When a combi boiler loses pressure, the first sign is often a cold radiator or a shower that will not run hot. Look at the boiler and you may find a fault code on the display and a needle sitting well below where it should be. Most modern combis refuse to fire once the pressure drops too far. That is a safety feature doing its job, not a breakdown.
Topping the pressure up takes minutes. Working out why it fell takes more thought, and that is what stops it happening again. This guide covers both, and where the job stops being yours. Related boiler and radiator guides are gathered in the combi boiler and radiator care section.
How a sealed system holds its pressure
A combi boiler almost always sits on a sealed heating system. Think of it as a closed loop of pipe and radiators, completely full of water, with four parts that decide what the pressure gauge shows:
- The water itself. Water barely compresses, so the gauge is really measuring how firmly the loop has been filled.
- The expansion vessel. A steel tank, usually inside the boiler casing, divided in two by a rubber diaphragm. One side holds system water; the other holds air or nitrogen at a set charge. As the water heats and expands, it pushes into the vessel and squeezes that cushion. Without it, the pressure would spike every time the heating came on.
- The pressure relief valve. A safety valve that opens if the pressure gets too high, typically at 3 bar on domestic systems, and sends water out through a copper pipe that usually ends on an outside wall.
- The filling loop. The only intended way for water to get in. It connects the mains cold supply to the heating circuit through one or two small valves.
Hold that picture in your head and most pressure problems make sense. Water can only leave through a leak, through the relief valve, or through a bleed point. Pressure can only rise because the water is hot, or because the filling loop is letting water in.
Reading the gauge
Most combis have either a round dial gauge on the front or underside, or a digital pressure reading on the display. Dial gauges are often marked with green and red zones, and some have a second, adjustable needle that the installer sets to show the correct cold pressure.
Read it when the system is cold, ideally first thing in the morning before the heating comes on. For most combi boilers the normal cold reading is between 1 and 1.5 bar. Your boiler's manual is the authority, though, and a few systems are set slightly differently, for instance where radiators sit on a floor well above the boiler.
When the heating runs, the needle should rise, because the water has expanded. A hot reading somewhere around 1.5 to 2 bar is typical. What matters is the pattern over time:
- Cold reading steady from week to week: healthy.
- Cold reading slowly falling: water is leaving the system.
- Hot reading climbing close to 3 bar: the expansion vessel is probably not doing its job.
- Cold reading rising on its own: water is getting in, usually through a filling loop valve that has not fully closed.
Keep a note of the cold reading for a week or two. A date and a number taped beside the boiler helps an engineer more than any description.
Decision guide: symptom, likely cause, next step
Find the description closest to what you are seeing.
The pressure dropped right after you bled the radiators
Likely cause: when air escapes through a bleed valve, the space it occupied is not refilled, so the loop is slightly less full than before.
What to do: top up through the filling loop as described further down. If the pressure then holds, there is nothing more to chase.
The pressure falls slowly over days or weeks and you can't see water anywhere
Likely cause: a small leak somewhere out of sight, or the relief valve letting water by.
What to do: first go outside and find the end of the relief valve pipe, usually a short copper pipe turned down towards the ground, close to where the boiler is on the other side of the wall. Look for a drip, a damp patch, or a white tide mark on the brickwork below it. Then check every visible joint and valve indoors by wiping it with dry kitchen paper; even a slow weep shows up as a damp mark. If both checks come up clean, the leak is likely under a floor, behind boxing or inside the boiler. The guide to signs of a hidden water leak lists the clues to look for in ceilings, walls and floors.
The pressure climbs high when the heating is on, then is low again the next morning
Likely cause: the expansion vessel has lost its air charge or its diaphragm has failed. Without the cushion, the expanding water drives the pressure up to the relief valve's limit, the valve opens, water is lost, and the system is short when it cools down.
What to do: this needs a heating engineer. Checking and recharging the vessel means taking the pressure off the boiler's water side, and replacing it means working inside the casing.
Water is dripping from a pipe on the outside wall
Likely cause: the relief valve is discharging, either because the pressure really is too high, or because the valve has opened once and a speck of dirt on its seat now stops it closing fully.
What to do: check that the filling loop is fully closed. Never cap, block or crimp the discharge pipe. Book an engineer to find out whether the vessel or the valve is at fault.
