Why does my boiler keep losing pressure?
Updated August 2026
A boiler that keeps needing a top-up is losing water somewhere, most often through a small leak, a waterlogged expansion tank, or a weeping pressure-relief valve. A sealed heating system should hold steady at about 12 to 15 psi cold and only drift slowly, so a gauge that sinks within days means water is escaping. Trace where before you keep refilling, because a hidden pipe or fitting leak can quietly rot floors and joists while you top the pressure back up.
Safety first: topping the pressure up is fine as a stopgap, but chronic loss means water is going somewhere, and a slow leak inside a wall, under a floor, or above a ceiling can rot structure and grow mold long before you see a stain. If you find active dripping, close the isolation valves to that section if you can, and get it fixed rather than just refilled. A boiler is also a gas appliance, so if refilling does not hold and you smell gas or the relief valve discharges repeatedly, shut it down and call a heating engineer.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Small leak in the pipework or a radiator | Most common | Check fittings, valves, and radiator bodies for damp, staining, or corrosion, then tighten or reseal the culprit. Hidden runs may need a plumber. | $0–500 |
| Waterlogged or failed expansion tank | Common | The tank's air charge is gone, so pressure swings wildly and the relief valve dumps water. A tech recharges or replaces it. | $150–400 |
| Weeping pressure-relief valve | Common | A relief valve passing water drips outside or into a tundish. Replace the valve, and find out why it lifted in the first place. | $100–300 |
| Recent radiator bleeding | Occasional | Bleeding air out lowers system pressure. Simply re-top the system to 12 to 15 psi. Not a fault if it happened once. | Free |
| Corroded auto air vent or fittings | Occasional | A leaking automatic air vent or an old compression fitting seeps slowly. Replace the vent or remake the joint. | $20–200 |
| Cracked heat exchanger | Rare | An internal crack lets system water escape into the combustion side, with no puddle to find. Serious, and usually points to replacing the boiler. | Often replace the boiler |
Check these first
- Read the gauge cold and warm. Normal is about 12 to 15 psi cold, rising a little when hot. Note how fast it falls after you refill.
- Walk every radiator and visible pipe run looking for damp patches, rust streaks, or mineral crust at valves and fittings.
- Check the discharge pipe from the pressure-relief valve, usually terminating outside. Water dripping there means the valve or expansion tank is the cause.
- Recall whether you recently bled radiators or drained anything. A single top-up afterward is normal, not a leak.
- Press the Schrader valve on the expansion tank. Water coming out instead of air means the tank has failed and lost its charge.
Prevent it next time
Pressure loss is easiest to catch early, which is why a quick monthly glance at the gauge matters; our note on how often to check boiler pressure makes it a habit. A slow drift of a psi or two over a month is normal, but a repeat refill every week is a leak worth chasing. An annual boiler service also tests the expansion tank charge and the relief valve, the two parts that most often cause mystery pressure loss, so they get caught on a $150 to $300 visit rather than after a $2,000 water-damage repair.
When to call a pro
You can handle the diagnosis and a simple top-up yourself, but re-charging an expansion tank, replacing a relief valve, or tracing a buried leak is plumber or heating-engineer territory. Call one when the pressure keeps dropping after a refill, when you find water at the relief discharge, or when no visible leak explains it. A relief-valve swap runs $100 to $300, an expansion-tank recharge or replacement $150 to $400, and leak tracing $150 to $500 depending on where it hides. Do not keep refilling a system that loses pressure daily, because you are pumping in fresh oxygenated water that accelerates internal corrosion.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
What pressure should my boiler be at?
For most sealed domestic systems, about 12 to 15 psi cold, which is roughly 1 to 1.5 bar on gauges marked that way. The needle should rise modestly when the system heats and settle back when it cools. Below about 10 psi many boilers lock out and refuse to fire to protect themselves, which is why a slow leak eventually leaves you with no heat. If yours needs topping above 15 psi cold to run, something is holding it low and should be checked.
Is it safe to keep topping up my boiler pressure?
As an occasional stopgap, yes, but not as a routine. Every refill adds fresh, oxygen-rich water, and that oxygen feeds internal corrosion and sludge that shorten the boiler's life and clog the pump and radiators. More important, constant loss means water is escaping somewhere, and a hidden leak damages the house even when the boiler seems fine. Top it up once to keep heat on, then find and fix the leak instead of making refilling a weekly ritual.
Why does my boiler lose pressure only when the heating is off?
Counterintuitively, that pattern often points at the expansion tank or a relief valve rather than an obvious leak. When the system cools it contracts, and a failed expansion tank cannot cushion the swing, so pressure can read low once things settle. A relief valve that only weeps at certain pressures adds to it. If the drop tracks the heating cycle rather than the calendar, have a tech check the expansion tank's air charge first, since recharging it is cheaper than chasing a leak that may not exist.
Can a boiler lose pressure without a visible leak?
Yes, and it is common. The water can escape into places you cannot see: a pinhole under floorboards, a fitting buried in a wall, or the relief valve quietly discharging outside where you never look. It can also leave the water side entirely through a cracked heat exchanger into the combustion chamber, which shows up as no puddle at all. A heating engineer can pressure-test the system to confirm a leak exists and narrow down where, which beats guessing.