Unvented Hot Water Cylinder Problems: What to Check Safely

Unvented hot water cylinders deliver strong mains-pressure hot water, but they contain pressurised water and safety controls. Problems such as running out of hot water, a dripping tundish or low pressure should be treated carefully rather than investigated by dismantling the system.

This guide explains what homeowners can check safely, what the main components do and when to arrange professional plumbing repairs. If water is discharging continuously or the cylinder appears to be overheating, turn off the heat source if safe and seek urgent help.

Common unvented cylinder problems

Hot water running out quickly can be caused by a timer setting, a failed immersion heater, a boiler fault or a cylinder that is not being heated fully. It can also happen when demand has increased, such as after guests stay or when several baths and showers are used close together.

No hot water may indicate a tripped electrical switch, incorrect programmer settings or a problem with the boiler, immersion heater or control thermostat. Low pressure at hot taps can be linked to a partially closed cold-water supply valve, a blocked pressure reducing valve or a wider mains supply issue.

A noisy cylinder may produce humming, clicking or kettling-like sounds as water heats. Some sounds are caused by normal expansion, but persistent or loud noises can point to scale, a failing valve, a problem with the expansion vessel or overheating.

• Check whether the immersion heater or boiler has power and whether a local switch has tripped.

• Confirm the timer and programmer are set for the times you expect hot water.

• Look for visible leaks around the cylinder, valves and nearby pipework.

• Check that the cold-water isolation valve is open, without forcing it.

Components explained in plain English

The pressure reducing valve controls incoming mains pressure so the cylinder and taps receive a suitable, steady supply. If it becomes blocked or fails, hot water flow may become weak or inconsistent.

An expansion vessel gives heated water somewhere to expand as it warms. The temperature and pressure relief valve is a safety device that can release water if temperature or pressure becomes too high. The tundish is the small, visible fitting that provides an air gap and lets you see when water is being discharged.

The immersion heater is an electric heating element fitted into the cylinder. The discharge pipework carries water safely away from relief valves, so it must never be blocked, capped or altered. A discharge pipe that is hot, dripping continuously or incorrectly installed is a safety concern.

Safe checks and the point to stop

Start with the easy observations: check the electrical supply to the immersion heater, review timer settings and see whether other appliances have normal cold-water pressure. You can also inspect the cylinder cupboard for dampness, unusual heat or water around the tundish, but do not remove covers or adjust valves.

If the tundish is dripping briefly while the cylinder heats, it may indicate a relief valve is lifting. Continuous discharge, forceful flow or a very hot discharge pipe needs prompt attention. Do not tie up the tundish, plug the discharge pipe or repeatedly reset a safety cut-out.

• Turn off the immersion heater or boiler heat source if the cylinder is overheating and you can do so safely.

• Keep clear of hot discharge water and do not touch exposed pipework unnecessarily.

• Arrange urgent professional help for continuous discharge, significant leaks or relief valve operation.

• Do not remove the cylinder’s service covers or change pressure settings yourself.

When a qualified engineer is needed

Unvented cylinders are controlled systems that combine mains pressure, stored hot water and several safety devices. Diagnosing a failed pressure reducing valve, expansion vessel, thermostat or temperature and pressure relief valve normally requires specialist knowledge, testing equipment and the correct replacement parts.

In England and Wales, work on unvented hot water storage systems is generally associated with an installer holding the relevant G3 qualification or an equivalent recognised competency. This reflects the safety risks involved, although exact legal and installation requirements can depend on the work being carried out and local regulations.

Arrange professional help if the relief valve is lifting, the tundish continues to discharge, the cylinder is overheating, pressure remains poor after basic checks or a leak cannot be isolated. A qualified engineer can test the controls, inspect the discharge pipework and confirm that the system is safe to use.

Frequently asked questions

Can I reset an unvented cylinder myself?

You may be able to reset an accessible electrical trip or programmer, following the manufacturer’s instructions. Do not repeatedly reset a safety cut-out, as this can hide a fault and create an overheating risk.

Is a dripping tundish always dangerous?

Any discharge deserves attention because the tundish is showing that a safety valve has opened. A short discharge may occur during heating, but continuous dripping or hot water flowing from the tundish should be checked by a qualified engineer.

Can low tap pressure be caused by the cylinder?

Yes. The pressure reducing valve, inlet strainer or cold-water supply may be restricting flow, although the mains supply or tap itself could also be responsible. An engineer can identify the cause without guessing or changing pressure settings.