TPRV Dripping on an Unvented Cylinder

Is a dripping TPRV dangerous?

It depends on the water. The temperature and pressure relief valve (TPRV) opens when the stored water gets too hot, or when pressure goes past its own rating, and sends the water to the tundish and out through the discharge pipe. If it is opening, something upstream has asked it to, or its seat is damaged.

Judge the water from a safe distance, without touching the valve or the pipe. Cool or lukewarm drips are a fault to get looked at this week. Steam, a steady flow, or water too hot to touch is a different situation: the cylinder is overheating and the valve is doing the one thing it exists for.

Do not plug, cap, tape or screw down any part of the valve or its discharge pipe. A relief valve that cannot discharge is no longer a safety device, and a cylinder holding hot water at mains pressure can fail violently.

How is the TPRV different from the expansion relief valve?

Most unvented cylinders have two relief valves, and the tundish cannot tell you which one is opening. The expansion relief valve sits on the cold inlet group, the brass block where the mains enters, and releases pressure caused by water expanding as it heats. The TPRV sits directly on the cylinder, where it can sense the stored water temperature.

That position is deliberate. Approved Document G describes a temperature relief valve or combined temperature and pressure relief valve as one of the two independent safety devices, placed so the stored water cannot exceed 100 degrees Celsius. The valve is fitted at the factory on packaged cylinders and is not meant to be disconnected except for replacement.

The practical consequence is that a failing expansion vessel normally shows at the expansion relief valve, not the TPRV. If the discharge is cool and appears on every heat-up, read waterlogged expansion vessel first. The opening pressures of both valves are printed on the valves and in the cylinder manual, and they differ between models, so use those figures and not a number from a website.

What makes a TPRV open?

Think of it as heat first and hardware second.

• Stored water that has gone past the thermostat setting because a control has failed: a stuck immersion thermostat, a motorised valve held open, or a boiler-side thermostat that no longer closes.
• A thermal cut-out that has not stopped the heat source before the water reached the TPRV limit. The cut-out is meant to act first, so a TPRV discharge with a heating fault points at two failures, not one.
• Two heat sources working against each other, such as a boiler coil and an immersion heater, with one control faulty.
• A seat scored by scale or grit, often after an earlier discharge, so the valve weeps although the water is not hot.
• An old valve. Relief valves are a spring and a seat, and both wear.

Hot discharge or cool drip: what each one tells you

A hot or steaming discharge, or a flow rather than a drip, is a heat-control fault until proved otherwise. Nothing else on the cylinder should be touched. The tripped cut-out is the next clue: see unvented cylinder tripped cut-out for how the two fit together.

A cool or lukewarm drip that is steady, whether or not the cylinder is heating, points at the seat. A seat that has been scratched will not close fully, and flicking the test lever may stop it for a day without curing it.

A cool drip that only starts a few minutes into a heat-up and stops as the cylinder cools is the expansion pattern, which belongs to the expansion relief valve and the vessel behind it, not to the TPRV.

What can I safely check myself?

Anything that stays outside the valves is fair game. Anything that involves the valves, the vessel or the electrical enclosure is not.

• Look at the end of the discharge pipe or the tundish and note whether the water is cold, warm or hot, and whether it runs in drops or a stream.
• Record when it happens and photograph the tundish with the time on the picture.
• Check the programmer and any boost setting. A cylinder heated by a boiler coil and an immersion heater together reaches its limit faster.
• Look at the cylinder base and the cupboard floor for staining or damp. Long discharge that finds its way back into the cupboard rots timber over months.
• Check the wet outlet without touching anything: a wet outlet on the inlet group points at the expansion relief valve, a wet outlet on the cylinder at the TPRV.

What does an engineer check for a dripping TPRV?

The order starts with heat, because replacing a TPRV that was reacting to overheating would hide a dangerous fault.

First, the stored water temperature against the thermostat setting, and the state of the thermal cut-out. If the water is hot, the fault is in the controls and nothing else is changed until it is understood.

Second, which valve is actually discharging, by following the wet outlet back from the tundish.

Third, only when temperature and controls are correct, the TPRV seat itself, and whether the drip continues once pressure is confirmed within the figures in the manual.

The same reasoning runs through unvented cylinder fault repair and the TPRV and expansion vessel repair page, and the tundish article shows how to read the discharge route.

Should the TPRV be repaired or replaced?

A relief valve is not a repairable component. Once the seat is damaged, it is replaced with a valve of the type and rating the cylinder manufacturer specifies. Approved Document G expects a temperature relief valve to be sized to discharge at least as much as the cylinder heat input, so a like-for-like valve from the manual is the point, not whatever fits the thread.

Nothing inside a sealed safety valve is a householder job, and replacing one is work for someone competent on unvented cylinders. When the cause was a control fault or the expansion side, replacing the valve alone cures nothing, because the new valve would open for the same reason. The repair or replace decision guide covers when the cylinder itself comes into question, and the TPRV and expansion vessel cost article covers what the parts typically cost.