Boiler Leaking When Heating Is On – Causes, Risks & What It Means
If your boiler only leaks water when the heating is switched on, that detail is critical.
It tells us the leak is being triggered by heat, pressure, or circulation, not by a permanent crack or failed seal that leaks all the time.
In most cases, this type of leak appears shortly after the boiler fires up, during warm-up, or once radiators start heating. When the system cools down again, the leak may reduce or stop altogether.
That on-off behaviour helps narrow the cause very quickly.
Why boilers leak specifically when the heating runs
When central heating is operating, three things happen at the same time:
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System water heats up and expands
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Internal pressure rises
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Water is forced to circulate through the boiler, pump, valves, pipework, and radiators
Any weak point in the system is far more likely to show itself under these conditions.
A boiler that stays dry when cold but leaks when heating is on is almost always reacting to thermal expansion or pressure stress.
The most common causes
1. Pressure relief valve (PRV) discharging
This is one of the most common causes we see.
As water heats, system pressure rises. If pressure exceeds the safe operating limit (usually around 3 bar), the pressure relief valve opens to protect the boiler.
When this happens:
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Water is released through the discharge pipe
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It may appear as a leak underneath the boiler
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It often only happens when heating is on
If the PRV has lifted repeatedly in the past, it may no longer reseal properly and can continue to drip even at normal pressures.
2. Expansion vessel issues
The expansion vessel is designed to absorb the increase in water volume as the system heats up.
If the vessel:
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Has lost its internal air charge
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Has a ruptured diaphragm
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Is undersized for the system
…pressure rises sharply when heating starts, forcing water out through the PRV or weak joints.
This fault almost always presents only during heating operation.
3. Pump or circulation-related leaks
When the heating is on, the pump is running continuously.
This can expose:
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Worn pump seals
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Hairline cracks in pump housings
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Leaks around isolation valves or unions
Because these components are only under full flow during heating, leaks may not appear when the boiler is idle.
4. Internal seals expanding and opening gaps
Some leaks only appear once components heat up.
Metal expands with heat. If internal seals, O-rings, or compression joints are worn or marginal, they may:
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Seal when cold
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Weep or leak when hot
This is common around:
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Heat exchangers
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Diverter valves
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Flow and return connections
5. System pipework or radiator valve leaks
Not all “boiler leaks” actually originate inside the boiler.
When heating is on:
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Radiator pipework expands
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Old joints can open slightly
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Drips may track back along pipes toward the boiler casing
This can make it look like the boiler is leaking when the source is elsewhere.
Is it dangerous to keep running the heating?
In short: yes, it can be.
Continuing to run a boiler that leaks under heat and pressure can lead to:
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Ongoing pressure loss
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Repeated PRV discharge (damaging the valve permanently)
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Corrosion of internal components
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Electrical damage if water contacts wiring or PCB components
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Sudden system shutdowns or lockouts
Even a small intermittent leak should never be ignored.
What you should do immediately
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Check the pressure gauge when the heating is off and again when it’s running
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If pressure rises rapidly or approaches 3 bar, stop using the heating
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Look for water at the PRV discharge pipe outside
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Avoid repeatedly topping up the system — this masks the problem and causes further damage
If you’re unsure where the water is coming from, turning the heating off and observing whether the leak stops can provide useful diagnostic information.
How an engineer will diagnose this properly
A competent engineer won’t guess. They’ll check:
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Static vs operating pressure
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Expansion vessel charge and integrity
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PRV condition
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Pump seals and unions
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Internal boiler components under heat
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System pipework and nearby radiator valves
Only once the true cause is confirmed should parts be replaced.
Related guidance
For a full breakdown of all boiler water leak types, including constant leaks, top and bottom leaks, and condensate-related issues, see the main guide:
→ Boiler Leaking Water – Causes, Risks & What To Do
If your boiler is losing pressure or leaking when the heating is on, it usually needs proper diagnosis rather than guesswork. A heating-related leak almost always means the system is reacting to heat or pressure — and that’s not something to ignore.