Why is my boiler losing pressure? Causes, fixes and when to call an engineer

Almost every combi and system boiler sold in the UK runs on a sealed circuit. Instead of a header tank in the loft topping the system up by gravity, the water in the radiators is held under pressure, and a small gauge on the front or underside of the boiler shows how much. With the heating off and the system cold, the needle should sit somewhere between 1 and 1.5 bar. When the heating has been running for an hour it will climb a little, often to around 2 bar, because water expands as it warms. That rise is normal.

Two readings matter. A cold reading below about 0.8 bar means most boilers will refuse to fire and show a low-pressure fault (E119 on Baxi, F22 on Vaillant, E1 18 on Worcester Bosch, and so on: the letters change, the meaning does not). A hot reading that pushes past 3 bar means the pressure relief valve will open and dump water outside through a small copper pipe, which then shows up as a drop when the system cools. So low pressure and high pressure are often the same story told at different times of day.

If your boiler has a digital display rather than a dial, the pressure is usually shown in the settings or information menu. The manual for your model will say where, and it is worth a two-minute look before assuming anything is broken.

Topping up is something homeowners are expected to do and it is safe if you follow the sequence. Find the filling loop: on most boilers it is a short braided silver hose with a small tap at each end, either under the boiler or built into the casing as a key or lever. Some newer models have an internal filling key that slots into a housing underneath.

1. Switch the boiler off and let it cool for a while so you are reading a cold pressure.
2. Open one tap fully, then open the second one slowly. You will hear water moving.
3. Watch the gauge. Stop at 1.2 to 1.5 bar. Do not chase the top of the green zone.
4. Close both taps firmly. On a keyed loop, remove the key and put it somewhere you will find it again.
5. Turn the boiler back on and reset the fault if it does not clear itself.

If the pressure shoots up quickly, you have overfilled. Bleed a radiator until the gauge comes back down. If it will not rise at all, the loop is not connected properly or an isolation valve is closed somewhere on the mains feed.

The rule of thumb that matters: topping up once or twice a year is fine. Topping up every fortnight is a symptom, and it needs looking at, because every top-up brings fresh oxygenated water into the system and that oxygen rusts the inside of the radiators.

When pressure keeps dropping and there is no puddle anywhere, the water is still going somewhere. A sealed heating system has dozens of compression joints, radiator valves, bleed points and pipe runs, and a weep from one of them that evaporates as fast as it leaks will never show as a wet patch. Over a week it can still take the pressure from 1.5 bar to zero.

The places to check first, with a dry piece of kitchen roll:

• Radiator valves, especially the nut where the valve meets the radiator tail and the spindle under the plastic cap. Slightly rusty streaks or a crusty white deposit give a slow leak away.
• Bleed valves at the top corner of each radiator. A bleed key left a quarter-turn open after the last bleed is a surprisingly common find.
• The boiler itself. Take the cover off the underside if it is designed to come off (many are) and look for staining on the pipework or drips from the pump, the diverter valve or the automatic air vent.
• The pressure relief pipe outside. If the end of the copper pipe on the outside wall is wet or has a green stain below it, the relief valve is passing.

Leaks under floors are harder. A radiator pipe that was chased into a concrete screed thirty years ago can corrode through from the outside, and the first sign is often a warm patch on the floor or a damp smell rather than water. That is the point where guesswork stops and water leak detection with thermal imaging and tracer gas earns its keep, because lifting the wrong floor is expensive.

Inside or beside most boilers is a red or grey drum called the expansion vessel. It is split by a rubber diaphragm: heating water on one side, a cushion of air or nitrogen on the other. When the water heats and expands, the cushion compresses and absorbs it, which is what keeps the gauge from swinging wildly between cold and hot.

Over the years the air charge leaks away through the diaphragm. With no cushion left, the system pressure climbs steeply as soon as the heating runs, the relief valve opens at 3 bar and vents water outside, and when everything cools the pressure has fallen to nothing. The homeowner tops it up, and the cycle repeats the next day. It looks exactly like a leak, and in a sense it is one, but the leak is the relief valve doing its job.

The tell-tale pattern is this: pressure fine when cold, high when hot, low again the next morning, and a damp relief pipe outside. An engineer can confirm it in minutes with a tyre pressure gauge on the vessel's Schrader valve. Recharging the vessel is a short job. If the diaphragm has split, the vessel is replaced, and on some boilers that means an external vessel plumbed in beside the unit because the internal one is buried.

A few things mimic a pressure loss without being one:

• A faulty pressure sensor or gauge. The boiler thinks the pressure is low and locks out, but a second gauge on the filling loop reads normal. Sensors are a modest part and quick to swap.
• A passing filling loop. The opposite problem: mains water creeps into the system through a loop that does not close fully, so the pressure keeps rising. Fit a new loop or replace the double check valve.
• Air in the system after work. New radiators or pipework leave air behind that finds its way to the top of the system over a few days. Bleeding it out lowers the pressure and looks like a loss. Top up once and it should settle.
• A cracked heat exchanger. Rare, and serious. Heating water leaks into the combustion side and either drips from the flue or steams off. If the boiler is old and the condensate seems excessive, this needs a gas engineer, not a top-up.

Call an engineer if any of these apply:

• You are topping up more than once a month and cannot find a wet joint anywhere.
• The relief pipe outside drips or stains, whatever the season.
• The gauge climbs past 2.5 bar every time the heating runs.
• There is a damp patch, a warm spot on a floor or a smell you cannot explain.
• The boiler is showing any fault other than the low-pressure code.

An engineer's visit for a pressure problem normally runs the same way: confirm the cold pressure with an independent gauge, check the vessel charge, inspect every joint on the boiler, walk every radiator, and if nothing shows, pressure-test the system with a sealed gauge overnight. From there it is either a quick repair or a proper leak trace. What it should not be is a quote for a new boiler on the first visit. A pressure fault is one of the cheapest things to put right on a boiler that is otherwise sound, and a good boiler repair engineer will tell you that.

One more thing: an annual service catches most of this before it becomes a lockout. The vessel charge, the relief valve and the joints under the casing are all on the checklist.