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Published by Mo's Plumbing and Heating
Why Does My Boiler Keep Losing Pressure?
You have topped it up, twice now, maybe more, and it keeps sliding back down. That is a different problem from a boiler that needed topping up once, and it has an answer. Water does not evaporate out of a sealed system. It is going somewhere, and there are only three somewheres.
The Short Answer
The water is leaving through a visible leak, through a leak you cannot see, or through the safety valve to the outside because the expansion vessel has failed. That third one is the one people miss, because it produces no damp anywhere indoors. Work out which by checking the discharge pipe outside the wall, then looking at radiator valves, then watching how the gauge behaves through a full heating cycle. The behaviour tells you more than the number.
How Fast Is It Going?
Before anything else, get a sense of the rate, because it narrows the field immediately and costs you nothing but noting a couple of readings.
A drop of around 0.1 bar over several months is not a fault. Sealed systems seep minutely at valve glands and pump seals, and every radiator you bleed takes a little out. Topping up once or twice a year is maintenance.
A drop of a few tenths over a week or two is a small leak, and it is usually findable. A drop from 1.2 to zero within a day or two is a leak of real size, and at that rate it will be doing damage somewhere, so be properly concerned rather than topping it up again.
And a gauge that falls only when the heating has been running, then sits still overnight, is pointing hard at the expansion vessel and the safety valve rather than at a leak. That distinction is free to observe and it saves a great deal of hunting.
Cause One: A Leak You Can Find
Start with the things that are visible, because a decent share of these are sitting in plain sight and get missed because people look at the boiler rather than the system.
Radiator valves are the commonest single culprit, specifically the gland where the spindle comes through. A weep there evaporates as fast as it appears on a warm radiator, so there is no puddle, only a faint crusty white or greenish staining on the valve body and sometimes a small rust mark on the floor beneath. Run a dry tissue around every valve on every radiator, top and bottom, both ends. It takes ten minutes for a whole flat and it finds a lot of these.
Then the boiler itself. Take the front cover off only if you are comfortable doing so, and otherwise look underneath it for staining or drips on the pipework where it emerges. An automatic air vent on top of the boiler that has started passing will leave a trail down the casing. A pump or a diverter valve that is weeping shows as a dried mineral crust.
And check the filling loop. A loop left slightly open does the opposite of this problem, but a loop with a failing valve can weep back out, and either way it is a two second thing to rule out while you are down there.
Cause Two: A Leak You Cannot See
This is the one everybody fears and it is less common than the fear suggests, but it does happen and it is worth being systematic rather than pulling up floors.
Buried pipework under floors is the usual location, and the usual cause is mechanical rather than age: a nail or a screw through a pipe when flooring went down, a joint that was never quite right, or a pipe that has been rubbing on a joist notch for years. The tell is a warm patch on the floor, a slightly lifting board, a musty smell in one spot, or a mark appearing on a ceiling below.
The important thing is that a pinhole under a floor can lose enough water to drop your pressure noticeably while producing almost nothing visible, because it evaporates into the void or soaks slowly into a joist. So the absence of a puddle does not clear it.
How this actually gets found, and it is handy to know so you are not sold a floor excavation on day one: the system is isolated in sections and pressure tested to see which leg holds and which does not, which narrows it to a run rather than a house. Thermal imaging and acoustic tracing help on some layouts. Lifting floorboards is the last step, not the first, and anyone reaching for a crowbar before narrowing it down is guessing.
Cause Three: The Expansion Vessel, and Why Nobody Finds It
This is the answer more often than people expect, and it is the one that looks like a mystery because there is genuinely nothing to find indoors.
The expansion vessel is a sealed container, usually inside the boiler on a combi, divided by a rubber diaphragm with pressurised air on one side. When the water in your system heats up and expands, it pushes into that vessel and squashes the air, which is what stops the pressure climbing. Over years the air charge leaks away or the diaphragm splits, and then there is nowhere for the expansion to go.
