Tell the three apart before you call anyone
Every falling gauge gets treated as a leak, and roughly a third of them are not. The three causes produce different patterns on the gauge, and the pattern is free to read.
| What the gauge does | What is almost certainly wrong |
|---|
| Falls steadily whether the heating is on, off, hot or cold | A genuine leak. Water is leaving the system. |
| Climbs hard when the heating runs, then sits below where it started once cold | Expansion vessel. The air charge has gone, so there is nowhere for the hot water to expand into. |
| Falls only after the boiler has been running, and the pipe outside is wet | Pressure relief valve. It is lifting, or passing on debris, and dumping to the drain. |
| Fell once after radiators were bled, then held | Nothing. That is the air you let out. |
The reading that settles it
- Read the gauge cold, first thing, with the heating off overnight. Write the number down. Ideal Heating puts the correct cold reading between 1 and 1.5 bar.
- Run the heating for a full hour with every radiator open, and read it again at full temperature. A rise of a few tenths is normal expansion; Ideal describes the hot reading climbing to around 2 bar.
- Read it cold again the next morning. It should come back to within a hair of your first number.
Now interpret it. If the hot reading barely moved from the cold one, the vessel is doing its job and you are looking for a leak. If the hot reading shot up near the top of the dial and the next cold reading is lower than the first, the vessel has failed and it has probably pushed water out through the relief valve on the way. If both cold readings are lower than they should be and the hot reading was unremarkable, water is escaping somewhere that does not care about temperature.
Frequency matters as much as the numbers. Worcester Bosch say a system will usually need topping up once or twice a year, and that anything much more frequent is a job for your installer. Weekly topping up is not a maintenance habit, it is a symptom.
Cause one: the system is genuinely losing water
A sealed heating system is closed. Water that leaves it has gone somewhere, and on the visible half of the system there are only so many candidates: radiator valve spindles, the nut where the tail meets the pipe, towel rails, the pump head, and the underside of any radiator carrying a rust bloom. Look along the skirting under each radiator with a torch held low, and run a dry tissue round each valve rather than a finger.
If all of that is dry, the loss is under a floor, inside a wall, or inside the boiler case. A pinhole in a 15mm pipe buried in screed can empty enough to drop the gauge every week and still produce no visible damp for months, because the screed absorbs it and the heating dries it back out. The first outward sign is usually a warm patch underfoot, a lifting board, or a stain on a ceiling below.
Ideal also make the point that matters for anyone topping up regularly: every refill dilutes the corrosion inhibitor in the system. Fresh mains water is oxygenated, and oxygen in a closed circuit makes more pinholes. Weekly topping up feeds the fault that caused it.
Cause two: the expansion vessel
The vessel is a sealed chamber split by a rubber diaphragm, with nitrogen or air on one side and system water on the other. Vaillant describe it as the component that absorbs the pressure rise when the boiler heats the water. When the charge escapes or the diaphragm perforates, the water has nothing to compress against, so pressure spikes when the system heats and collapses when it cools.
Two stages are worth separating. Early on, the vessel swing is all you get: a wide gap between cold and hot readings and nothing actually leaving the system. Once the hot peak passes the top of the safe band, the relief valve lifts to protect the boiler and water does start leaving, out through the discharge pipe. At that point you have a real loss, but the cause is still the vessel. Replacing a length of pipework would not fix it.
Vaillant put vessel life at five to ten years, and note that hard water shortens it by attacking the diaphragm. In most of London that is the shorter end of the range. Ideal describe the failure plainly: if the rubber bursts or tears, water fills the air gap and the pressure drops.
Cause three: the pressure relief valve
The relief valve is the boiler's safety device. Vaillant describe it as the valve that lets pressurised water out safely, through a pipe that runs from the boiler to the outside wall or down towards the ground. Ideal say the same thing from the other direction: that outlet pipe should normally be dry.
Two things make it wet. Either something upstream is genuinely over-pressurising the system, which usually means the vessel or a filling loop left open, or the valve itself is passing. Baxi name debris inside the valve as a cause, and say a Gas Safe registered engineer will need to clean it out or replace it.
The check takes one minute. Find the small copper or plastic pipe emerging from the wall near the boiler and put your hand under it after the heating has run. Damp means the valve has been open. Limescale staining on the brickwork below it means it has been open for a long time.
Things you can fix yourself, and should
- A filling loop left open. Both valves closed, and the loop detached if yours comes off. An open loop lets mains pressure creep in and can look like a vessel fault.
- Air after bleeding. One drop after bleeding radiators is expected. Top up and forget it.
- A weeping radiator valve nut. A quarter turn on the gland nut often stops it. If it does not, that is a radiator valve job, not a detection job.
Anything inside the boiler case is not a DIY item. Under regulation 3 of the Gas Safety (Installation and Use) Regulations 1998, work on a gas fitting has to be done by someone competent and, in practice, registered. Recharging or replacing a vessel and swapping a relief valve both sit on that side of the line.
When this is not our job
If the reading and the outside pipe both point at the vessel or the relief valve, you want a Gas Safe heating engineer with the parts, not a leak detection visit. Say so when you call anyone, including us, and we will tell you the same. We are the right call when the pattern says water is leaving the system and nobody can find where.
What a detection visit actually does
Guessing at a hidden heating leak means lifting floor until you find water. The alternative is a sequence. The circuit is isolated and pressure-tested, which confirms the loss is real and sizes it. The pipe runs are followed with thermal imaging, which reads the warm line of a flow pipe through screed and tile and shows where the pattern goes wrong. Where thermal cannot see, the circuit is drained and tracer gas is pushed through, then picked up at the surface. The floor is opened at one point, once, after the leak has been located. More detail on the repair side is on the boiler and heating leak repair page, and the rest of this cluster sits under boiler and heating pressure.
Related reading if your gauge is falling and everything looks dry: boiler losing pressure and boiler pressure keeps dropping with no visible leak.
What it costs
Detection is charged at £150 per hour, agreed before we attend. If we do not find the leak you do not pay for the time we spent looking. Because we repair the pipe and reinstate the floor as well, the trace does not end with a report and a hole.
