Hot water guides

Why hot water has its own cluster

Hot water is where the most safety-critical domestic plumbing lives, and where the largest gap sits between what people assume and what is actually required.

A vented system is inherently safe: the cylinder is open to atmosphere through a vent pipe, so it cannot become pressurised. An unvented cylinder is sealed and pressurised from the mains, which is what gives you mains pressure at every outlet – and it means the safety devices are the only thing standing between stored hot water and a serious failure.

Unvented or vented sets out how the two differ, and what Part G3 actually requires covers the regulatory side without hand-waving.

Start here, depending on what you are dealing with

The two faults that account for most of it

The expansion vessel

Water expands when heated, and in a sealed system that expansion has to go somewhere. An expansion vessel – or on some cylinders an internal air bubble – absorbs it. When the vessel loses its charge, or the bubble is lost, the expansion has nowhere to go and pressure climbs until a safety valve lifts.

The symptom people notice is water discharging from a pipe outside, usually in the afternoon or evening when the cylinder has reheated. It is frequently misread as a leak or dismissed as normal. It is neither – it is the safety device doing its job because something upstream has failed, and left alone it wastes water continuously and eventually the valve itself fails.

Expansion vessel failure is the most common unvented fault there is.

The discharge pipework

The discharge arrangement from the safety valves – the D1 and D2 pipework – is where a large share of installations fall down, and it is precisely the part that has to work on the one day it is ever needed. Sizes, gradients, permitted lengths, the number of bends, the tundish position and where the pipe terminates are all specified.

D1 and D2 discharge pipework covers it in detail. If you are checking somebody else’s work, this is the thing to look at.

Servicing, which really is not optional

An unvented cylinder needs an annual service, and unlike some maintenance recommendations this one is not a manufacturer trying to sell visits.

• The expansion vessel charge is checked and re-pressurised. This is the item that fails silently.
• The safety valves are tested, because a valve that has sat undisturbed for years may not lift when it is needed.
• The immersion thermostats and thermal cut-outs are checked.
• The discharge arrangement is inspected for blockage or damage.
• Any anode is checked, where the cylinder has one – it is what protects the vessel from corrosion and it is consumed.
• The whole thing is looked at by somebody competent to spot what is developing.

Skipping it also generally voids the manufacturer’s warranty, which on a cylinder is a substantial sum. The unvented cylinder annual service sets out what a real one covers.

See also unvented cylinders, cylinder replacement and the cylinder sizing tool.