What happens next is the signature. You heat the system, the pressure has nowhere to expand into, so it climbs steeply, often to 3 bar. The safety valve does its job and lets water out through the discharge pipe to the outside. The system cools, and now it has less water in it than it started with, so the pressure reads low. You top it up. Tomorrow it happens again.
So the pattern is: fine when cold, very high when hot, low again the next morning. And the evidence is outside rather than inside. Find the small pipe that comes through the external wall near the boiler and points at the ground. If it is dripping, damp, stained, or has a limescale trail, you have found where every top up has gone.
Recharging a vessel is a routine job and inexpensive relative to what people fear when their pressure keeps dropping. Where the diaphragm has gone the vessel is replaced, or an external one fitted alongside where there is no room inside the case.
Narrowing It Down Yourself
You can get a long way before anyone comes out, and doing so changes the visit, because it turns an open-ended hunt into checking a hypothesis.
- Note the gauge cold, first thing in the morning, for several days running. A steady overnight reading with drops only after heating cycles points at the vessel and the safety valve.
- Go outside and photograph the discharge pipe. Wet, stained, or dripping is close to conclusive.
- Tissue test every radiator valve, top and bottom, both ends, on every radiator. Look for staining as much as for wet.
- Check the airing cupboard and under the boiler for crusty white deposits, which are the fingerprint of a slow weep that keeps drying out.
- Turn the radiator valves off at both ends of one radiator at a time for a day if you have a suspect, and see whether the drop stops. Crude, but it isolates a leg.
- Write down the dates and pressures. Four dated readings tell an engineer more than any description, and almost nobody has them.
Where Glasgow Pipework Tends to Give Up
The pipework matters here because so much of the city's heating was retrofitted into buildings that never had any, and retrofits leave their own signature. In a converted flat the heating pipes were threaded through joists, along skirtings and under floors that were already a hundred years old, often with limited access, and the compromises made then are where the weeps show up thirty years later.
Microbore is the specific one to know about. A lot of Glasgow flats fitted or upgraded from the late seventies onward run eight or ten millimetre soft copper in long single runs from a manifold, and it was a sensible choice at the time. It is thin walled, it work hardens where it has been bent tight, and it is unforgiving of a joist notch that lets it rub. When microbore fails it tends to be a pinhole at a bend rather than a burst, so it loses pressure slowly and quietly and it is hard to find without isolating sections.
The other pattern is the cold hallway. Glasgow flats often have a long internal hall with a single radiator on it, and the valve at the far end of a run that is only occasionally hot goes through more expansion and contraction cycles than the rest. Gland weeps turn up there disproportionately in our experience, and it is the radiator people never think to check because that room is never the one that feels cold.
Finally, worth naming because it produces a false alarm every winter: on an upper floor flat the safety valve discharge pipe sometimes terminates high on a gable or into a hopper rather than at ground level where you can see it. If you cannot find yours, that does not mean the vessel is fine. It means you cannot see the evidence from your own window, and it is a reasonable thing to say when you call.
When to Call Someone Out
Worth calling once the rate is faster than a few tenths of a bar a month, or as soon as the discharge pipe outside shows wet, because that is a vessel or a safety valve and neither improves. Also call rather than continuing if you have topped up more than three or four times, since each top up brings fresh oxygenated mains water into a sealed system and that is what drives internal corrosion and sludge over time. Muhammad does most of the pressure testing on these and will isolate the system in sections to find which leg is losing before anything gets lifted. Bring your dated readings to the conversation, they shorten it.
Boiler Repair in GlasgowCommon Questions
A little, yes, and all sealed systems do it. A tenth of a bar over a few months is ordinary seepage at valve glands and pump seals, and bleeding radiators takes some out deliberately. Once or twice a year of topping up is maintenance rather than a fault. The question is always the rate, not whether it moves at all.
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Read Next
Full guide
What Should the Pressure in a Boiler Be?
On a sealed system, which is nearly every combi and most modern system boilers, aim for 1.0 to 1.5 bar when the heating has been off for a few hours and the system is cold.
How to Repressurise a Boiler
Turn the boiler off and let it cool.
Is Low Boiler Pressure Dangerous?
Low pressure is not dangerous.
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