Boiler Keeps Losing Pressure

Reviewed by the DCI Leak Detection team · Last updated June 2026

A boiler that won't hold its pressure is one of the more wearing faults to live with. You top it up, the heating works for a while, then the gauge sinks again and you're back where you started. The cause is almost always one of a handful of things, and most are easy enough to spot. This guide walks through the seven most common reasons a boiler keeps losing pressure, the checks you can safely do yourself, and the point at which it's time to bring in a professional.

What pressure should a boiler be?

Before you can decide whether something's wrong, it helps to know what "right" looks like. On most UK combi and system boilers, the pressure gauge should read between 1 and 1.5 bar when the heating is cold. As the system warms up the water expands and the pressure rises a little, often to around 2 bar when hot, which is perfectly normal (Worcester Bosch; Ideal Heating).

If the needle sits below about 0.5 bar, the boiler has lost too much water and many models will lock out and stop firing until you top it back up. So the question isn't really "is my pressure low?" It's "why does it keep going low?"

Why does my boiler keep losing pressure?

One principle explains almost every case: a sealed heating system shouldn't lose water. If the pressure keeps dropping, either water is escaping somewhere or the part that's meant to absorb pressure changes has failed. That narrows it down fast.

A one-off drop after you've bled the radiators is normal and nothing to worry about. A slow fall that needs a top-up once or twice a year is also considered normal by manufacturers. But a repeated, ongoing loss, every few days or weeks, almost always means a genuine leak or a faulty component, and that's worth getting to the bottom of rather than topping up forever.

The 7 most common causes of a boiler losing pressure

1. A leak at a radiator valve or pipe joint

This is the single most common reason a boiler loses pressure. A weeping radiator valve, towel-rail fitting or pipe joint lets water out slowly, and the gauge follows it down. Look for damp patches, light rust or tide marks around valves and beneath radiators (Ideal Heating). Even a tiny seep is enough to drop the pressure over a few days.

2. A failed or undercharged expansion vessel

The expansion vessel is a sealed tank that absorbs the extra pressure as your heating water heats up and expands. It holds a cushion of air or nitrogen behind a rubber membrane. Over time that charge can leak away, or the membrane can split. Once it does, the system can't manage pressure properly and the gauge drifts down with no water escaping anywhere you can see (Vaillant). It's a classic "losing pressure with no visible leak" cause.

3. A weeping pressure relief valve (PRV)

The pressure relief valve is a safety device that releases water if the system pressure gets too high. If it sticks or wears, it can weep continuously instead of sealing shut. The tell-tale sign is water dripping from a small copper pipe (the overflow) that runs through an outside wall. Check the wall outside near the boiler for staining or a steady drip.

4. Recently bled radiators

If you've just bled your radiators, a small pressure drop afterwards is expected. You've let water and trapped air out, so the gauge falls. This is a one-off, not a fault. Top the pressure back up to 1 to 1.5 bar and it should hold. If it keeps falling after that, the bleeding wasn't the real cause.

5. A hidden leak under floors or in walls

Not every leak leaves a puddle. Heating pipes buried in screed, run under floorboards or chased into walls can leak quietly for weeks. They show up only as falling pressure and, eventually, a warm patch or damp on a wall. These are the leaks worth taking seriously, and the ones non-invasive central heating leak detection is built to find.

6. A corroded or leaking part inside the boiler

Components inside the boiler, such as the heat exchanger and the internal seals and joints, can corrode or loosen with age and leak internally. You won't see this from the outside, and it isn't a DIY job: the casing should only ever be removed by a qualified engineer (Vaillant).

7. An auto air vent or filling loop left open

Less common, but worth a look: an automatic air vent that's failing can let pressure escape, and a filling loop that hasn't been closed fully can quietly affect pressure too. Both are quick for an engineer to check and rule out.

When the boiler pressure keeps dropping but there's no visible leak

This is the situation that catches people out. You've checked every radiator, dried under every valve, and found nothing, yet the pressure still falls. When that happens, you're usually looking at one of two things: a failed expansion vessel (no water escapes, so there's nothing to see), or a concealed leak under the floor, beneath screed or inside a wall. On systems with a stored hot water cylinder, a leaking hot water cylinder can quietly drain pressure in much the same way.

The mistake is to keep topping up and hoping. Every time you add fresh water you also add oxygen and dilute the corrosion inhibitor that protects your system, so constant re-pressurising slowly damages the very boiler you're trying to keep running. A concealed heating leak won't fix itself, and it can track along joists and cause real damage before it ever shows. This is exactly where professional detection earns its keep. Thermal imaging and acoustic equipment can pinpoint a hidden leak to within centimetres, so any access is minimal rather than lifting a whole floor on guesswork.

Quick fixes you can try safely

There are a few sensible checks and fixes a homeowner can do without any specialist tools:

Topping the pressure up is the only "repair" that's safe to do yourself. Anything involving the valve, the expansion vessel, internal parts or pipework should be left to a registered engineer.

When to call a Gas Safe engineer

Topping up via the filling loop is homeowner-safe. Beyond that, the law and good sense both say to bring in a professional. Call a Gas Safe registered engineer if any of the following apply (Vaillant; Ideal Heating):

• The pressure keeps dropping despite topping up.
• The pressure relief valve is weeping from the outside overflow.
• You suspect the expansion vessel needs recharging or replacing.
• There's any sign of a leak from the boiler itself.
• You'd need to remove the boiler casing or work on pipework, which is never a DIY job.

If the leak is on the heating pipework rather than the boiler, a leak detection specialist can find exactly where it is first, so your engineer (or you) can repair the right spot with the least disruption. That's where we come in: finding the leak, fast, before anything gets opened up.

Tracing heating leaks in Cornwall & Devon

Across Cornwall and Devon we trace hidden heating leaks every week using non-invasive thermal, acoustic and tracer-gas equipment, so we find the source with the least possible damage. If the water seems to be coming from the unit rather than the system, our guide on why your boiler is leaking water helps you tell the two apart. And if you've already pinned the loss to the heating circuit, our central heating leak detection service finds it and gives you the report you need, including for an insurance claim. No find, no fee on residential leak detection, subject to terms.

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