7 minute read
Boiler losing pressure but no visible leak? Where the water goes
A sealed heating system is a closed loop. If the gauge keeps falling, water is leaving it, even when every floor and ceiling is dry. It is going to one of five places, and you can rule out two of them yourself in ten minutes.
First, make sure it really is losing water
Two things lower the gauge without anything being wrong. Bleeding radiators lets air out, and the pressure falls with it, so a top-up after bleeding is normal. And the reading always drops a little as the water cools, so compare cold with cold: read the gauge first thing in the morning before the heating comes on, top it up to between 1 and 1.5 bar, and read it again at the same time the next day.
If it has dropped noticeably, and you have not bled anything, the system is losing water. Topping up once or twice a year is normal. Every few weeks is not, and every few days means water is going somewhere quite quickly.
1. Out through the pressure relief valve pipe
This is the one people miss, because the water never comes inside. The pressure relief valve protects the boiler by letting water out if the pressure climbs too high, through a small pipe, usually 15 mm copper, that goes through the wall. Outside, it normally ends turned back towards the wall, often low down, although on some houses it comes out high up near the gutter.
Do not confuse it with the condensate pipe. Modern condensing boilers also have a wider plastic pipe, usually grey or white, that carries away acidic water produced while the boiler burns gas. That pipe is meant to drip or trickle when the boiler is firing, and it tells you nothing about pressure loss.
Go outside and find it. Look under its end for a drip, a wet wall, a green or white mark on the brickwork, or a damp patch on the path. Check again with the heating running. If water comes out, the valve is either faulty or doing its job because something else is pushing the pressure up.
That is exactly what happened in Bracknell. The homeowner was topping up constantly, with no damp anywhere in the house. The engineer found water escaping at the relief valve outlet, proved it with tracer gas, and then showed the rest of the pipework held pressure for 45 minutes at 2 bar. Read the Bracknell case.
2. A failed expansion vessel
The expansion vessel takes up the extra volume as the water heats. When its membrane fails or it loses its air charge, the pressure climbs sharply every time the heating comes on, the relief valve opens, and when the system cools the gauge sits lower than before. The water leaves through the same outside pipe, so this often looks like point 1.
Watch the gauge from cold to hot. Some rise is normal, typically up to about half a bar, and your boiler manual gives the working range. A jump towards 3 bar, with the relief valve pipe dripping, points at the vessel. On a combi boiler the vessel is inside the casing, so recharging or replacing it is a job for a Gas Safe registered engineer; on other systems it is a job for a heating engineer. Either way, it saves you looking for a leak that is not there.
3. The filling loop
The filling loop is the braided hose or small lever under the boiler that lets mains water in. If it has not been closed fully, it can creep the pressure up until the relief valve lets water out, which can make the pressure look unstable rather than simply falling. Check both valves are firmly shut.
4. Radiators, valves and towel rails
Radiator valves can weep so slowly that the water evaporates before it marks the floor. Take the head off each thermostatic valve and feel for moisture around the pin. Look for rust or a white crust at every joint, rust along the bottom seam of older radiators, and a stain on the carpet or floor under each one.
ADI's own advice is a sheet of blue paper towel under the boiler, left for an hour with the heating on and then for a few days. Drips from a boiler fitting show up clearly on it.
5. Pipes in the floor, walls or ceilings
If none of the above explains it, the leak is probably on pipework you cannot see. In houses where the downstairs heating pipes are set into a concrete floor, the screed can soak up a slow leak for months. Signs to look for:
- A warm patch on a tiled or stone floor when the heating is on
- Laminate or wooden flooring lifting, or a door sticking on a swollen floor
- A skirting board coming away from the wall, or damp low on a wall
- A brown ring on a ceiling under a bedroom or bathroom
- A musty smell downstairs with no bathroom nearby
In Market Deeping the boiler kept going to zero after several engineer visits found nothing. An air pressure test proved a small leak, a damp meter found the spot in the hallway, and the pipe was inside a plastic sleeve that had stopped tracer gas reaching the surface. Read the Market Deeping case.
What the gauge is telling you
| What you see | Most likely | What to do |
|---|---|---|
| Drops after you bled the radiators, then holds | Air let out; no sign of a leak | Top up to 1 to 1.5 bar cold |
| Climbs towards 3 bar when hot, relief pipe drips | Expansion vessel or filling loop | Close the filling loop, then call a heating engineer |
| Falls steadily, relief pipe drips when cold too | Faulty relief valve | Needs replacing, and the rest of the system testing |
| Falls steadily, relief pipe dry, house dry | Hidden leak in floor, wall or ceiling | Leak detection survey |
| Falls to zero within a day | A significant leak, or a relief valve stuck open | Check the relief valve pipe, turn the boiler off and call |
What not to do
Do not pour in leak sealer to buy time. It can seal a tiny weep for a while, but it also makes tracer gas and pressure testing much less reliable, and it can cause problems in the boiler itself. Why leak sealer makes things harder.
When to call
Call when the relief valve pipe is dry, the radiators and valves are dry, and the pressure still falls. Tell the office how often you top up and what you have already checked. It goes straight into the quote and the engineer's plan for the day.
Quick answers
Why does my boiler lose pressure overnight?
Water shrinks slightly as it cools, so the gauge always reads lower in the morning than it did with the heating on. That drop is normal. The test is to compare cold with cold: if the reading before the heating comes on is lower each morning than the morning before, the system is losing water.
Can a boiler lose pressure without a leak?
Yes. Bleeding radiators lets air out, and the pressure falls with it. A failed expansion vessel makes the pressure swing so high when the heating is on that the relief valve lets water go. Neither is a leak in the pipework, although in the second case water does leave the system, through the relief valve pipe outside.
Where does the water go if there is no damp?
Two common places: out through the pressure relief valve pipe to the outside, or into a concrete floor that soaks it up before it can show. The first is a quick check you can do yourself. The second needs thermal imaging, tracer gas and pressure testing to find.