Hot water cylinder tundish dripping: is it normal?

A drip from a safety discharge deserves attention. Start by finding where it comes from.

The answer in 2 lines. On an unvented hot water cylinder, water running through the tundish — the small open fitting in the discharge pipe — should be investigated. Do not assume that dripping during heating is harmless. Keep clear of the discharge, record when it happens and arrange a check by someone competent in unvented hot water systems. If there is steam, unusually hot tap water or loud bubbling, switch off the cylinder's heat sources using accessible, dry controls and seek urgent help. Do not operate safety valves or dismantle the system to diagnose it.

The first steps

Locate the water without touching it. Is it passing through an open tundish, leaking from a connection, or appearing beneath the cylinder? Photograph the location from a safe position.
Record the circumstances. Note whether it happens while the cylinder heats, after hot water use or continuously. Report any unusual noise or change in tap-water temperature.
Arrange the right inspection. Explain that the fault concerns a hot water cylinder and possibly its safety discharge. Ask specifically for competence in unvented systems if yours is unvented.

Stop: danger

Steam, excessively hot water or loud bubbling: stop using the hot water, switch off accessible heat-source controls if safe, and seek urgent professional attention. Keep people away from the cylinder and discharge.
Water near electrical equipment: do not touch wet switches, wiring or the immersion-heater cover. Do not cross a wet floor to reach an isolator.
Never block or restrict the discharge. Do not fit a cap, close an unidentified valve or redirect hot discharge into a bucket.

First identify the system

This guide concerns stored domestic hot water, particularly an unvented cylinder supplied under mains pressure. A vented cylinder has a different arrangement, usually involving a cold-water storage cistern. A combi boiler without a hot water cylinder is another system again.

Read the cylinder label and existing paperwork without removing covers. If the system type is unclear, send a photograph to the person arranging the inspection rather than guessing. The official English hot-water safety guidance explains the distinction: open the current document and look under hot water supply and systems.

What your observations can tell you

What you notice	What to do
Drips through the tundish during heating	Report the pattern; do not dismiss it as routine expansion.
Water runs continuously	Arrange prompt investigation and describe whether the heating is on.
Water escapes around the tundish	Keep clear and report both the discharge and the escaping water.
No visible discharge	This alone does not prove that the safety devices work.

These observations help an engineer investigate; they do not identify a failed component. Avoid buying a replacement valve before the system has been assessed.

What not to do

Repeatedly turning a relief-valve lever to try to clear a suspected blockage.
Adjusting pressure controls, recharging an expansion vessel or opening electrical covers yourself.
Continuing to heat a cylinder that is producing steam or other overheating warning signs.

What to ask the professional

Provide the cylinder model, photographs, service history and your observations. Ask for the cause of the discharge, the proposed repair and confirmation that the safety arrangements have been checked. A boiler service should not be assumed to include the cylinder.

For installation or replacement work, use the GOV.UK competent-person guidance: follow its register link and check that the installer's registered work covers the proposed job. For fault-finding, ask separately about current unvented-cylinder competence. Keep the written findings and service record.

If you rent in England

Report the fault to your landlord or managing agent in writing, attaching photographs and describing any overheating signs. Ask who will attend and what you should do with the heating meanwhile. Do not authorise alterations on the assumption that you will automatically be reimbursed.

Read GOV.UK's repairs guidance, particularly the heating and hot-water responsibilities and the section on repairs that are not done. Wales, Scotland and Northern Ireland have different housing rules.

Good to know

Leave the tundish visible and accessible; stored belongings can hide a developing discharge.
Follow the maintenance instructions for the actual cylinder rather than a generic valve-exercising routine.
For overheating warning signs, read the HSE domestic hot-water safety alert and its warning-sign section; it particularly addresses immersion-heated systems with a plastic storage cistern.

Illustrative example: Sam notices water passing through an open fitting beside the cylinder whenever the hot water reheats. He photographs it without touching the pipe and sends the service record to the engineer. He leaves the valves alone and asks for the discharge to be investigated, rather than requesting a particular replacement part.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 6 November 2026, for England. Confidence: high for keeping safety discharges unobstructed and recognising overheating warnings; medium for interpreting a particular drip without inspecting the installation. The English building guidance incorporates 2024 amendments. This guide cannot diagnose a valve or expansion fault remotely.