Temperature and Pressure Relief Valve Explained

What does a temperature and pressure relief valve actually do?

An unvented cylinder holds water at mains pressure with no open vent to the loft. That is what gives you strong showers, and it is also why the cylinder needs protecting. Water heated well above 100°C under pressure stays liquid, and if the cylinder split it would flash to steam in an instant. The temperature and pressure relief valve, usually called the T&P valve, exists so that can never happen.

The valve screws into a boss near the top of the cylinder, where the water is hottest. A wax or fluid-filled probe sits in the water. When the water reaches the valve’s set temperature, the probe expands and lifts the valve off its seat. Cold mains water then flows into the cylinder to replace what is dumped, so the temperature falls. The same valve also has a spring set to a pressure limit, so it will open if pressure alone climbs too high.

It is the third of three independent safety layers that Building Regulations Part G (section G3) requires on an unvented cylinder. The first is the control thermostat that you set, normally around 60°C. The second is a non-self-resetting overheat cut-out that switches off the boiler supply or the immersion if that thermostat fails. The T&P valve only acts if both of those have failed. So when it opens, two other things have already gone wrong.

Is your cylinder losing water through it right now?

Every unvented cylinder discharges into a tundish, a small funnel with a visible air gap, normally in the airing cupboard just below the valves. Two valves can feed it: the T&P valve and the expansion relief valve on the cold inlet. Which one is running, and how hot the water is, tells you how urgent this is.

• Hot water pouring or running steadily through the tundish, often with the pipes hissing or banging: the T&P valve has opened. Treat this as an emergency.
• Cool or lukewarm water dripping, mostly while the cylinder is heating: this is usually the expansion relief valve, and a sign the expansion vessel has lost its charge. It needs fixing within days, not tonight. Our page on water running into the tundish covers that pattern in detail.
• Nothing in the tundish but water on the floor under the cylinder: that is a leak from a fitting or the cylinder itself. See hot water tank dripping.

If hot water is discharging, switch off the boiler and the immersion heater at their switches. Do not turn off the cold mains to the cylinder until the discharge runs cold, because the incoming cold water is what is cooling it. Keep people away from the outside discharge point, which may be spilling scalding water.

Why did the T&P valve open?

Because the T&P valve is the last device in the chain, the real fault is almost always upstream of it. These are the usual suspects:

• A two-port motorised valve stuck open on the boiler coil. The cylinder thermostat asks it to close, but the boiler keeps pumping hot primary water through the coil. The overheat cut-out should break the supply to that valve; if it has also failed, the water keeps climbing.
• A failed immersion heater thermostat with its contacts welded shut, together with a cut-out that has not tripped or has been bypassed. This is more likely on direct electric cylinders and on older combined thermostat and cut-out units.
• A thermostat pulled out of its pocket, so it reads the wrong part of the cylinder and never tells the heat source to stop.
• Pressure rather than temperature. A failed pressure reducing valve lets full mains pressure into the cylinder, and if the expansion relief valve is also sticking, the T&P valve lifts on pressure. The pressure reducing valve problems guide explains that part.
• The valve itself. After years of scale, a T&P valve can let by at normal temperatures, giving a slow warm drip. That is a worn seat, not an overheating cylinder, but it can only be confirmed by checking the actual water temperature.

What an engineer does when a T&P valve has lifted

Work on the safety devices of an unvented cylinder should be done by someone who holds the unvented hot water (G3) qualification. It is not a householder job, because a wrong part or wrong setting removes the one thing that stops the cylinder failing violently.

A proper visit starts with making it safe, then finds out which layer failed. The engineer will measure the stored water temperature, test the cylinder thermostat and the overheat cut-out, and check the motorised valve closes fully when the thermostat is satisfied. On the cold side, they check the incoming pressure after the reducing valve and the charge in the expansion vessel.

If the valve was forced open by a genuine overheat, the fix is the component that failed, plus a check that the T&P valve has reseated and no longer drips. If the valve is letting by on its own, it is replaced. The replacement must match the setting stamped on the original and be one the cylinder maker approves, usually from their own spares list. A generic valve with the right thread is not a like-for-like swap.

Replacing a T&P valve means isolating the cylinder and partly draining it, so most of the cost is time rather than the part. Genuine T&P valves for common UK cylinders are typically a few tens of pounds to around a hundred pounds depending on the brand, as a market guide. The repair price is agreed with you on site before any work starts, and the call-out is set out on our pricing page.

T&P valve, expansion relief valve and boiler relief valve: which is which?

People mix these up, and the confusion leads to the wrong fix. A quick way to tell them apart:

• The T&P valve sits on the cylinder body near the top, has a probe going into the cylinder and a small lever or knurled cap. It protects against overheating and overpressure together.
• The expansion relief valve sits on the cold inlet group, next to the pressure reducing valve and the expansion vessel. It responds to pressure only, usually rated a bar or two below the T&P valve. On a Megaflo Eco, for example, the inlet group reduces the mains to 3 bar, the expansion relief valve is rated at 8 bar and the T&P valve at 90°C and 10 bar.
• The pressure relief valve on a combi or system boiler protects the heating circuit, not the hot water, and usually discharges through a copper pipe to the outside wall at about 3 bar. If that one is dripping, read boiler pressure relief valve leaking instead.

Should the T&P valve ever be tested?

Yes, but as part of the annual service the cylinder maker asks for, not as a casual check. During a service the engineer twists or lifts the cap on each relief valve briefly to confirm water flows freely to the tundish, then checks that it snaps shut and stops dripping. A valve that has not moved for years can stick shut, or can fail to reseat once opened because scale lodges on the seat.

That second risk is why homeowners are usually told not to do it themselves. If a valve does not reseat after a test, you are left with a continuous discharge and no one to fix it. Most unvented cylinder warranties also depend on a yearly service with a record kept, so it is worth booking one through our cylinder page if the last one is overdue.