A boiler losing pressure now and then is ordinary. The gauge wants to sit somewhere between 1 and 1.5 bar when the system is cold, and topping it up once or twice a year is nothing to worry about.
Losing pressure every week is a fault. Something is letting water out, and the water is going somewhere. This page covers how to put the pressure back safely, why it keeps dropping, and the point at which topping it up stops being a fix and starts causing damage of its own.
What the gauge should be reading
Cold system, heating off, everything at room temperature: between 1 and 1.5 bar. Most gauges have a green band marking roughly that.
Once the heating fires up, the water expands and the needle climbs. Seeing 2 bar or a little over on a hot system is normal and not a problem. What is not normal is the needle reaching 3, because at that point the pressure relief valve opens and dumps water outside to protect the boiler.
If that happens, the system will be short of water once it cools, and you have found your leak: it is the boiler letting go of water deliberately.
Below about 0.5 bar most boilers will lock out and refuse to fire at all. That is a designed safety response rather than a breakdown.
Putting the pressure back, safely
You need the filling loop. On most systems it is a short braided silver hose underneath the boiler with a small valve at each end.
On a lot of Ideal and Worcester models it is built in instead, operated by a key or a plastic lever hidden under a flap on the underside of the boiler.
If yours is one of those and you cannot work out which part you are supposed to turn, stop and ask rather than forcing anything.
Turn the heating off and let the boiler cool. Filling a hot system gives you a false reading and you will overshoot.
Check both ends of the loop are properly connected.
Open one valve, then open the second slowly. You should hear water moving.
Watch the gauge, not the clock. It moves faster than people expect.
At about 1.2 to 1.5 bar, close both valves fully. Not almost closed. A loop left slightly open keeps feeding the system and will push the pressure past 3 bar in the night, at which point the relief valve dumps it all outside and you are back where you started, or worse.
If it is a detachable loop, disconnect it once you are done.
If you overshoot, you can let a little out by bleeding a radiator briefly with the key and a cloth.
The point where topping up stops helping
Once a month or more often, and you are no longer maintaining a system, you are feeding a leak. That matters more than it sounds.
Every top-up puts fresh mains water into a sealed system. Fresh water carries dissolved oxygen, and oxygen inside a heating system means corrosion.
Corrosion means magnetite, the black sludge that blocks radiators, wrecks pumps and blocks up heat exchangers. So a boiler topped up weekly for a year is quietly being damaged by the fix.
If you are refilling it more than a couple of times a year, the leak is the thing to sort out, not the pressure.
Why your boiler keeps losing pressure
A leak somewhere on the system
The most common answer, and often the least visible. Radiator valve spindles and the joints under them weep slowly.
Pipework under floorboards or buried in a screed can drip for months into a joist or a concrete slab and never show itself on a carpet.
A leak that loses a boiler a bar of pressure over a fortnight can be an amount of water you would struggle to see.
The expansion vessel has lost its charge
This is the classic, and it has a signature you can spot yourself. The pressure climbs high when the heating runs, sometimes tripping the relief valve, and then sits well below 1 bar once everything is cold. Up sharply, down too far, over and over.
The expansion vessel is a sealed chamber with a rubber diaphragm and a cushion of air behind it, and its job is to absorb the expansion of hot water.
When the air charge leaks away, there is nothing to absorb anything, so the pressure swings wildly. Often it can simply be re-charged with a pump and a gauge, which is not an expensive job. Sometimes the diaphragm has failed and the vessel needs replacing.
The pressure relief valve is passing
The relief valve is designed to open at 3 bar and vent outside through a small copper pipe, usually near where the boiler is. If grit or scale stops it seating properly afterwards, it never fully closes and dribbles continuously.
Worth checking, because it is easy: find that pipe outside and look at the end of it. Wet, dripping, or a limescale trail down the brickwork means the valve is passing, and no amount of topping up will hold.
A leak inside the boiler itself
Less common, and the one that matters most. Internal joints, the automatic air vent, or in the worst case a corroded heat exchanger. Sometimes there is a stain or a drip visible under the boiler, sometimes the water simply evaporates off hot metal and you never see a drop.
What you can safely check, and where to stop
Fair game, and genuinely useful before anyone comes out:
Look under every radiator valve, and run a dry tissue around the spindles and joints. A tissue finds weeps that an eye misses.
Check the pipe outside from the relief valve, as above.
Look at the ceiling below any upstairs pipework for staining, and feel along skirtings and carpet edges for damp.
Check the boiler is not sat in its own small puddle.
Note down what the gauge reads cold and what it reads with the heating running for an hour. That pair of numbers tells an engineer a great deal before he has touched anything.
Where to stop. Do not take the boiler casing off: there is nothing behind it you can lawfully or safely work on unless you are Gas Safe registered. Do not force the relief valve, and do not leave the filling loop open to keep pace with a leak.
What happens on a pressure-loss callout
The first question is which side of the system the water is leaving from, because the answer sends the job in completely different directions.
Joe isolates the boiler from the rest of the pipework and watches what the pressure does. If it holds with the boiler isolated, the leak is out in the house and it becomes a leak trace. If it drops with everything shut off, the problem is inside the boiler.
Then the expansion vessel gets checked properly with a gauge rather than guessed at, the relief valve gets looked at, and the visible pipework gets gone over. You get told which of those it is, what it costs to put right, and whether it is worth doing, before anything is started.
Excellent communication. Always provides detailed reasons for chosen repair. Good value.
A vessel re-charge is at the cheap end. A replacement vessel is a modest job. A leak under a floor is honest work and honest time, and anyone quoting that blind over the phone is guessing.
When pressure loss means the boiler is finished
One case is different from the rest. If the leak is a corroded heat exchanger on a boiler that is already fifteen years old, you are looking at the single most expensive part in the appliance, on something near the end of its life anyway.
That is the point where the sensible conversation is not about the repair at all. Joe will tell you plainly if that is where you are, and you can weigh the cost of the part against a new boiler with a warranty on it.
Sometimes the repair still makes sense. Often it does not, and being told so straight is worth more than a cheaper quote.
If your pressure will not hold and you are somewhere around Barnsley, Rotherham, Sheffield, Doncaster or Wakefield, ring 07791 793415 and describe what the gauge is doing. That description alone usually narrows it down.
