Small Under-sink Water Heaters

These units are typically mounted uur-of-sight directly under a sink to take advantage of the blumbing for waste water than is normally present there. However because the units operate at mains pressure the location of the heater is a purely a matter of convenience. Whilst the specific details may vary from product to product, some features are generic as outlined below:

Electrical Connection (A)

These units use electric heating elements rated at 3kW or less and may be wired to a domestic ring main. The electrical power requirements are similar to a typical electric kettle -- there is normally no need to connect directly to the electrical consumer unit.

Cold Water Inlet Connection (B)

The water supply connection is made by tee-ing from the cold water mains. It is recommended that a service valve is also fitted at the time of installation to facilitate future maintenance or re-siting of the unit.

Regular Taps (C)

A useful benefit of these water heaters is that they are suitable for operation with regular taps, proving high mains pressure flow rates for both hot and cold water outlets. (Please note that the alternative vented class of undersink water heater requires the use of special vented taps which can be expensive to purchase if not already available).

Multiple Outlet Capability (D)

Unvented water heaters can supply a number of sinks, with the practical limitation often being determined by capacity of the water storage unit.

Accessories (E)

Depending on on the specific details of the installation, unvented water heaters may require accessories to be fitted to the cold inlet pipework. The requirements may include an external  safety pressure relief valve and may also include a small expansion vessel plus check valve to accommodate expansion of water during the heating process, and a pressure reducing valve if the local water mains pressure is unusually high. These accessories are similar across the range of unvented water heaters and are a requirement of UK regulations for capacities over 15 litres.


This guide provides a high level guide to the general features of products in this range. Installation details vary between products. All installations must be carried out in accordance with the manufacturer's specific requirements and with relevant regulations. Installation must always be carried out by competent persons.