The short answer
If your boiler pressure keeps dropping, the most likely cause is a leak somewhere in the sealed heating circuit, a pipe, radiator, valve or the boiler itself. A failed expansion vessel or recent radiator bleeding can also be to blame. Repeated top-ups only mask the problem; a steady fall usually means a leak that needs tracing.
It is one of the most common heating worries we hear about in Devon homes. You glance at the pressure gauge, see the needle has dropped into the red, top it up, and within days or weeks it has fallen again. The cycle is frustrating, and it usually points to a real fault rather than a quirk of the boiler.
Your central heating is a sealed system. Once it has been filled and pressurised, the same water circulates round and round. In a healthy system that water cannot escape, so the pressure should stay roughly steady for months on end. When the pressure keeps falling, water is going somewhere it should not, or a component that manages pressure has stopped doing its job. This guide explains the usual causes, how to tell a leak from the alternatives, and when it is worth having the leak traced.
What counts as normal boiler pressure?
For most combi and system boilers, the gauge should read somewhere between 1.0 and 1.5 bar when the heating is cold. It is normal for the reading to rise a little when the heating is on and the water has warmed up, then settle back down as it cools. That gentle movement is the system doing its job.
Trouble usually starts when the cold pressure drifts below around 0.8 to 1.0 bar. Many boilers will lock out and stop firing if the pressure falls too low, which is a safety feature rather than a breakdown. Topping it back up to the right level should restore heating. The real question is why it dropped in the first place, and whether it is going to keep happening.
The three usual causes
1. A leak in the sealed circuit
A leak is the single most common reason pressure keeps falling. Because the system is sealed, even a slow weep will gradually lower the pressure over days or weeks. The leak can be anywhere water flows: a radiator valve, a pipe joint, the pump, an automatic air vent, or inside the boiler itself.
Some leaks are easy to spot. Look for damp patches, green or white staining around brass fittings, rusty marks under radiator valves, or a faint musty smell near pipework. Many leaks, though, leave no trace you can see, especially where pipes run under floors, beneath screed, or behind walls. A pinhole losing a teaspoon of water at a time can be enough to keep dragging the pressure down without ever showing a wet patch.
2. A failed expansion vessel
Inside or near your boiler sits an expansion vessel: a small tank with a rubber diaphragm and a cushion of air. As water heats and expands, the vessel absorbs the extra volume so the pressure does not climb too high. Over time that air charge can be lost, and the diaphragm can perish.
When the vessel fails, the pressure often behaves oddly. It can climb sharply when the heating is on, sometimes high enough to push water out through the pressure relief valve and the small copper overflow pipe outside the house. Then, once that water has been lost and the system cools, the cold pressure reads low. If you keep seeing the pressure swing high while hot and then sit low when cold, or you notice water dripping from an outside overflow pipe, a tired expansion vessel is a strong suspect. This is a job for a Gas Safe registered engineer to test and recharge or replace.
3. Recent bleeding or top-up work
The most innocent explanation is simple maintenance. If you have recently bled trapped air out of your radiators, you will have let a little water out at the same time, and the pressure naturally drops. The same goes for any work an engineer has done on the system. This kind of drop is a one-off. You top the pressure back up, and it holds. If you bled the radiators a week ago and the gauge has stayed put since, you can relax. If it keeps falling, the bleeding was a coincidence and something else is going on.
How to tell a leak from the other causes
A few simple observations will usually point you in the right direction before any work begins:
- Watch the pattern. A steady, gradual fall over days or weeks, whether the heating is on or off, points towards a leak.
- Note when it drops. Pressure that shoots up while hot then sits low when cold leans towards an expansion vessel fault rather than a leak.
- Check outside. Water dripping from the small overflow pipe on an outside wall suggests the pressure relief valve is releasing, often linked to the vessel or a stuck valve.
- Look for damp. Stains, rust around valves, lifting flooring or a musty smell all hint at water escaping somewhere.
- Time the refills. If you are topping up more than about once a month, and definitely if it is weekly, treat it as a leak until proven otherwise.
