An electric CPSU is a central heating system providing space and domestic water heating. A thermal store containing primary water is heated mainly during off-peak times to approximately 75°C in summer and between 85°C and 95°C in winter. The space heating circuit operates in the same way as a wet central heating system, with controls appropriate for wet systems. For domestic hot water, secondary water flows directly from the cold mains into a heat exchanger, where it is heated by the hot water in the store before being delivered to the taps.
Determining fuel costs (external to DEAP): The CPSU volume should be sufficiently large to meet most of the space and water heating demand during on-peak times from heat stored during off-peak times, otherwise the on-peak fraction will be high. For a 10-hour off-peak tariff providing three off-peak periods per day, available in some other countries, a volume of at least about 270 litres may be appropriate. For the night-rate tariff available in Ireland, providing one off-peak period each night, a larger volume would be appropriate.
The heat losses from the CPSU are calculated, as for other hot water storage vessels, on the ‘Water heating: options and storage’ tab, using data from Table 5.1.