Daikin puts forward the case for air-to-water heat pumps, saying they can provide the solution for residential systems
The age of the heat pump has well and truly arrived in the UK, driven, in the main, by the need to cut energy usage and costs.

There are already thousands of heat pump installations in the UK. But heat pump usage here still lags a long way behind that of Germany, France and Scandinavia, where the number of installations is in the millions.

In the domestic-heating sector, heat pumps - electrically operated air-to-water heat pumps in particular - provide an environmentally conscious and energy-efficient alternative to fossil-fuel boilers. They are generating increasingly lucrative business for heating installers. Air-to-water heat pumps, using low-temperature heat from the atmosphere in the form of renewable solar energy, are clean, easy to install and service. They are also highly efficient, with a coefficient of performance (COP) of about 4.0.

Positive benefits
There is the added appeal of low-emission operation, a readily accessible heat source, and low investment cost compared with ground-to-water units. Factor in excellent low ambient operational characteristics - and the ability to reduce high-value energy input by 40-80% - and it all adds up to a compelling reason for use in residential applications.

The most sophisticated air-to- water units combine a host of features that translate into positive benefits for installers and home- owners alike. At the heart of the system is the compressor, through which heat is taken from the outside air and transferred to the building's water circuit.

In order for this water circuit to operate efficiently, it must carry the correct amount of heat to meet the required duty. In spring and early autumn, the heat needed to get a building up to temperature is obviously less than in deep winter.

Traditional methods - in which a burner turns on and off, overheats the water, turns off the flame and then lets it cool down below the set point before relighting the gas - uses an excess of energy.

The heat pump modulates the speed of the compressor so that it matches the heating required at any time by means of inverter control. The main benefits of this are that the heated space does not experience wide swings in temperature. The running costs are kept to the minimum and the environmental benefit is reflected in reduced levels of emitted CO2.

Another modern feature is weather compensation, by which the outdoor temperature is sensed, along with the water temperature supplied to the heating system. If the weather warms up, the system senses that the level of heat needed is less. It then throttles back the compressor in order to reduce the water temperature. This saves energy, reduces CO2 and ensures comfort levels are maintained.

Heat pumps incorporating features such as these include the Daikin air-to-water Altherma, which can also handle the domestic hot-water requirements throughout the year.

This is an economic consideration in the summer months when heat drawn from the air is more plentiful. At this time, the relationship of heat output to power input - COP - becomes high. Even in the winter, Altherma continues to return effective COPs at temperatures well below zero - units in Norway regularly operate efficiently at -15˚C to -20˚C.

The heating concept of Altherma is based on the provision - via underfloor heating and/or radiator circuits - of low-temperature water regulated to match the heating load of the building. The domestic hot water option can heat to 65˚C, or above, via a purpose-built stainless-steel tank.

The lower part of the tank is heated to 55˚C by the heat pump. The electric booster heater, located in the upper part of the tank, can boost the water to higher temperatures if required.

Special kit
This latter option has been extended for use in conjunction with solar heating panels. This is by means of a special kit which enables the water tank to be connected to a solar collector.

Built-in control is able to prioritise the production of hot water via the solar collector. If this is unable to generate sufficient energy, the system switches automatically to heat pump operation, or the electric booster heater in the water tank.

Between 30% and 70% of the energy required annually for domestic hot water can therefore be obtained from the sun.