Electric Boilers.

Original Article
February 11th, 2018


Electric boilers are generally found in areas where gas is not available or in flats. In RdSAP there are four options for electric boilers. We shall look at each in turn in this article.

Direct acting electric boiler or electric flow boiler

The direct acting boiler (also called electric flow boilers) can solely use on-peak electricity. If there is a dual meter, RdSAP assumes that some of the heating requirement uses off-peak electricity. However, the off-peak fraction will only be 10-30 per cent depending on the tariff and therefore it cannot be compared to a storage system.

The direct acting boiler will often be a tall thin white unit, about 1m tall and 0.1m square. Although it can be a wall hung more rectangular unit. 

The system can provide hot water as well as heating if used in conjunction with an indirect cylinder. The controls for these boilers have the same options as gas boilers. To enter this boiler into NES One, choose heater type ‘boiler’, boiler ‘standard’, fuel ‘electricity’ and system ‘direct acting’.

Electric CPSU

The electric combined primary storage unit differs from its gas counterpart by the fact that the thermal store for the electric CPSU must be a minimum of 270 litres (for gas this minimum is 70 litres.) 

It uses a dual tariff and the majority of the heating is done at the off-peak rate. 

The electric CPSU works by using electricity to heat a store of water that is then piped to the radiators. The hot water is provided indirectly via a heat exchanger within the store. 

In contrast to the small and narrow design of the direct acting boiler, the electric CPSU will be around 1.8m2 and 0.6m2 as it encompasses a minimum 270 litre thermal store.   The units are often housed in a cupboard. The controls for this system have the same option as the gas CPSU.

To enter an electric CPSU into NES One, choose heater type ‘boiler’, boiler ‘CPSU’, fuel ‘electricity’ and system ‘electric CPSU in heated space’.

Electric dry core boiler

Electric dry core boilers are quite rare and mostly large units, similar in size to a fridge and are often found under the counter in the kitchen. 

Water storage boiler

The water storage boiler is similar to the dry core boiler in that it heats up using off-peak electricity overnight. 

However, rather than thermal bricks, the heat energy is stored in water. The units are quite large as they incorporate the water store. The main difference between this and the electric CPSU is that it is made to operate solely on a off-peak tariff and the domestic hot water is provided directly from the water store. 

The most common brand is ‘GEC Nightstor’. The dry core boiler contains bricks that are heated overnight on off-peak rate (in the same way as storage heaters). A fan blows hot air from the bricks onto an air-to-water heat exchanger to provide hot water for the space and water heating. These boilers require a dual meter. To enter a dry core boiler into NES One, choose heater type ‘boiler’, boiler ‘standard’, fuel ‘electricity’ and system ‘dry core in heated space’.

The water storage boiler provides the hot water via a heat exchanger within the thermal store.

To enter this boiler in NES One, choose heater type ‘boiler’, boiler ‘standard’, fuel ‘electricity’ and system ‘water storage in heated space’. One of the manufacturers of this type of boiler is Headwall.