Heat Pumps: Fresh look at air-to-water HWS heat pumps for UK

One of the easiest and most cost-effective ways of taking advantage of heat pump technology in homes and offices is to install an integrated air-to-water hot water service heat pump. Manufacturers can supply packaged units where the heat pump components are mounted above a highly insulated large capacity storage calorifier, all housed in a smart jacket. Stephen Laws, technical director at Clyde Combustions, explains how such units can be applied.
Heat Pumps: Fresh look at air-to-water HWS heat pumps for UK

AIR-to-water heat pump technology for supplying potable hot water in homes and offices has been around for some time. Heat pumps of this type take free heat from the air and transfer it to raise the temperature of the stored potable water. Indeed, on the Continent these appliances are a very popular way of providing a hot water service, especially in new and refurbished premises.

So why have they not been popular in the UK? One reason has been the low cost of natural gas in the UK. It has been cheaper to heat potable hot water using a gas boiler as the prime heat source rather than electrical power. It can be argued this is also the reason why alternative technology has had such a hard time getting established in the UK market. With recent and projected increases in gas prices this will be less of a factor in the future.

Continental European winter temperatures can be much lower than those experienced in the UK, so domestic boilers tend to be larger than units specified here. This is why, on the Continent, boilers are usually installed in large utility rooms or in cellars which are much more common on the other side of the channel. Utility rooms and cellars are ideal places for a HWS heat pump as warm air can be drawn directly into the appliance. Incidentally, you may have seen photographs recently published by some manufacturers showing units in large rooms that double up as home gyms which are not common in the UK!!

With energy saving requirements enshrined in the latest British Building Regulations, all house builders and architects will at last be obliged to take into account the spaces needed for low carbon technology products such as heat pumps. An integrated air to water heat pump is about the same size as a domestic fridge freezer, commonly 1.7 metre high, requiring a floor area measuring 1.5 x 0.7 metres plus space for pipe work and access at the front. So as long as the space is available, this fuel efficient appliance should be considered for the hot water service provision.

Installing an air to water Hot Water Service heat pump in a commercial premises plant room can bring instant cost benefits to the owner. The more sophisticated heat pump units have two primary heating coils inside the vessel, these can be connected, for example, to a boiler circuit and a solar panel array.

Clyde endeavours to take the technology and match it to the UK market requirements.

The air to water HWS heat pump package from Clyde has a D tube condenser (10), wrapped around the outside of the calorifier vessel (23), this brings boiled R134a refrigerant from the evaporator (25) to heat the stored water from 10°C to 55°C in a few hours. Once heated, the calorifier will provide a flow rate of 926 litres every 24 hours. A 2kW electric immersion heater is included in the specification, so if desired the unit can be installed independent of any other heat source as a part of a de-centralised hot water service scheme. The onboard controls include a legionella prevention programme which can be set to raise the temperature of the stored water for one hour to 65°C, once a week. The two auxiliary heating coils mounted inside the vessel can be connected to a geothermal heat pump, solar panels, a boiler system or a combination of these heat sources. For example, with the upper coil connected to solar panels to boost heat input further, the lower coil could be connected to a boiler return circuit, a combination that would bring 10°C stored water to 55°C in less than 25 minutes.

The Clyde heat pump has an exceptional Coefficient of Performance (CoP) indicating a heat output of 3.33kWth for every 1kWe input of electrical energy consumed. This makes the unit a cost efficient alternative when compared with fossil fuel sources for providing a potable hot water service in energy efficient homes and commercial premises.

Operational overview of a Clyde air to water HWS heat pump.

Low pressure refrigerant is boiled at the evaporator (24) by the heat content of the incoming air (1). The refrigerant vapour is compressed from 6 to 18 bar and circulated around the loop of the D-tube condenser coil (10) that surrounds the hot water vessel (25). The refrigerant vapour condenses and transfers sensible heat to the cooler potable water store.

1 April 2006


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