Summary

"The pressure relief valve is discharging" is one of the most common — and most commonly misdiagnosed — calls a heating engineer or plumber gets. Customers describe it in different ways: water dripping from a pipe outside the house, a puddle under the boiler, a hissing or spraying noise, or simply "there's water coming from somewhere near the boiler." The first job on arrival isn't fixing anything — it's correctly identifying which safety device is discharging, because the central heating PRV and the unvented cylinder's TPR/ERV are different devices, governed by different standards, with different urgency profiles and different root causes.

This matters because the two devices look superficially similar (a valve with a discharge pipe running somewhere safe) but mean very different things. A central heating PRV discharging is almost always a sign of an expansion vessel or filling loop fault on the heating circuit — annoying, water-wasting, but not immediately dangerous once isolated. An unvented cylinder's TPR discharging scalding water is a genuine safety device doing exactly its job to prevent a catastrophic pressure/temperature failure of a stored hot water vessel — a discharging TPR should never simply be capped off or ignored, and repeated discharge points to a fault that, left unaddressed, has a small but real risk of a serious cylinder failure.

This guide sets out how to identify which system is discharging, the immediate safe response for each, the underlying causes to check, and when the job is a straightforward repair versus when it needs to stop and be escalated. It complements the existing system pressure and expansion vessels and boiler pressure too high articles, which cover ongoing pressure-loss diagnosis in depth — this guide is specifically about what to do when discharge is happening now.

Key Facts

Quick Reference Table

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Observation Likely source Immediate risk Typical root cause
Dripping from a pipe near the boiler, warm to lukewarm water Central heating PRV Low — nuisance/water waste Failed expansion vessel, filling loop left open, or PRV seat fouled
Continuous discharge, system pressure gauge reads high (>2.5 bar hot) Central heating PRV, actively venting overpressure Low-moderate — investigate same day Expansion vessel diaphragm ruptured
Discharge from a pipe near an airing cupboard/unvented cylinder, hot/scalding water Cylinder TPR or ERV Moderate-high — do not ignore Cylinder thermostat fault, immersion stat fault, or ERV/expansion vessel fault on the cylinder circuit
Visible staining/limescale trail with no active drip today Either — past event Low now, but confirms an intermittent fault exists Same causes as above — investigate before it recurs
Hissing or spraying (not just dripping) Either, at higher flow than a simple weep Moderate — treat as urgent Valve seat failure or major overpressure event in progress
Discharge coincides with heating firing up, stops when it cools Central heating PRV Low-moderate Expansion vessel undersized or failed — classic thermal-cycle pattern
Discharge with no clear pattern, random timing Either Investigate Filling loop passing, or intermittent leak elsewhere raising system pressure

Detailed Guidance

Step 1: identify which system is discharging

Water discharging from a pipe near/under the boiler or on an
external wall
        |
        v
Trace the discharge pipe back to source
        |
        +-- Traces to boiler / heating manifold --------> CENTRAL HEATING PRV
        |        (usually small bore copper/plastic,             |
        |         no tundish visible)                             v
        |                                          Check system pressure gauge
        |                                          Go to "Central Heating PRV" branch
        |
        +-- Traces to an unvented hot water cylinder ---> TPR / ERV (G3 device)
                 (often via a visible tundish —                   |
                  open funnel-style fitting)                      v
                                                     STOP if not G3 qualified
                                                     Go to "Unvented Cylinder" branch

If you cannot trace the pipe with certainty, do not guess. Central heating PRV discharge and cylinder TPR discharge require different competencies and carry different urgency — a wrong assumption here can mean missing a genuine safety-critical fault on an unvented cylinder.

Central heating PRV branch — diagnosis sequence

  1. Read the system pressure gauge cold (system off, cooled for at least an hour). Normal is 1.0–1.5 bar. If it's reading high already, that's your first clue.
  2. Check whether the filling loop is closed. A filling loop left open after a top-up is one of the most common and easiest-to-miss causes — mains pressure (typically 2–5 bar) will continuously push the system pressure up until the PRV vents. Close both valves fully.
  3. Heat the system and watch the pressure rise. A normal rise from cold to hot is roughly 0.3–0.5 bar. A rise that pushes past 2.8–3.0 bar and triggers the PRV strongly suggests a failed expansion vessel.
  4. Test the expansion vessel. Isolate, drain the system to zero pressure, and depress the Schrader valve on the vessel with a tyre pressure gauge. Water instead of air means the diaphragm has ruptured — replace the vessel. See system pressure and expansion vessels for the full vessel test procedure.
  5. Inspect the PRV seat after the root cause is fixed. If the PRV has discharged repeatedly, debris or limescale can prevent it reseating cleanly even once the vessel is replaced — a continued weep after a vessel fix means the PRV itself now needs replacing, not just the vessel.
  6. Confirm the discharge pipe terminates safely. It should run to a visible, safe location — not into a void, not somewhere that could scald a passer-by, and with a continuous fall so it can't trap water and freeze in winter.

