Read the whole cycle, not the low reading
People notice the low number and describe the fault as losing pressure. The diagnostic information is in what happened before it.
Warm water occupies more space than cold water, so pressure in a sealed system always rises when the heating runs. Ideal describe the normal picture: 1 to 1.5 bar cold, rising to around 2 bar hot. Vaillant give the safe operating window as 0.7 to 2.5 bar. A gentle rise and a return to the same cold figure is a system working correctly.
What you have is different. The needle climbs a long way, the boiler may lock out at the top, and when everything cools the reading is below where it began. Water left the system during the hot part of the cycle. The question is how, and there are three answers.
Answer one: the expansion vessel has lost its charge
This is the common one. Vaillant describe the vessel as a chamber divided by a rubber diaphragm, with gas on one side to absorb the pressure rise as the boiler heats the water. Ideal describe the failure: if the rubber bursts or tears, water fills the air gap.
Once the air pocket has gone or shrunk, expanding water has nothing to compress. The same amount of heat now produces a much larger pressure rise, and the peak overshoots the top of the safe band. The relief valve then does its job and lets the excess out, which is why the cold reading afterwards is lower than the cold reading before. Nothing is leaking. The system is being vented by its own safety device, every cycle, and topping it up guarantees a bigger overshoot next time.
Vaillant put vessel life at five to ten years and name hard water as a cause of diaphragm failure, which is relevant across most of London. A vessel can sometimes be recharged with air rather than replaced, but Baxi and Ideal both place the work with a Gas Safe registered engineer, and regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 is the reason.
Answer two: the relief valve is passing on its own account
A relief valve that has picked up debris will weep below its opening pressure. Baxi name exactly this, and say the valve needs cleaning out or replacing. The difference from a vessel fault is the shape of the rise: with a healthy vessel and a dirty valve, the hot peak is normal and the system still ends up short, because the valve has been dribbling the whole time.
Either way the evidence is outside. Vaillant describe the discharge pipe as running from the boiler to the outside wall or down towards the ground; Ideal say it should normally be dry. Check it within a few minutes of the heating running, not the next day.
Answer three: a leak that only opens when the heating is on
Less common, and real. Two mechanisms produce it.
A hairline crack or a tired joint can seal itself when cold and open as the pipework warms and moves. Copper expands measurably along a long run, and a joint that has been taking that movement for twenty years is where it shows.
The other is circuit-specific. If part of your system is only pressurised or only flowing when the pump runs, a leak on that branch appears to switch on with the heating. Systems with zone valves, an old gravity circuit, or a towel rail on its own loop can all behave this way.
Both cases produce a loss with no matching overshoot at the top of the cycle, which is what separates them from the vessel.
The test, in one heating cycle
- Cold reading with the system off and cool. Write it down.
- Run the heating for an hour, every radiator open, and read the peak.
- Check the discharge pipe outside while the system is still hot.
- Let it cool fully and read again.
| Pattern | Conclusion |
|---|
| Large rise, wet pipe outside, low afterwards | Expansion vessel. The valve is only doing what it is there for. |
| Normal rise, wet pipe outside, low afterwards | Relief valve passing. Clean or replace. |
| Normal rise, dry pipe outside, low afterwards | A leak that opens when hot. Detection job. |
| Large rise, dry pipe, and the same reading when cold | Vessel weakening but not yet venting. Worth pre-empting. |
What not to do meanwhile
- Do not top up to the top of the green band. If the vessel has failed, a higher cold fill produces a higher hot peak and more venting.
- Do not bleed radiators to bring the pressure down. It works for one cycle and adds air to the system. Bleeding is the right fix only for a system that was simply overfilled, which boiler pressure too high explains how to recognise.
- Do not cap or plug the discharge pipe. It is the safety route on a pressurised appliance.
Who you actually need
On the first two patterns in the table, a Gas Safe heating engineer with a vessel and a valve on the van will fix it in a visit, and a leak detection firm will not. That is worth saying plainly, because the search results for this problem are full of detection ads. Call us when the pipe outside is dry and the rise is normal, which means water is leaving somewhere that nobody can see.
What we do then
Isolate the circuits one at a time and pressure-test each, which finds the branch in the first hour and proves whether the loss is temperature-dependent. Thermal imaging on a running system is at its most useful here, because a leak that only opens when hot is also a leak that is warm when it opens. Where the run is buried and thermal cannot resolve it, the circuit is drained and traced with gas. Then the pipe is repaired and the floor, wall or ceiling made good, which is set out on the boiler and heating leak repair page.
Neighbouring symptoms: boiler losing pressure, boiler pressure keeps dropping with no visible leak, and the rest of the boiler pressure cluster. Work is £150 per hour, agreed before we attend, no find no fee.
