Can I Replace a Hot Water Cylinder Myself?

Does it matter whether the cylinder is vented or unvented?

It decides everything. A vented cylinder is fed from a cold water storage tank, usually in the loft, and has an open vent pipe that runs back over that tank. It operates at the low pressure that tank gives, so if something goes wrong the water has somewhere harmless to go. An unvented cylinder is fed straight from the mains, sealed, and designed to run at mains pressure with the water expanding as it heats. Its safety depends on a set of devices working together.

If you are not sure which you have, look for the cold water tank in the loft and the vent pipe at the top of the cylinder. No tank and no vent pipe, with a tundish somewhere nearby and a valve group on the cold feed, means unvented. The vented or unvented guide covers the checks in more detail.

Many people searching this question have an unvented cylinder, because unvented cylinders are common in newer installations. That is the case to be most careful about.

Where does DIY stop on an unvented cylinder?

It stops at the start. Approved Document G, the guidance to the hot water safety part of the Building Regulations in England, treats installing an unvented hot water storage system as notifiable work, to be notified to building control directly or handled through a competent person scheme, and tells owners to choose installers who have the necessary skills. Manufacturer guarantees often depend on how the cylinder is installed and commissioned, so a home-fitted cylinder may also lose its guarantee. Check the manufacturer’s own terms.

The reason is mechanical. A sealed cylinder stores a large volume of water at temperature with nothing to vent it if the heat source fails to cut out. Several independent devices stop that from becoming dangerous: the thermostat, a manual-reset over-temperature cut-out on the heat source, the temperature and pressure relief valve, the expansion relief valve and the expansion vessel. A competent installer has to fit them correctly, connect each to a discharge route that works, and test that each one operates.

If any of those devices is missing, mis-set or blocked, the cylinder can overheat or overpressurise. That is why the rule is firm: steam or very hot water coming from an unvented cylinder means turn off the heat source, keep clear and call an engineer.

What does the discharge pipework have to do?

This is where home installs most often fail the standard. The temperature and pressure relief valve discharges through a visible tundish, and from there a pipe has to carry any water to a safe place. Approved Document G sets out the pipe sizes, the maximum run length for a given valve size, a vertical drop below the tundish, the fall and the termination. The short pipe from the valve to the tundish is metal, and the tundish has to be visible so that a drip is noticed.

A cylinder that is swapped for a larger or a different type without checking the discharge route can end up with a pipe that is too long, too small, or goes nowhere safe. The moving a cylinder guide covers why the route is surveyed before a cylinder is repositioned.

A second regulation also applies. The Water Supply (Water Fittings) Regulations 1999 govern the fittings connected to the mains, including how stored water is stopped from flowing back into the supply. Pressure reducing valves, non-return valves and expansion devices on the cold feed are part of meeting that.

What about the electrical side?

Most cylinders are heated by one or two immersion heaters on a dedicated circuit. The heaters carry a high load, and a poor terminal connection heats up and burns. Isolate at the consumer unit and do not rely on the switch beside the cylinder alone, but treat reconnecting heater terminals as work for a competent electrician. Altering the fixed wiring or the circuit is an electrician’s job, and in a dwelling some new electrical work is notifiable under Part P.

Nothing inside the electrical enclosures on the cylinder, such as the thermostat and cut-out unit, is a householder repair. If your cylinder has an immersion that trips or a hot cable, stop and have it tested. See immersion heater replacement for how that is done.

What can you do yourself, and what must a competent installer leave behind?

On a vented cylinder, a confident DIYer can plan a like-for-like swap. The sequence is to isolate the electrics and water, drain, disconnect, fit the new cylinder, refill, purge air, set the thermostat and check every joint. Two hazards deserve more weight than they get. First, a full cylinder is heavy: water weighs a kilogram a litre, so a 150-litre cylinder carries well over 150 kg once full. Second, draining a hot cylinder can scald. See the draining a cylinder safely guide.

Even on a vented swap, think about whether the old cylinder is worth replacing at all. The repair or replace guide explains how to tell. If you are moving from vented to unvented, the job changes class entirely and is not a swap.

What a competent installer should leave behind is a checklist you can ask for. A commissioned cylinder with the thermostat and cut-out set, a tundish that runs to a safe termination, the expansion vessel charged, the valve group in place, the discharge route tested, a Building Regulations notification and the manufacturer’s documents. The unvented cylinder fitting page describes what the work covers, and the like-for-like replacement page covers a straight swap.