Customer Job: Boiler Pressure Relief Valve Leaking in West Horsley

The Challenge

A homeowner in West Horsley called us out because their boiler was leaking. On arrival the “leak” turned out to be the boiler’s pressure relief valve (PRV) discharging into a bucket. The PRV discharge pipe had never been run to the outside or to a drain, so every time the valve opened, water collected indoors.

A PRV only opens when the system pressure climbs too high, so the bucket was a symptom, not the fault. The job was to find out why the pressure kept building and fix that, so the valve would stop discharging.

The Solution: Recharge the Expansion Vessel

We isolated and drained down the boiler, then recharged the expansion vessel. The vessel had lost its charge, which was causing the pressure to build as the water heated up and forcing the PRV to discharge.

With the vessel recharged, we refilled the system to the correct pressure and tested the heating and hot water — both working correctly. Because the boiler is around 15 years old, we also checked the flue gas readings while on site, and explained to the customer how the system works so they know what normal pressure looks like.

Why a Boiler’s Pressure Relief Valve Discharges

Water expands when it is heated. In a sealed heating system the expansion vessel — a tank with an air cushion on one side of a rubber diaphragm — absorbs that extra volume. Over the years the air charge leaks away, and the vessel stops doing its job:

• Pressure climbs: With no air cushion left, each heat-up pushes the system pressure higher.
• The PRV opens: At its set limit the pressure relief valve releases water to protect the boiler. That is it working correctly, not failing.
• Recharge, then refill: Draining down, recharging the vessel and refilling to the correct cold pressure fixes the cause.
• Discharge must go outside: PRV water can be very hot. The discharge pipe should run to the outside or a suitable drain — never into a bucket indoors.

The Repair Process

1. Find the source: The PRV was found discharging into a bucket, with its discharge pipe not connected to the outside or a drain.
2. Drain down: The boiler was isolated and drained.
3. Recharge the vessel: The expansion vessel was recharged to stop the pressure building.
4. Refill and test: The system was refilled to the correct pressure and the heating and hot water tested — both working.
5. Health check: Flue gas readings were checked on the ageing boiler and the customer was shown how the system works.

Recommended Follow-Up

• PRV discharge pipe: Connect the PRV discharge pipe so it runs safely to the outside or to a suitable drain. Discharge water can be very hot and should not be left running into a bucket.
• Boiler service: A full strip-down boiler service, given the boiler’s age.
• Bath valve: Replace the bath valve cartridge.

We will quote for these separately.

Boiler Pressure Questions

Why is my boiler’s pressure relief valve dripping?

Usually because the system pressure is getting too high when the boiler heats up, and the most common cause is an expansion vessel that has lost its air charge. Recharging the vessel and resetting the pressure normally stops it, as it did here.

Is it safe to let the PRV drain into a bucket?

No. PRV discharge water can be very hot. The discharge pipe should be run to the outside or to a suitable drain, where it can release safely and you can see if it is discharging.

Do you cover West Horsley?

Yes. Talis covers West Horsley and nearby Guildford, Cobham, Woking and Dorking for boiler repairs and plumbing. For related jobs, see our leaking boiler repair in Godalming and our commercial PRV discharge call-out in Westminster. If your boiler is losing pressure rather than gaining it, see our boiler losing pressure repair in West Kensington.