There is a damp patch, a stained ceiling or a drip at a radiator valve
Likely cause: a weeping valve gland, a loose union nut, or a joint failing under a floor.
What to do: a weep from the spindle of a radiator valve can sometimes be stopped with a gentle quarter turn of the small gland nut just below the head. A union nut can sometimes be nipped up a fraction. Don't overtighten brass fittings, which crack. For anything behind a ceiling or under a floor, a technician with the right instruments can usually find it without lifting large areas, and electronic leak detection explains how that works.
The pressure only falls while the heating runs, and there is a faint hiss from the boiler
Likely cause: the automatic air vent, a small valve on top of the pump inside the boiler, letting out droplets or vapour along with air. Small amounts can evaporate inside the casing without ever leaving a visible puddle.
What to do: this is inside the boiler. Leave it to an engineer.
The gauge reads normal but the boiler still shows a low pressure fault
Likely cause: a faulty pressure sensor or switch.
What to do: call an engineer, and don't add water to a system that is already at the right pressure.
The needle drops within hours and you can hear running water
Likely cause: a significant leak.
What to do: turn the boiler off. Look for the water and, if it is coming from one radiator, close both of that radiator's valves. If water is near light fittings, sockets or the consumer unit, switch the electricity off at the consumer unit only if you can reach it without touching anything wet. Then get a plumber or heating engineer out.
Repressurising through the filling loop
The details vary between boilers, so read the manual first. Some combis have a built-in filling key or lever underneath; others use an external braided hose with a small valve at each end. The general sequence is:
- Turn the heating off and let the system cool. A reading taken on a hot system will mislead you.
- Find the filling loop. It is usually directly under the boiler, near the gauge. If it is a removable hose, connect both ends hand-tight.
- Note the current reading and decide on your target, usually somewhere between 1 and 1.5 bar.
- Open one valve fully. Then open the second valve slowly. You should hear water hissing into the system and see the needle begin to rise.
- Watch the gauge the whole time. Close the second valve as soon as the needle reaches your target.
- Close the first valve. Make sure both are fully shut, not just nearly shut.
- Wait a couple of minutes with your eye on the needle. If it keeps creeping up, one of the valves is still passing.
- If the loop is meant to be removed, disconnect it with a towel underneath to catch the drips. In many places, water rules expect a temporary filling connection to be disconnected after use to protect the drinking water supply; your manual or local regulations will say.
- Reset the boiler if its display shows a fault, following the manual, and turn the heating back on.
If you overshoot, bleed a radiator slowly into a cloth until the gauge drops back. It takes very little water to move the needle.
What not to do: don't leave the filling loop open, don't block the relief valve pipe, don't take the boiler casing off, and don't pour a leak-sealer additive in without asking an engineer whether it suits your boiler. Above all, don't treat regular topping up as maintenance. It is a symptom.
Why a slow loss is worth chasing
It is tempting to top the pressure up every few weeks and carry on. The trouble is what each top-up brings with it. Fresh mains water carries dissolved oxygen, which feeds corrosion inside steel radiators, and in hard water areas it brings minerals that scale up the boiler's heat exchanger. Every top-up also dilutes the inhibitor that protects the system. Over time, a system that is constantly refilled tends to fill with black sludge, which is the subject of the guide to radiator flushing.
The leak itself can do more harm than the boiler fault. Water dripping slowly into a ceiling or floor void can soften plasterboard and rot timber long before a stain shows, and a falling gauge is often the earliest warning.
When to stop and call an engineer
Book a qualified heating engineer if:
- you are topping up more than occasionally and can't find a visible leak;
- the relief valve pipe outside drips, or shows signs of having discharged;
- the pressure climbs towards 3 bar when the heating is on;
- the cause appears to be anything inside the boiler casing;
- the boiler shows a pressure fault while the gauge reads normal.
Anyone who opens up a gas boiler must be qualified and registered under whatever scheme applies in your country, and the rules differ from place to place. If at any point you smell gas, don't operate light switches or appliances, open the windows, leave the property and call your local gas emergency number.
Selin Aydemir
Selin writes about central heating – combi boilers, radiators, underfloor loops and the small habits that keep a sealed system running quietly. She likes explaining why a fault happens before saying what to do about it.
General guidance only. Gas, electrical and structural work must be done by a suitably qualified person, and rules differ from country to country.
