Why Your Boiler Keeps Losing Pressure
A sealed system that keeps dropping below 1 bar is telling you water is leaving it somewhere — a leak, a discharging PRV or a failed expansion vessel. The cure is finding which, not just topping it up.
A boiler that loses pressure and needs repeated topping up is one of the most common heating callbacks. The pressure gauge falling means water is leaving the sealed system, and repeatedly refilling it only masks the problem while dragging in fresh oxygen that accelerates corrosion.
This guide sets out the usual causes — visible leaks, a discharging pressure relief valve, a failed expansion vessel, and a leaking heat exchanger — how to tell them apart, and where the line sits between customer top-up and work for a Gas Safe registered engineer. Any work on the sealed system or the appliance itself must be carried out by a qualified engineer.
Key takeaways
- Pressure loss means water is leaving the system — the job is to find where, not just to top up repeatedly.
- The three big causes are a leak somewhere on the circuit, a discharging pressure relief valve, and a failed expansion vessel.
- A PRV that weeps outside means the vessel has likely failed or the fill pressure is too high — check the vessel pre-charge.
- Pressure that climbs steeply on heat-up then drops when cold is the classic signature of a failed expansion vessel.
- Frequent top-ups accelerate corrosion by introducing oxygen — fix the cause rather than living with the symptom.
How a sealed system holds pressure
A sealed heating system is filled to around 1–1.5 bar cold and closed off. As the water heats and expands, the expansion vessel absorbs the extra volume so the pressure only rises modestly; on cooling it returns to the cold fill figure. If the gauge steadily falls over days or weeks, water is escaping the closed loop somewhere.
That 'somewhere' is what the diagnosis is about. It is either leaving through a leak on the pipework or components, being pushed out through the pressure relief valve, or a failed expansion vessel is making the pressure swing so violently that the PRV lifts and discharges.
Leaks on the system
The most obvious cause is a genuine leak: a weeping radiator valve, a corroded radiator, a compression joint, or a fitting under the boiler. A slow leak may only show as a faint stain, a green verdigris deposit on brass, or a patch of corrosion — worth inspecting valves, visible joints and under the boiler carefully.
Some leaks hide under floors or in walls and never show as a visible drip, evaporating before they pool. If the pressure falls steadily but nothing is visible and the PRV and vessel check out, a concealed leak is the likely culprit and may need pressure testing or a leak-detection additive to trace.
Rule out the obvious first
Before condemning the vessel or heat exchanger, walk the system: every radiator valve, visible joint, the pump, the auto air vent and under the boiler. A weeping valve is a five-minute find that saves an hour chasing the wrong fault.
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A discharging pressure relief valve
The pressure relief valve (PRV) is a safety device that opens if system pressure exceeds a set figure — typically 3 bar — venting water outside through a discharge pipe. If that pipe drips or the wall below the outlet is stained, the PRV is lifting, and every discharge drops the system pressure.
A PRV lifts either because the pressure genuinely got too high — often because the expansion vessel has failed and can no longer absorb the expansion — or because the valve seat has been contaminated by debris and no longer seals. Either way it is a symptom to trace, and work on the valve is for a qualified engineer.
A failed expansion vessel
The expansion vessel is the single most common cause of mysterious pressure loss. It uses an air cushion behind a diaphragm to absorb the expansion of heated water. If the air charge is lost or the diaphragm splits, there is nothing to absorb the expansion, so pressure spikes hard on heat-up, lifts the PRV, and then reads low once the system cools and the discharged water is gone.
The tell-tale signature is a gauge that climbs steeply toward the PRV setting whenever the heating runs, then sits low when everything is cold. The fix is to recharge the vessel to the correct pre-charge, or replace it if the diaphragm has gone — both jobs for a Gas Safe registered engineer, who will drain the system side before checking the charge.
The heat-up test
Watch the gauge from cold as the heating fires. A healthy system barely moves; a failed vessel drives the needle up toward 3 bar and trips the PRV. That pattern points straight at the vessel before you touch anything else.
A leaking heat exchanger
Less common but more serious is a leaking primary heat exchanger inside the boiler. Water can escape internally without ever appearing as an external drip — sometimes evaporating away, sometimes showing as condensation or staining inside the case. This is a boiler fault, not a system fault.
A heat exchanger leak is diagnosed by a Gas Safe engineer once external leaks, the PRV and the vessel have been ruled out. It usually means a repair or, depending on the appliance's age and value, replacement — and it is firmly qualified-engineer territory.
From guidance to action
Frequently asked questions
Is it safe to keep topping up my boiler?
As a short-term measure to keep heating on, an occasional top-up is fine. But repeated topping up is a symptom, not a fix — each refill introduces fresh oxygen that accelerates corrosion. If it needs topping up more than very occasionally, get the cause found by a Gas Safe registered engineer.
Why does my boiler lose pressure only when the heating is on?
That pattern usually points at a failed expansion vessel. With no air cushion to absorb the expansion, pressure spikes on heat-up and lifts the pressure relief valve, so water discharges outside and the system reads low once it cools. The vessel needs recharging or replacing.
Can I fix a boiler losing pressure myself?
A homeowner can top up the pressure using the filling loop and check for obvious weeping valves. Anything beyond that — the PRV, the expansion vessel, the heat exchanger or the sealed system — is work for a Gas Safe registered engineer, as it involves the appliance and gas safety.
How do I know if it's the expansion vessel or a leak?
Watch the gauge from cold. A leak causes a slow, steady fall regardless of whether the heating is on. A failed vessel makes the pressure climb steeply on heat-up toward the PRV setting, then read low when cold. If the PRV discharge pipe is dripping, suspect the vessel.
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