A combi boiler loses pressure for one of two reasons: water is leaving the sealed system (a leak, often hidden in the heating pipework) or a component is misbehaving (pressure relief valve, expansion vessel). Repressurise to 1 to 1.5 bar using the filling loop. If it keeps dropping, the water is going somewhere, and it needs finding.
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A combi is a sealed loop. The same water goes round and round, so the pressure gauge should sit steady week after week. When it keeps sliding towards zero, that water is escaping somewhere, and because a combi has no tank topping things up automatically, the gauge is the first place the truth shows. This guide covers the combi-specific causes, how to repressurise safely, and how to read the speed of the drop.
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How a combi holds its pressure
Unlike an older heat-only system with a feed tank in the loft, a combi runs as a pressurised sealed circuit: boiler, radiators and the pipework connecting them. It is filled once through the filling loop, pressurised to around 1 to 1.5 bar cold, and then closed off. Manufacturers including Vaillant and Viessmann quote that same 1 to 1.5 bar range as the healthy resting zone, with a modest rise when the heating is running.
Because the loop is sealed, pressure only falls when water leaves it. That is worth holding on to: a dropping gauge is not a quirk, it is a measurement of water going missing.
The usual causes, most likely first
| Cause | The tell |
|---|---|
| Recently bled radiators | You let air out, so the pressure dropped. One repressurise fixes it and it holds afterwards. Entirely normal. |
| A leak in the heating circuit | The most common cause of repeated pressure loss. Often invisible: a weeping joint under a floor, a corroded pipe in a wall, a pinhole under the kitchen units. The gauge keeps falling however often you top up. |
| Pressure relief valve letting by | The safety valve discharges outside through a small copper pipe. If that discharge pipe drips, the valve is passing water out of the system. |
| Expansion vessel failure | Pressure swings high when hot, then the PRV dumps the excess and the system reads low when cold. Our guide to expansion vessel problems covers this one. |
| A fault inside the boiler | Heat exchangers and internal joints can leak too. One for a Gas Safe engineer. |
Repressurising safely, step by step
The process below matches the manufacturers' own guidance (Vaillant, Viessmann and British Gas publish the same steps):
- Switch the boiler off and let it cool.
- Find the filling loop. Usually a flexible braided hose beneath the boiler with a small valve at each end. Check both ends are attached properly.
- Open the valves slowly. You will hear water entering the system. Watch the gauge.
- Stop between 1 and 1.5 bar. Close both valves fully.
- Switch the boiler back on and, if your model needs it, reset the fault code.
What the speed of the drop tells you
- Holds for months: healthy system. Bleeding radiators or seasonal temperature swings explain the occasional nudge.
- Drops over a few weeks: a small weep or a tired component. Worth investigating before winter, while it is still small.
- Drops overnight or daily: water is leaving the system quickly. That is a real leak or a failing valve, and it will not fix itself. This is the pattern we covered in depth in boiler keeps losing pressure.
When the leak is hidden
Here is the frustrating part of combi pressure loss: in most homes the heating pipework runs under floors and through walls, and a leak of warm water onto a warm surface evaporates before it ever shows as a damp patch. People spend months topping up daily with not a drop of water to be seen anywhere.
That is exactly the job specialist detection exists for. Tracer gas introduced into the heating circuit surfaces through floors precisely above the leak; acoustic equipment hears the escape; thermal imaging maps the hot water where it should not be. The leak gets pinpointed to a small area first, so one neat opening is made in the right place instead of floors coming up on guesswork. It is the difference between surgery and exploratory digging.
If your combi needs topping up daily and your engineer has ruled the boiler itself healthy, our central heating leak detection service finds the escape point without wrecking the house to do it.
Frequently asked questions
What pressure should a combi boiler sit at?
Around 1 to 1.5 bar cold. It rises a little as the heating warms, which is normal.
Is it safe to keep topping the pressure up?
Occasionally, yes. Regularly, no. The water you keep adding is going somewhere, and constant fresh water accelerates corrosion and sludge inside the system.
How quickly should a combi lose pressure before I worry?
A healthy system holds for months. A top-up every few weeks deserves a look. Overnight or daily loss means a live escape that needs finding.
My combi loses pressure but there is no visible water. Why?
Hidden pipework plus evaporation. Most heating leaks under floors never show a wet patch, which is why detection uses tracer gas, acoustics and thermal imaging rather than eyes alone.
Could it be the boiler itself rather than a leak?
Yes: the pressure relief valve, expansion vessel or an internal component. A Gas Safe engineer checks the boiler; if it tests healthy, the loss is in the pipework.
Topping up every morning?
DCI Leak Detection pinpoints hidden central heating leaks across Cornwall and Devon with tracer gas, acoustics and thermal imaging, so the repair is one small opening in the right place. No find, no fee on residential work, subject to terms.
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