Why Does My Boiler Keep Losing Pressure?
A boiler keeps losing pressure for one of a few reasons: a small leak somewhere in the system, a waterlogged or failed expansion tank, a weeping pressure relief valve, or air escaping after you bleed the radiators. A slow, steady drop over weeks usually points to a hidden leak or a tired expansion tank. A sudden drop often follows bleeding radiators or a valve discharging. Topping up the pressure is a short-term fix — finding why it falls is the real repair, and most causes need a technician to locate and correct.
Boiler pressure loss: cause, sign, and fix type
| Cause | Sign | Fix type |
|---|---|---|
| System leak | Damp spots, stains, or corrosion near pipes | Professional seal or pipe repair |
| Failed expansion tank | Pressure runs high hot, low cold | Tank recharge or replacement |
| Leaking relief valve | Water dripping from the discharge pipe | Valve replacement by a tech |
| Air after bleeding | Pressure fell right after bleeding radiators | Simple repressurize via filling loop |
| Loose fittings | Slight moisture at joints or the pump | Tighten or reseal by a tech |
What draws the pressure down
Pressure loss almost always means water is leaving the closed loop, or the system can no longer manage its own expansion. A true leak is the first suspect. Look for damp patches, rust stains, or mineral crust around radiator valves, pipe joints, and beneath the boiler. Even a pinhole leak can bleed pressure over a week or two while barely leaving a mark.
The expansion tank is the next suspect. It holds a cushion of air that absorbs the pressure swing as water heats and expands. When that tank waterlogs or its internal bladder fails, pressure spikes when hot and collapses when cold. Diagnosing a leak versus a tank fault is where professional boiler repair earns its keep, since the two fixes are very different.
- Check radiator valves, pipe joints, and the boiler base for damp or rust.
- Suspect the expansion tank if pressure runs high hot and low cold.
- A dripping discharge pipe outside points to the pressure relief valve.
How to stop the pressure loss for good
If the pressure only fell once, right after you bled the radiators, you likely just lost a little water and air. Top it back up through the filling loop to 1 to 1.5 bar and watch it for a few days. If it holds, you are done. If it keeps sliding, our guide on why a boiler stops producing heat covers what low pressure does next.
Recurring loss means a part is failing, and that is technician territory. Expansion tanks, relief valves, and hidden leaks all need proper tools and sometimes a drain-down. Knowing what boiler repair costs helps you weigh the fix. And if you ever smell gas near the boiler, stop, leave the house, and call the utility before doing anything else.
- Repressurize once and watch; a stable gauge means it was just bled air.
- Repeated loss needs a tech to find the leak or replace the tank or valve.
- Smell gas near the boiler? Leave and call the utility before any repair.
Questions homeowners ask next
Why Does My Boiler Keep Losing Pressure?
The common causes are a leak in the pipes or radiators, a failed expansion tank, a leaking pressure relief valve, or air lost after bleeding radiators. A slow drop suggests a leak; a sudden drop often follows bleeding. Recurring loss usually needs a technician.
Is it safe to keep topping up boiler pressure?
Refilling once or twice is fine, but doing it often masks a real problem. Constant pressure loss means water is escaping somewhere. Have a technician find the leak or failing part before it gets worse.
How often should I need to add pressure?
A healthy boiler should hold pressure for months with no top-ups. If you are refilling weekly or monthly, something is leaking or the expansion tank has failed and needs service.