The short version
- A sealed system that loses pressure has a leak, an expansion vessel fault or a discharging valve
- Repeated refilling accelerates internal corrosion and sludge
- Some leaks are outside the house and easy to miss
- Pressure that only drops when the heating runs points at a different cause than a constant slow loss
The four causes, in order of likelihood
Almost every case we attend falls into one of these. Knowing which pattern you have narrows it before an engineer arrives.
- A visible leak on a radiator valve, towel rail or pipe joint, usually leaving a small stain or a crusty deposit
- A hidden leak under floors or in a screed, often showing as an unexplained warm patch
- A failed expansion vessel, which shows as pressure that climbs high when hot and drops below normal when cold
- A discharging pressure relief valve, which sends water out of a small pipe through the outside wall
The outside pipe nobody looks at
There is a small copper pipe running through the wall near the boiler. If it drips or shows limescale streaking down the brickwork, the pressure relief valve is passing water and that alone explains a steady loss. It is easy to miss because the evidence is outdoors and dries between checks.
How the pattern tells you the cause
Note when the pressure falls. A slow, steady drop whether the heating runs or not points at a leak. A drop that only appears after the system has been hot and cooled again points at the expansion vessel or the relief valve. A sudden drop after work has been done on a radiator points at the joint that was disturbed.
Why topping up is not a fix
Fresh mains water carries dissolved oxygen and minerals. Every refill introduces more of both, which speeds up corrosion inside radiators and leaves the black sludge that eventually blocks heat exchangers and kills pumps. A system topped up weekly for a year has effectively been refilled several times over, and the damage shows up later as cold spots and circulation faults.
- Corrosion products build up as magnetite sludge
- Inhibitor is diluted with every refill until it stops protecting anything
- Circulation faults and overheat lockouts follow
What we do to find it
We check the gauge behaviour hot and cold, test the expansion vessel charge, inspect the relief valve discharge, and where nothing is visible we use leak detection rather than lifting floors speculatively. Once it is found, the repair is usually far smaller than people expect.
Where the line is
Never remove the boiler casing, adjust gas controls or work on any gas component yourself. Those are Gas Safe engineer jobs by law. If you smell gas, call the National Gas Emergency Service on 0800 111 999 and turn the gas off at the meter.
Common questions
How often should I need to top up a combi?+
Once or twice a year at most, and often not even that. Anything more frequent means something needs finding.
Can low pressure damage the boiler?+
The boiler will normally lock out rather than damage itself, but running with low pressure and repeatedly refilling does damage the system through corrosion.
Is an expansion vessel repairable?+
Often it can be recharged, and where the internal membrane has failed it is replaced. Either way it is a straightforward job.
What pressure should the gauge show?+
Most systems sit around one to one and a half bar when cold, and the correct band is marked on the gauge itself. Check yours rather than assuming.
Could a leak be under the floor?+
Yes, particularly in properties with pipework in screed. That is where proper leak detection saves money against guesswork.
Tired of topping up the boiler?
We will find where the water is going instead of refilling it again. Tell us how often you are topping up and what the gauge does when hot.