Why Your Boiler Keeps Losing Pressure
If you have been topping up your boiler with the filling loop every few weeks, you have been treating a symptom for months and possibly years.
A sealed heating system holds a fixed volume of water. It is a closed loop. Under normal conditions the pressure stays where it was set, indefinitely, without anyone touching it. If it keeps falling, water is leaving the system, and it is going somewhere.
This guide covers where.
- Gas Safe registered 981362
- Written from jobs we have actually done
- Milton Keynes and 20 miles around
- 8am – 8pm, Mon to Sat
Last updated
First, what normal looks like
Most sealed systems sit at roughly 1 to 1.5 bar when cold, rising to somewhere around 2 to 2.5 bar when the heating is running and the water has expanded. Your boiler’s own documentation gives the figures for your appliance and they vary.
- A small rise when hot and a fall back when cold is normal. That is water expanding and contracting, and it is what the expansion vessel exists to accommodate.
- A slow fall over many months is not unusual and is worth watching rather than panicking about.
- A fall you have to correct every few weeks is a fault.
The three causes, in order of likelihood
One: the expansion vessel has failed
The most common cause, and the one most often missed because there is nothing to see.
Water expands as it heats. In a sealed system that expansion has nowhere to go, so an expansion vessel absorbs it. Inside the vessel is a rubber diaphragm with a charge of air behind it, and the air compresses to make room for the expanding water.
Over years the vessel loses its air charge, or the diaphragm perishes. When that happens there is nowhere for the expansion to go, so pressure climbs sharply when the heating runs, and the pressure relief valve opens to dump the excess. That water discharges to a small pipe outside, usually copper or plastic, terminating on an external wall.
The tell: pressure swinging widely between cold and hot, and a discharge pipe outside that drips or shows staining and limescale below it. Go and look at yours. A lot of people have a pipe quietly dribbling on a path and have never noticed.
Usually repairable by recharging the vessel, or replaceable if the diaphragm has gone.
Two: there is a leak on the system
Somewhere in the pipework, the radiators or the boiler itself.
Heating water is not fresh water. It has been in the system for years, it is oxygen depleted and it is usually dark with magnetite. So a heating leak leaves a dark, rusty stain rather than a clean pale one, which is a useful way to distinguish it from a mains water leak.
Where they hide:
- Radiator valve connections, particularly thermostatic valves that have been operated a lot
- Towel rail connections in bathrooms, where the leak evaporates before it pools
- Under floors, which is the expensive one and the reason tracing properly matters
- The boiler itself, internally, which may not be visible without removing the case
If you can see a stain, photograph it. If you cannot, that does not mean there is no leak.
Three: the pressure relief valve is passing
Once a relief valve has opened and passed debris across its seat, it may not close fully afterwards. Then it weeps continuously, and the system slowly loses pressure through it.
Same tell as a failed expansion vessel: a wet or stained discharge pipe outside. The distinction between the two is a job for an engineer, and the fix is different.
Why topping it up is making things worse
This is the part that matters and almost nobody is told it.
You introduce fresh, oxygenated mains water into the system
Oxygen is what corrodes steel radiators from the inside. That corrosion produces magnetite, the black sludge that settles in radiator bottoms, blocks heat exchangers and wears out pump bearings. A system that gets topped up monthly is being fed a fresh dose of oxygen and dissolved minerals every month.
So a pressure loss you have been managing for two years has probably also given you a sludge problem, and in a hard water area it has given you a scale problem too.
That is why the honest advice is to find the cause rather than keep the loop handy.
What you can check yourself
- Look at the pressure gauge cold and again after the heating has been running for half an hour. Note both. A wide swing points at the expansion vessel.
- Go outside and find the pressure relief discharge pipe. Usually a small pipe through an external wall near the boiler. Look for dripping, staining or limescale below it.
- Check around every radiator valve and behind towel rails for dark staining.
- Check the filling loop itself is fully closed. A loop left slightly open causes the opposite problem, pressure that creeps up.
- Note how often you top up. Weekly, monthly, twice a year. That single number tells an engineer a great deal.
When it is urgent
- Water visible around the boiler rather than just low pressure.
- Pressure dropping to zero repeatedly since most boilers lock out below a threshold and running dry is not good for a pump.
- Any staining on a ceiling below where heating pipework runs.
Otherwise it is not an emergency and it is not something to leave for another winter.
Frequently asked questions
How often is it normal to top up a boiler?
Ideally never, or perhaps once a year after work has been done on the system. Every few weeks is a fault.
Can I keep topping it up?
You can, and each time you do you introduce fresh oxygenated water that accelerates corrosion inside the system. It is managing a symptom while worsening the cause.
What pressure should my boiler be at?
Typically around 1 to 1.5 bar cold, though it varies by appliance. Your boiler’s documentation gives the correct figure.
Water is dripping from a pipe outside near my boiler. Is that normal?
No. That is almost certainly the pressure relief valve discharging, which usually means a failed expansion vessel or a valve that is not seating. Worth looking at.
Can a leak be under the floor?
Yes, and that is why tracing properly matters. Lifting the wrong floor on a guess is expensive.
Will a new boiler fix it?
Not if the cause is a leak in the system or a failed vessel, both of which are outside the appliance. Worth diagnosing before spending.
What to do about it
Get it diagnosed
Call 07514 312117 or send a WhatsApp with a photo of your pressure gauge and, if you can find it, the discharge pipe outside. Gas Safe registered 981362, CIPHE member, fully insured, covering Milton Keynes and a 20 mile radius.