What is a TPR?

A TPR valve (Temperature and Pressure Relief valve) is the final safety device fitted to an unvented hot water cylinder. Its purpose is to prevent the cylinder from exploding if both the temperature and pressure inside the cylinder become dangerously high.

How it works

The valve continuously monitors both:

Water temperature inside the cylinder.
Water pressure inside the cylinder.

If either reaches a dangerous level, the valve opens automatically and discharges hot water through the discharge pipework (typically terminating safely outside the building).

Typical operating values in the UK are:

Temperature: approximately 90–95°C
Pressure: typically 7 bar or 10 bar, depending on the cylinder manufacturer's specification.

The valve will open if either limit is exceeded—it doesn't require both to occur at the same time.

Why is it needed?

An unvented cylinder stores water under mains pressure. If a thermostat failed and the water continued to heat, the pressure inside the cylinder would rise rapidly. Without a functioning TPR valve, the stored energy could cause the cylinder to rupture violently.

For this reason, the TPR valve is a mandatory safety device under UK Building Regulations and must never be removed, blocked or isolated.

How is it different from an expansion relief valve?

Many people confuse the two:

Expansion Relief Valve
Protects against excessive pressure caused by expansion
Usually fitted in the cold water combination valve
Opens first during most pressure faults

TPR Valve
Protects against excessive temperature and pressure inside the cylinder
Screwed directly into the cylinder near the top
Acts as the final backup safety device

The expansion relief valve is designed to operate before the TPR valve. If it has been blanked off or is faulty, the TPR valve becomes the only remaining means of pressure relief.

Signs that a TPR valve may be faulty
Water constantly dripping from the discharge pipe (it may need replacing, or another fault may be causing it to lift).
No discharge during servicing when the test lever is operated.
Corrosion or limescale around the valve.
The valve has been capped, plugged or isolated (this is extremely dangerous).
Servicing

During an annual G3 service, the engineer should:

Inspect the valve for signs of corrosion or leakage.
Check that the discharge pipework is correctly installed.
Test the valve using the manual test lever (where appropriate).
Ensure the valve reseats correctly after testing.
Replace the valve if it fails to operate or continues to leak after testing.

As someone specialising in unvented cylinders, you'll know that a TPR valve should be regarded as the last line of defence. If you ever find one that has been blanked off, tied shut or removed, the cylinder should be isolated immediately until the fault has been rectified, as this represents a serious safety risk.

See https://easyflow.co.uk/services/safety-valve-replacement