Unvented cylinder TPR/ERV branch — diagnosis sequence

Stop and confirm G3 competency before proceeding with any repair. Diagnosing what's happening is fine for any competent tradesperson to observe, but opening up, adjusting, or replacing safety devices on an unvented cylinder requires G3 (unvented hot water systems) qualification under Approved Document G3.

  1. Confirm the discharge is genuinely from the cylinder's safety devices, not from a nearby central heating pipe routed through the same area — trace back to the tundish if one is fitted.
  2. Check for scalding risk immediately — do not let anyone touch the discharge pipe or the water. If it's actively flowing, treat this as more urgent than a central heating PRV weep.
  3. Ask when it started and how often. A single discharge event that hasn't recurred may have been a transient fault; continuous or repeated discharge means an active, unresolved fault in the safety device chain.
  4. A G3-qualified engineer should check, in sequence: the cylinder thermostat and immersion heater thermostat (overheat can trigger the TPR thermally), the expansion vessel on the cylinder's cold feed (same failure mode as a heating circuit vessel — lost pre-charge or ruptured diaphragm), the pressure-reducing valve on the incoming mains feed, and the TPR/ERV valves themselves for seat damage.
  5. Never cap, plug, or remove a discharging TPR or ERV as a "fix." These are the last line of defence against a Boiling Liquid Expanding Vapour Explosion (BLEVE) — an extremely rare but genuinely catastrophic failure mode for pressurised hot water storage. If in doubt, isolate the cylinder from both cold feed and power and get a G3-qualified engineer on site the same day.
  6. Check the discharge pipework itself (D1 and D2 runs, tundish) is correctly installed per Approved Document G3 — undersized or badly routed discharge pipe can itself be a factor in unsafe discharge, separate from the device fault. See part g sanitation for the full G3 safety device chain and discharge pipe specification.

Distinguishing a one-off event from an ongoing fault

Not every discharge event needs an emergency response, but every discharge event needs investigation. A single PRV weep after a very cold night (system pressure creeping up as it re-warms) with no repeat in the following days can sometimes be a low-level, self-resolving event — but it should still be logged and the vessel checked at the next visit. Repeated discharge, discharge that coincides reliably with the heating firing, or any TPR/ERV discharge on an unvented cylinder should always be treated as an active fault requiring same-visit or next-day attention, not "keep an eye on it."

What NOT to do

Frequently Asked Questions

My customer says water is "dripping from a pipe outside" — is this an emergency?

Trace it first. If it traces back to the central heating PRV, it's a fault that should be fixed promptly (same day or next available slot) but isn't typically an immediate danger once the filling loop is confirmed closed and the system isolated if pressure is climbing uncontrolled. If it traces back to an unvented cylinder's TPR or ERV, treat it with more urgency — scalding water discharge from a safety device on a pressurised, heated vessel should be assessed by a G3-qualified engineer the same day, not scheduled for "next week."

Can I just replace the PRV and move on without checking the expansion vessel?

No — this is the single most common misdiagnosis in this area. In most cases the PRV is functioning correctly; it's opening because the expansion vessel has failed and there's nowhere else for the expansion water to go. Replacing only the PRV without fixing the vessel means the new valve will start discharging again within days or weeks. Always test the vessel pre-charge before concluding the PRV itself is at fault.

The discharge has stopped by the time I arrive — what do I check?

Look for physical evidence: limescale or staining on the discharge pipe or the wall beneath it, a wet patch that's since dried, or calcium deposits around the valve body — all indicate a genuine past discharge even without an active drip. Test the system pressure through a full heating cycle (cold, then hot) to see if the pattern recurs while you're on site, and test the expansion vessel pre-charge regardless, since an intermittent fault is still a fault.

Is it dangerous to touch a discharge pipe to check if it's warm?

For a central heating PRV discharge pipe, brief contact to check temperature is generally low-risk once you're confident it's the heating circuit, not the cylinder — but be cautious, as water can still be hot. For anything you suspect is a cylinder TPR/ERV discharge, do not touch the pipe or attempt to feel the flow — that water can be at or near boiling point and the risk of scalding is real.

Do I need to notify Building Control if I replace a cylinder's safety devices?

Any work on an unvented hot water storage system's safety devices should be carried out or supervised by a G3-qualified person, and depending on the scope of work, notification to Building Control (directly, or via a competent person scheme such as WaterSafe) may apply — see part g sanitation for the full notification requirements. Simple like-for-like replacement of a failed device by a G3-qualified engineer is generally more straightforward than a full new installation, but check current requirements before assuming no notification is needed.

Regulations & Standards