Domestic hot water (DHW) accounts for 14% of heat used in the UK home. This compares with 63% in space heating but is much more challenging to a major reduction than is space heating due to its relationships to hygiene and health. In the UK, DHW accounts for 80 TWh delivered energy per year and with the move towards a low-carbon future, it can be expected that a combination of improved building insulation and the application of efficient space heating will drastically reduce carbon emissions. However, efficient heating systems such as heat pumps are unable to supply instantaneous hot water, whereas a combination boiler (‘combi’) can deliver 20–30 kW or more for this purpose and heat pumps sized to meet typical house space heating demand are rated around the 10 kW level. A larger heat pump would be uneconomic and would have an unacceptable start-up time to meet normal requirements. Thus, the only way that present heat pump systems can provide hot water is to charge a conventional DHW tank. This would not be an issue except that new build houses tend to be designed without space for tanks and opt for instantaneous hot water via a gas combi boiler. Also, there is a strong trend when replacing gas boilers to choose combi boilers and discard the storage tank to provide more space within the home. At present almost two-thirds of the 23 million gas boilers in UK homes are ‘combi’ boilers with no large hot water tank. It should be noted that combi condensing boilers do not deliver DHW with laboratory test efficiency due tostart-up transients, hot water left in pipe runs, etc. The mean efficiency of a trial set of combination boilers tested for the EST was 82.5%. The dynamics of state-of-the-art combi condensing boilers when used for hot water production at 55°C from a 10°C inlet was achieved in 100 s. Much is made of ‘hybrid’ heat pumps to provide both space and water heating but essentially they are an electric heat pump plus gas boiler with an integrated control system. When supplying DHW the load is met by the gas boiler and is no more efficient than a combi. Off-gas grid consumers using direct electricity are also disadvantaged by higher fuel costs. A ‘hidden’ DHW load is the demand by appliances such as dishwashers and washing machines which use 14% of domestic electricity. In recent years, the trend has been towards cold-fill appliances with electric heating. The argument in favour of this is that efficiency is higher if an electric heater is used and losses occur from the DHW cylinder and pipework between appliance and cylinder. This should be revisited if DHW is supplied by a high-efficiency heat pump and also if future appliances can request water at the required temperature from a smart supply. More efficient alternatives must be made available to respond to these market drivers.
