These also need servicing

It is commonly known that your boiler needs regular servicing to make sure it is running efficiently and safely. What is not commonly know is your hot water cylinder may do as well. Unvented hot water cylinders are the hot water system of choice for many new build properties with more than one shower coming from the hot water system. This is because they provide enough water flow for more than one shower to be used at the same time. If you have a hot water cylinder but do not have any water tanks in the loft or high up in a cupboard, then you’ll have an unvented hot water cylinder which looks similar to the pictures below.

Example of unvented hot water cylinder

Unvented hot water system










These systems have certain safety devices which in effect prevent your cylinder from exploding. As extreme as that sounds it is the truth. When hot water is in a sealed environment the water has to be able to expand. This is the job of the expansion vessel. If however the expansion vessel should fail there will be a pressure relief valve that will automatically open to let the hot water out and the cold water in. This will in turn reduce the pressure in the cylinder. If another fault should occur with either your heating controls or boiler which would cause your hot water to remain on, there is a valve that would automatically open should the hot water temperature get too high. If an unvented hot water cylinder did not have functional safety devices then the picture below will show you what could happen.

unvented cylinder explossion

This explosion happened in America where unvented hot water cylinders are more widely used. I’m happy to say the UK have better regulations on who can fit and service unvented hot water cylinders which means explosions like this are extremely rare in the UK.

A common symptom of an un-serviced hot water cylinder is a dripping is seen or heard when the water in the cylinder is being heated. The dripping will be seen from outside or the tundish, see below picture. If the tundish is wet then your unvented hot water cylinder needs a service urgently.


Make sure your service engineer is qualified to work on your unvented hot water system. The engineer must have a ‘Domestic Hot Water Storage Systems’ or ‘Hot Water Systems & Safety’ qualification. These comply with Approved Document G of the Building Regulations.


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