Why topping up only masks the problem
Reaching for the filling loop every few days feels like the easy answer, and the occasional top-up is perfectly fine. The trouble is that repeated refilling treats the symptom and ignores the cause. Every time you add fresh, oxygen-rich water to the system, you give corrosion a fresh supply of the oxygen it needs. Over months that can mean more sludge, more wear, and more pinholes forming on the inside of pipes and radiators.
If the underlying issue is a leak, constant topping up also lets water keep escaping into your floors, ceilings or walls. A slow leak that nobody chases can do quiet damage to timber, plaster and insulation long before it becomes obvious. Finding and fixing the source is almost always cheaper than living with the consequences. You can read more about the warning signs of a hidden leak in our guide on spotting a hidden water leak.
Why leak sealant is only a temporary fix
Chemical leak sealant, added through the system, is sold as a quick way to stop a heating leak. It can work on the very smallest weeps, sealing a pinhole or hairline crack as the liquid reaches air at the leak point, and the seal may hold for weeks or months. For a tiny, hard-to-reach leak it can buy time.
It is not a cure, though, and it carries real drawbacks. Sealant cannot fix a damaged pipe, a failed joint or a corroded fitting, and it will not hold against anything more than the smallest weep. It can also settle in the wrong places, potentially clogging the pump, narrow heat exchanger passages or radiator valves, and some boiler manufacturers warn it may affect the warranty. Just as importantly, once sealant is in the water it can interfere with tracer gas leak detection, the most accurate way to pinpoint a hidden leak. That can make a later professional survey harder and more expensive. The sensible order is always to find and repair the leak properly first, and treat sealant as a short-term measure at most.
When to have the leak traced
If you have ruled out recent bleeding, your engineer has checked the expansion vessel and pressure relief valve, and the pressure still keeps dropping with no visible leak, the water is almost certainly escaping from a pipe you cannot see. This is the point to have the leak traced rather than guessing where to lift floorboards.
Modern leak detection finds hidden heating leaks without tearing the house apart. Using thermal imaging, acoustic listening equipment, tracer gas and moisture mapping, the source can usually be pinpointed to a small area, even under a solid floor. That means a targeted repair instead of pulling up a whole room. If the leak runs beneath flooring or screed, our page on central heating leak detection explains what the process involves, and our dedicated central heating leak detection service covers homes right across Devon.
It is also worth checking your home insurance. Many policies include a provision called trace and access, which can cover the cost of locating a leak and reaching it, though it is not on every policy and limits vary. If you think you have a leak, report it to your insurer promptly, as most policies expect you to act as soon as you reasonably can. Always check the specific terms with your insurer.
Frequently Asked Questions
A very small drift over many months can be normal, and an occasional top-up is fine. What is not normal is pressure that keeps falling every few days or weeks. A sealed heating system should hold its pressure for a long time, so repeated drops usually mean a leak or a faulty component rather than ordinary wear.
As a rough guide, needing to top up more than about once a month suggests a problem worth investigating. If you are refilling weekly, or the pressure falls noticeably within a day or two, treat it as a likely leak. Constant topping up also adds fresh oxygen to the water, which can speed up internal corrosion.
Yes. Many heating leaks are completely hidden, especially where pipes run under floors, beneath screed or behind walls. A pinhole can lose just enough water to lower the pressure without ever showing a damp patch. A failed expansion vessel can also cause low pressure with no leak at all, which is why proper testing matters.
Sealant can temporarily seal the very smallest weeps, but it will not fix a damaged pipe or joint, and it can clog parts of the system. It may also interfere with tracer gas detection later, making a professional survey harder. The better approach is to find and repair the leak properly first, and treat sealant as a short-term measure at most.
A Gas Safe registered engineer should check the boiler, expansion vessel and pressure relief valve. If the pressure still drops with no visible cause, a leak detection specialist can trace a hidden leak using non-destructive equipment. Devon Leak Detection covers homes across Devon and can be reached on 07897 027775 or hello@devonleakdetection.co.uk.
Worried About a Leak in Devon?
If your boiler pressure keeps dropping and you cannot find the cause, we can trace the leak for you, with no need to tear up floors guessing.