Low pressure isn’t the problem — this is

The radiator might be cold, the gauge might be low, and you might be staring at a boiler pressure drop like it’s the whole story. But in most homes, the real plot is the expansion vessel and whether it’s quietly doing its job of absorbing heat expansion without dumping stress into the system. This matters because “topping up” pressure can feel like a fix, while the underlying fault keeps nudging you towards leaks, lockouts, and repeated call-outs.

You can hear it if you listen. The filling loop hisses, the needle rises, the heating runs, and then a day later you’re back where you started-slightly anxious, slightly colder, pretending it’s just winter being winter.

Low pressure is a symptom, not a diagnosis

Boiler pressure moves. It should rise a bit when the heating is on, and settle when it’s off. A small swing is normal; a steady drift down is the clue.

The trap is treating the gauge like a tyre. People top up to 1–1.5 bar, get heat back, and call it sorted. But pressure doesn’t “get used up”; it escapes, or it’s being forced out by something else.

Here’s the simplest way to frame it: a pressure drop usually means either water is leaving the sealed system, or the system is relieving pressure on purpose and you’re only seeing the after-effect.

The usual suspect: the expansion vessel losing its cushion

The expansion vessel is a metal tank with a rubber diaphragm inside: water on one side, air (or nitrogen) on the other. Its job is to give expanding hot water somewhere to go so the system doesn’t spike in pressure every time the boiler fires.

When that air cushion is lost-through a failed diaphragm or a low pre-charge-the system has no shock absorber. Pressure shoots up when hot, the boiler protects itself by venting water through the pressure relief valve (PRV), and then when everything cools down you’re left with… low pressure. Not because the boiler is “weak”, but because it dumped water to survive.

You’ll often spot the pattern without tools:

• Pressure is fine when cold, then climbs quickly when heating is on.
• It approaches 2.5–3 bar, then you may hear discharge outside.
• Later (or next morning) pressure is low again and the boiler may lock out.

It’s a bit like wiping up a puddle without turning off the tap. The gauge is the puddle.

A quick set of checks you can do without guessing

You don’t need to dismantle anything to gather good evidence. You just need to watch the system behave.

1. Note the cold pressure (first thing, heating off). Many systems sit around 1.0–1.5 bar cold.
2. Run the heating for 10–15 minutes and watch the rise. A gentle lift is normal; a jump towards 2.5–3 bar is not.
3. Look for PRV discharge. There’s usually a copper pipe that exits outside; if it drips after heating, that’s a big sign.
4. Check for visible leaks at radiator valves, towel rail bleed points, and under the boiler. A slow weep can be silent for weeks.
5. Notice how often you top up. If it’s more than “once in a blue moon”, it’s not a lifestyle choice-it’s a fault.

Let’s be honest: nobody really tracks this until the house goes cold. But these few observations tell an engineer far more than “it keeps losing pressure”.

What topping up is hiding (and why it can make things worse)

Using the filling loop repeatedly can mask the real failure long enough for extra damage to happen. Fresh water brings oxygen and minerals, which can accelerate corrosion and sludge. And if the PRV has started to weep, it can get stuck slightly open after repeated operations, turning a one-off pressure event into a постоян drip you never see until the wall outside stains.

If you’re stuck and need heat, topping up is fine as a temporary move. Just treat it like a temporary move: get the house warm, then get the cause handled.

“Pressure isn’t the problem. Pressure is the message,” as one heating engineer put it to me, wiping a thumb across a rusted PRV outlet like it was a confession.

What the fix typically involves (and when to call someone)

If the expansion vessel pre-charge is low, a Gas Safe engineer may be able to re-pressurise the air side and restore the cushion-but only after the water side is safely depressurised and the vessel is tested. If the diaphragm has failed (often indicated by water at the Schrader valve), the vessel usually needs replacing, or an external vessel added if access is awkward.

In parallel, the PRV often needs attention. If it has been venting, it may not reseal properly. Replacing a tired PRV can stop the “pressure up when hot, pressure down when cold” loop from continuing even after the vessel is corrected.

Call a professional promptly if:

• Pressure climbs close to 3 bar when heating runs.
• The PRV outlet outside drips after the boiler fires.
• You’re topping up weekly (or more).
• You see any signs of water inside the boiler casing (don’t open it yourself).

Keeping the system calm once it’s sorted

When the root cause is fixed, good habits keep it that way. Bleed radiators only when needed, and always check pressure afterwards. Don’t ignore small stains at valves. And if you have to top up, note the date-your boiler loves a paper trail more than it loves guesswork.

A sealed system is meant to be boring. When the gauge turns into a routine, it’s telling you the system isn’t sealed any more, or it’s been forced to unseal itself.

Clue you can see	What it often points to	What to do next
Pressure rises fast when heating is on	Expansion vessel issue	Book an engineer to test vessel pre-charge/diaphragm
Dripping pipe outside after heating	PRV venting (often due to vessel)	Don’t ignore; likely PRV/vessel work needed
Pressure slowly falls even when cold	Small leak somewhere	Check valves/rads/boiler; engineer if not obvious

FAQ:

• Why does my boiler pressure drop overnight? Often because the system heated up earlier, over-pressurised, and discharged through the PRV; when it cools, you’re left with low pressure. A small leak can also show up more clearly over longer idle periods.
• Is it safe to keep topping up the pressure? As a short-term measure to restore heating, usually yes, but repeated topping up can accelerate corrosion and may worsen a leaking PRV. Treat it as temporary and get the cause fixed.
• How do I know if the expansion vessel has failed? A common sign is pressure shooting up when heating runs, then dropping later, sometimes with water from the PRV pipe. Only an engineer should test the vessel properly (including checking the Schrader valve and pre-charge).
• What pressure should my boiler be at? Many homes aim around 1.0–1.5 bar when cold, but your manual may specify differently. The key is stability: small, predictable movement is normal; big swings and регуляр drops are not.
• Could it just be air in the radiators? Air can reduce heating performance and may affect readings after bleeding, but it doesn’t usually cause a recurring pressure drop on its own. If bleeding “fixes” it for a day and then it returns, suspect a leak or vessel/PRV issue.