Repeat Pressure Loss

Boiler Losing Pressure — Diagnosis & Repair

Topping up pressure once isn't unusual — sealed central heating systems lose a small amount over time. Topping it up every few weeks means the water is going somewhere, and that's a fault to find, not a routine to repeat. The three usual suspects are a leak somewhere in the system (visible or hidden), a faulty pressure relief valve letting water out, or a failed expansion vessel that can no longer absorb normal pressure changes.

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Our engineers trace where the pressure is actually being lost rather than just refilling the system and leaving. That can mean checking radiator valves and joints, examining the pressure relief valve discharge pipe for signs it's been weeping, and testing the expansion vessel's pre-charge — each points to a different fix.

A boiler that keeps losing pressure will eventually lock out or refuse to fire at all, so this is worth treating as a fault to solve rather than a top-up to keep repeating. Left unaddressed, a leak inside a wall or under a floor can also cause damage well beyond the heating system.

Diagnosis identifies the cause first; the repair — whether a valve, a joint, or the expansion vessel — is quoted once we know what needs fixing, not guessed in advance.

What you get

Traces the actual source of pressure loss instead of just refilling and leaving.
Checks the pressure relief valve, expansion vessel and visible pipework and joints.
Identifies hidden leaks affecting flooring or walls where relevant, before they cause further damage.
Gas Safe registered engineers for any work involving the boiler itself.
Explains whether the fix is a component repair or a sign of a wider system issue.
Advice on correct pressure range for your specific system going forward.

How it works

A method, not a guess

How often and how much

How frequently you're topping up and by how much tells us a lot before we even arrive — a slow drop over months is different from losing pressure within days.

Visual and system checks

The engineer checks visible pipework, radiator valves and joints for leaks, and examines the pressure relief valve discharge for signs it has been letting water out.

Expansion vessel test

If there's no obvious leak, the expansion vessel's pre-charge is tested — a failed vessel is a common, well-understood cause of repeated pressure loss that a top-up alone will never fix.

Fix and confirm

Once the cause is confirmed and you approve the repair, the engineer fixes it, repressurises the system correctly, and checks it holds pressure before finishing.

Questions

Losing Pressure FAQs for London homeowners

I keep topping up the pressure — is that a problem?

If it's a one-off, no. If you're doing it repeatedly — every few weeks — the water is leaving the system somewhere, and it won't stop on its own. It's worth having diagnosed rather than treating the top-up as routine maintenance.

I can't find a visible leak — could it still be losing pressure from a leak?

Yes. Leaks under floors, inside walls, or from a pinhole in a radiator can be invisible day to day while still slowly draining the system. If nothing is visibly wet, the pressure relief valve and expansion vessel are the next most likely causes, both of which an engineer can test.

What is a pressure relief valve and why would it cause this?

It's a safety valve designed to release water if system pressure gets too high — that's exactly what it should do. The problem is when it starts releasing water at normal pressure because it's worn or has debris in the seat, which slowly drains the system without an obvious flood.

What is an expansion vessel and how do I know if it has failed?

It's a sealed component that absorbs the natural expansion of heated water so pressure doesn't spike. When its internal diaphragm or pre-charge fails, the system can no longer absorb that expansion properly, and pressure behaves erratically — often needing repeated top-ups. It's tested with a pressure gauge, not guessed at.

Is it dangerous to run a boiler that keeps losing pressure?

Most boilers are designed to lock out below a minimum safe pressure rather than run unsafely, so the more common outcome is simply losing heating, not a safety event. That said, an unexplained leak should always be found and fixed rather than managed indefinitely with top-ups.

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