A temperature & pressure relief valve (T&P valve) is the last-line safety device on a sealed hot water cylinder, opening automatically if the stored water gets dangerously hot or the vessel over-pressurises. It exists because sealed water can be heated far past 100 °C without boiling — at 3 bar gauge water boils at about 144 °C — and would flash to steam, expanding roughly 1,600-fold, if containment failed.
What does a T&P valve do?
A T&P valve protects a sealed hot water cylinder against the one failure mode that valves earlier in the safety chain cannot catch: stored water that is both hot and pressurised beyond safe limits. It combines two independent triggers in one body — a thermostatic element that opens on excess temperature and a spring-loaded seat that opens on excess pressure — and discharges stored water through a tundish to a safe visible point, drawing cooler mains water in behind it.
It is fitted directly into the top section of the cylinder, where the hottest water sits, so its temperature probe senses the true worst-case stored temperature rather than a pipework average.
Why do sealed cylinders need temperature protection as well as pressure protection?
Because pressure relief alone cannot prevent a steam explosion. In a sealed vessel, water can be heated well beyond 100 °C without boiling — at 3 bar gauge the boiling point is about 144 °C. If a thermostat and its backup cut-out both fail, a cylinder full of superheated water becomes a stored-energy hazard: any breach or sudden pressure drop lets the entire volume flash to steam, expanding around 1,600 times in volume. The T&P valve removes that scenario by dumping water and capping the stored temperature long before the danger zone, regardless of what the pressure side is doing.
This is why the device is non-negotiable on an unvented cylinder and why products certified to EN 12897 carry it as standard equipment, with the valve itself specified under EN 1490.
How is a T&P valve different from the expansion relief valve?
The two valves answer different questions and normally never open together:
| Valve | Trigger | Normal role | Discharge means |
|---|---|---|---|
| Expansion relief valve | Pressure only | Relieves everyday expansion if the expansion vessel fails | Service fault — usually lost vessel pre-charge |
| T&P relief valve | Temperature or pressure | Last line against over-temperature and gross over-pressure | Control failure — isolate and investigate immediately |
A drip from the expansion relief valve is a maintenance nuisance; a discharge from the T&P valve is a warning that the control thermostat and thermal cut-out may both have failed. The distinction matters when diagnosing a wet tundish.
What settings do T&P valves use?
T&P valve settings are fixed by the manufacturer to match the cylinder's certification, and they vary by market, product standard and approval scheme — there is no single universal opening temperature or pressure. The correct values are stamped on the valve body and stated in the cylinder documentation. Two rules follow: never replace a T&P valve with anything other than the specified model, and never attempt to adjust one. The valve's rating is part of the pressure vessel's certified safety case.
How often should a T&P valve be tested?
Once a year, as part of the cylinder's annual service: the test lever is lifted briefly to confirm the valve opens, flows freely to the tundish and reseats without weeping. Limescale is the main enemy — in hard water areas a valve that is never exercised can seize or block, which is one reason unvented cylinder servicing should not be skipped. A valve that weeps after testing, or discharges in normal operation, needs investigation the same day; the causes are covered under common problems with unvented cylinders.
Frequently asked questions
What makes a T&P valve open?
Either of two independent triggers: stored water temperature exceeding the valve's rated limit, or cylinder pressure exceeding its rated pressure. Temperature-driven discharge signals a failed control thermostat and cut-out; pressure-driven discharge points to a failed expansion provision or pressure-reducing valve.
Can I adjust or swap a T&P valve for a different rating?
No. The valve's temperature and pressure ratings are part of the cylinder's certified safety case and vary by market and product approval. Replace like-for-like with the manufacturer-specified valve only, and never fit a plug or a lower-rated substitute.
Why is my T&P valve discharging hot water?
Hot or steaming discharge means the stored water has reached the valve's temperature limit — a genuine over-temperature event. Isolate the heat source, leave the discharge route clear and call a competent installer immediately; the control thermostat and thermal cut-out need checking before the cylinder is reheated.
Do vented cylinders have T&P valves?
Generally no. An open-vented cylinder relieves expansion and over-temperature up its open vent pipe to atmosphere, so it does not rely on mechanical relief valves. T&P valves are the signature safety device of sealed, unvented storage.