Hot water cylinders and water heaters: choosing and replacing

A vented cylinder is fed from a cold water tank in the loft, and gravity provides the pressure. It is simple and tolerant, but pressure on upper floors can be weak and the loft tank has to be kept in good order.

An unvented cylinder is fed directly from the mains and stores water at mains pressure, so showers upstairs and downstairs run strongly without a pump and the loft tanks can go. In exchange, the cylinder carries safety devices that must be installed correctly and checked regularly, and it only works as well as your incoming mains allows.

Before recommending an unvented cylinder, the installer should measure the flow rate and pressure of your incoming main with water running, not just the static pressure. An old or undersized supply pipe, common in older terraces, can make an unvented cylinder disappointing, especially if several outlets run at once.

It also needs somewhere for the discharge pipe to go: the safety valves must discharge safely to a visible, safe point outside or into a drain. Where that route is impossible, the choice of cylinder or its position may have to change.

• measured mains flow rate and dynamic pressure
• number of bathrooms and showers likely to run together
• cylinder size and recovery time with the planned heat source
• discharge pipe route and termination point
• what happens to the old cylinder and loft tanks
• the Building Regulations route and certificate for an unvented cylinder

Installing an unvented hot water storage system is notifiable under the Building Regulations in England. The installer must be competent to fit it, which in practice means holding the relevant unvented qualification, and either notify Building Control or self-certify through a competent person scheme. Ask for the certificate and keep the benchmark or commissioning record, because manufacturers' warranties usually require annual servicing by a competent person.

This is not a job for general handymen. The temperature and pressure relief valves exist to stop a cylinder failing dangerously, and fitting them wrongly defeats them.

Most cylinders have an electric immersion heater as backup. A failed immersion element is a common, contained repair; a cylinder that is leaking from its body is usually replaced rather than repaired.

Instantaneous electric water heaters serve a single sink or shower without any storage. They save running a long pipe from the cylinder to a distant cloakroom, but a shower unit draws a heavy current and needs its own suitably sized circuit, which is electrical work that may be notifiable.

If you plan to fit a heat pump or solar thermal panels, the cylinder should be chosen for that now. Heat pump cylinders have a much larger internal coil to transfer heat at lower temperatures; a standard cylinder replaced today may have to be replaced again when the heat pump arrives. Tell the plumber about any plans so the cylinder is not a dead end.

Describe the current system, the problem and what you would like. We offer the request, without your contact details, to plumbers and heating engineers in your area, and only one can buy it. Before it sees your details, we email you its name, address and phone number. The service is free. Offrano does not check qualifications, so ask to see the unvented qualification card and the competent person scheme registration before work starts.