideal heat source, since in mid-winter, when the heating demand is highest, the air is at its coldest. However, a vast number of days in the year are somewhere between ‘mild’ and ‘chilly’, where air-source efficiency is good.
At outside air temperatures below around 6 or 7°C, ice or frost will tend to form on the heat exchanger. That said, frost build up may be very slow in extremely cold dry weather. Frosting restricts the air passages and reduces the efficiency, but a de-frost mechanism is deployed that reverses the system to melt the ice. This process is not as wasteful as may be first thought, but it still contributes to a reduction in energy-efficiency (in the region of 10%). It also reduces the maximum heat output in certain cold humid conditions. In general terms the most sophisticated and energy-efficient defrost methods are usually fitted to more expensive heat pumps.
Exhaust-air heat-recovery heat pumps take their heat from ventilation air extracted from a building. In countries like Sweden, where electricity is more commonly used for heating, exhaust-air heat pumps are more common. This may in part be due to their ‘cleaner’ electricity-generating network which has a large input from hydro power stations.
There are many ‘passive’ air-air heat recovery systems that can recover up to 90% of the otherwise wasted heat. These have no heat pump and are very cheap to run. These must be properly installed in air-tight buildings so can be well suited to new-build houses. Such systems can be energy-efficient if designed well, but there is always a danger that badly installed ventilation systems increase the demand on the house’s heating system. i.e. ventilation is in excess of requirements, and the heating system (e.g. radiator) load goes up.
Possibly, the most viable ventilation recovery system would be in large buildings where central extraction ducting is used.
The small air-to-water domestic hot water heaters (dhw) are sometimes optimised for exhaust air above say 15°C. i.e. for use with exhaust ventilation air. Others can use colder outside air. This type of unit may not work very well in winter, but if much of the year is warm, the average efficiency can be reasonably good.