​Gas fired instantaneous multi-point hot water heaters

​Electric instantaneous multi-point hot water heaters

​With these types of hot water heater, cold water is taken from the water undertaker’s main and heated in a heat exchanger as demand requires before being distributed to the outlets.


As long as the tap is running, hot water will be delivered to the taps.
There is no limit to the amount of hot water that can be delivered.

There is no storage capacity.

​Expansion of water due to being heated is normally accommodated by back pressure within the cold water main.
However, if this is not adequate or the cold water system contains pressure reducing valves or check valves, then an expansion vessel must be fitted.

​Combi boilers are dual function appliances.

They provide instantaneous hot water and central heating within the same appliance.

​Gas or oil fired combination boilers

​In normal working mode, combination boilers are central heating appliances, supplying a proportion of their available heat capacity to heat the central heating water.

When a hot tap is opened, a diverter valve diverts the boiler water around a second heat exchanger, which heats cold water from the water undertaker’s cold water mains to supply instantaneous hot water at the hot taps.

​Thermal stores

​Sometimes called water jacketed tube heaters, thermal stores work by passing mains cold water through two heat exchangers which are encased in a large storage vessel of primary hot water fed from a boiler.