Start from the fact that the system is closed
Almost every heating system fitted in London in the last thirty years is sealed. It holds a fixed volume of water at a pressure you top up through a filling loop. Nothing evaporates out of it, nothing is consumed by running it. So when the gauge falls, water has physically left the pipework.
That single fact does most of the diagnostic work. It rules out the explanations people are usually offered — cold weather, an old boiler, "they all do that" — and it turns the question into a much narrower one: which component is letting go, and can you see it?
There are really only six answers, and they need six different fixes:
- Radiator valves — the spindle, the tail nut, or the lockshield
- The radiators themselves — corroded from the inside, usually leaking low down
- Towel rails — disproportionately common, for reasons of where they sit and how they are made
- The circulating pump — a weeping head gasket or shaft seal
- Buried pipework — under floorboards, in screed, behind plaster
- The boiler's own internals — heat exchanger, auto air vent, diverter valve, pressure relief valve
And one case that looks exactly like a leak and is not: a failed expansion vessel. A system with no working vessel pushes water out through the pressure relief valve every time it heats up. You lose water, the gauge drops, and there is no leak anywhere in your pipework. Rule that out before anyone talks about lifting a floor.
Three checks that narrow it, in about ten minutes
1. Read the gauge cold, then hot
Take the reading with the heating off and the system cool. Then run the heating for half an hour and read it again. A normal system rises roughly 0.3 to 0.5 bar as the water expands, and settles back when it cools.
If it climbs to 2.5 bar or into the red and then sits below 1 bar once cold, the vessel is the suspect, not the pipework. If the cold reading is simply lower every few days regardless of use, water is leaving. What the reading should actually be covers the bands by boiler type.
2. Ask whether the loss tracks with the heating being on
This is the most useful question in the whole exercise, and almost nobody asks it.
| Pattern | What it points at |
|---|---|
| Drops only when the heating runs | A joint that opens as the pipe expands, or the expansion vessel. Pipework moves several millimetres when it heats — a marginal joint weeps hot and seals cold. |
| Drops steadily whether the heating runs or not | A genuine hole. Gravity does not care about the timer. Buried pipe, radiator, or valve. |
| Drops overnight in summer with everything off | A leak below the level of the boiler, usually a ground-floor circuit or a pipe under a screed. |
| Only after someone bled radiators | Not a leak. Expected. Top up once and watch it. |
We go into the overnight pattern in detail on pressure that drops when the heating is on.
3. Count how often you top up
Once or twice a year is normal. Every month or two is a weep. Weekly is an active leak. Daily, or it will not hold overnight, means a hole big enough to be doing damage somewhere you have not looked yet. The frequency also tells you how urgent the repair is, which matters more than the exact litres.
Whatever the answer, stop treating the top-up as the fix. Every refill puts fresh oxygenated mains water into the system, and oxygen is what turns a weep into a pinhole and a pinhole into a burst. If you have been repressurising for months, see how to repressurise a boiler, and when not to.