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uk Home & property emergencies pressure relief valve dripping • tpr valve leaking • tundish dripping • unvented cylinder discharge • hot water cylinder relief valve • water heater safety valve • intermittent discharge pipe • hot water safety discharge • relief valve releasing water • cylinder overpressure concern • overheating hot water cylinder • expansion vessel failure • pressure reducing valve issue • water heater making hissing • relief pipe hot water outside • unknown leak from cylinder • unvented system g3 concern • safety valve discharge warning • scalding risk hot water leak

What to do if…
your water heater’s pressure relief valve is releasing water intermittently and you’re unsure why

Short answer

Treat intermittent relief-valve discharge as a safety warning: keep people away from the discharge, and switch off the heat source to the hot water system now. Then arrange an engineer to check the system before you ignore it or try to “stop the drip”.

Do not do these things

  • Do not cap, plug, clamp, tape, or otherwise block the relief valve outlet, tundish, or discharge pipe.
  • Do not wedge any valve handles open/closed “to see what happens”.
  • Do not touch or put your hand under the discharge (burn/scald risk), even if it looks like “just warm water”.
  • Do not keep turning the thermostat up/down repeatedly or “resetting” things while it’s discharging.
  • If you have an unvented hot water cylinder, do not attempt DIY repairs: use an engineer trained/competent in unvented systems (often evidenced by an “unvented hot water” qualification).

What to do now

  1. Create a safe buffer around the discharge point.
    Keep children/pets away and keep your face/hands away from the flow. If safe, put a bucket under the end of the discharge pipe (without restricting the opening), or clear the area so hot water can’t splash someone.

  2. Identify what you’re looking at (so you don’t ignore the wrong thing).
    Many UK unvented cylinders discharge via a tundish (a small clear/visible fitting) so you can see when water is passing. If water is passing through the tundish intermittently, treat it as a sign the safety system is operating and needs checking — not something to “live with”.

  3. Switch off the heat source feeding the hot water (do this first).

    • Immersion heater: switch it off at the local isolator/fused spur.
    • Boiler-heated cylinder / combi making hot water: turn hot-water heating off at the controller, and if needed turn the boiler off.
      This reduces the risk of overheating while you work out next steps.

  4. If water continues after the heat is off, reduce refilling pressure — only if you’re sure of the correct valve.
    After step 3, if you can confidently identify the cold-water supply valve feeding the cylinder, you can turn it off to reduce ongoing discharge/refilling.
    If you are not sure you’ve found the right valve, don’t guess — leave it and move on.

  5. If the discharge becomes heavy, steaming, or violent, escalate immediately.
    Keep clear of the discharge route and cylinder cupboard. Ensure the heat source is off. Call an emergency plumber/heating engineer.
    If you believe there is immediate danger and you cannot safely control the situation, leave the area and call 999 (or 112).

  6. Arrange the right kind of help (important for UK unvented cylinders).
    Ask for an engineer experienced/competent in unvented hot water systems to check likely causes (commonly pressure/expansion control, inlet pressure control, thermostat/control faults, or a failing relief valve). If you’re a tenant, report it to your landlord/agent as a safety repair.

  7. Document what you’re seeing (30 seconds).
    Take a quick photo/video of the tundish/discharge and note: when it happens (only during heating? after hot water use?), and whether it stops once heating is off. This helps the engineer avoid guesswork.

What can wait

  • You do not need to diagnose the exact cause right now.
  • You do not need to drain the whole cylinder unless a professional instructs you to.
  • You do not need to replace parts “just in case” or order anything before a proper check.
  • You do not need to decide whether to replace the cylinder — many causes are system/valve/control related.

Important reassurance

A relief discharge is designed to “fail safe” by letting water out instead of allowing pressure/temperature to build. It’s understandable to feel alarmed — your job right now is to keep it unblocked, reduce heating, and get it checked.

Scope note

This is first-steps-only guidance to stabilise the situation and avoid dangerous mistakes. An engineer may need to test controls/pressure regulation/expansion arrangements and verify the discharge setup.

Important note

This guide is general information for an urgent home safety situation, not a substitute for an in-person inspection. If you’re unsure about any valve, switch, or system type, prioritise switching off the heat source and getting qualified help rather than experimenting.

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