Boiler pressure too high

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When the heating runs, the water in the system gets hot and expands. That expansion has to go somewhere, and where it goes is the expansion vessel. It's a sealed tank, usually inside the boiler, with air on one side of a rubber diaphragm. The air squashes up, the water gets the room it needs, and the gauge barely moves.

When that vessel stops doing its job, the water has nowhere to go. So every time the boiler fires, the pressure climbs.

This page covers what normal looks like, what you can check yourself in ten minutes, and roughly what it should cost, so you know before anyone quotes you.

Cold, before the heating has been on, you want somewhere around 1 to 1.5 bar.

Running, it lifts a bit. Up to 1.8 or 2 bar is nothing to worry about.

Cooled down again, it should come back to roughly where it started.

The pattern that matters is this one: fine when cold, climbing to 2.5 or 3 bar once the heating has been on a while, then back down to normal as it cools. That's expansion with nowhere to go, and it's the expansion vessel nearly every time.

Take the reading twice. Once cold, before the heating comes on, then again twenty minutes after it fires. Write both down, because the two numbers together tell you more than either on its own.

Then go outside and find the discharge pipe. It's a small copper pipe coming through the wall near the boiler, pointing down. If it's dripping while the pressure is high, or there's staining on the wall below it, the pressure relief valve has been lifting to let water out. That's the safety doing its job, and it's also telling you this has been going on a while. And if that valve has been lifting for a long time it often stops seating properly afterwards, so it weeps constantly and you end up losing pressure instead. Same fault, other direction.

You can bleed a bit of water off a radiator to bring the pressure down and keep the heating running. That's fine as a stopgap. Just know you're treating the gauge rather than the fault, and every time you do it you lose a bit of the inhibitor that stops the system rusting from the inside.

Two things I'd leave alone.

Don't try to recharge the expansion vessel yourself. On most combis it's inside the case, and getting at it means dropping the system to nothing and taking the boiler apart. That's not a DIY job and it isn't worth the risk to save an hour of someone's time.

And don't keep letting water off week after week without getting to the bottom of it. If the relief valve has been lifting regularly, there's a second problem waiting for you further down this page.

There are two different faults here and they look similar on the gauge. When the pressure rises tells you most of it, though it isn't quite a clean split.

It only climbs when the heating is running, and settles back down as the system cools. That's usually expansion with nowhere to go, so the expansion vessel. This is the common one.

It climbs while everything is cold, or creeps up overnight with the heating off. That's mains water getting into the heating circuit, and usually it's the filling loop passing.

The one that catches people out is a filling loop passing slowly. It creeps while the system is cold, but you often only notice once the heating runs and tips it over the top, so it looks like a vessel fault. Before anyone quotes you for a vessel, make sure both filling loop valves are properly shut. I've seen one boarded in behind a panel, quietly topping a system up for months while everybody blamed the vessel.

If the loop is definitely shut and it still creeps when cold, on a combi it can be the plate heat exchanger letting mains across into the heating side, and that's a different job entirely.

Taking the reading cold and hot, like I said earlier, is what tells these apart. It's the single most useful thing you can do before you ring anyone.

A radiator with a decent amount of air trapped in it will act as an expansion vessel.

Air squashes. Water doesn't. So a rad that's been sat half full of air for years has been quietly doing the vessel's job, and the gauge looked fine the whole time.

Then somebody bleeds the radiators properly, or takes one off to decorate, and suddenly the pressure is going to 3 bar and nobody can work out what changed. Nothing did. The vessel has been dead for a long time and the air in that radiator was covering for it.

So if this started right after you bled the system, drained it down, or had a radiator off, you almost certainly haven't broken anything. You've just uncovered something that was already broken.

Worth ruling this out before you spend money on it.

If your boiler shows a pressure reading on its display as well as having a gauge on the front, compare the two. They should agree. If the display says 2.8 and the gauge says 1.4, one of them is wrong, and it's usually the sensor rather than the gauge.

That matters because a sensor reading high can lock a boiler out and make you think you've got a pressure problem when the water side is perfectly fine. It also works the other way, and I've seen people chase a pressure fault for weeks that only ever existed on a display.

Old analogue gauges stick too. Tap the glass gently and see if the needle moves.

Here's the honest bit. Nearly every boiler has a fault code for pressure being too low, and very few have one for pressure being too high.

So more often than not, high pressure shows you nothing at all. The boiler carries on, the relief valve quietly lets water out onto the wall outside, and the first you know about it is either the gauge or a damp patch. Some boilers will lock out on an overheat code instead, because the water can't move the way it should.

If your boiler does throw a code, note it exactly and tell whoever you speak to. But don't assume you haven't got a problem just because there's no code on the screen. On this fault, no code is normal.

• The gauge is going over 2.5 to 3 bar every time the heating runs
• The discharge pipe outside is dripping or has staining below it
• The pressure climbs while everything is cold
• You've bled it down more than once or twice and it keeps coming back
• The boiler is locking out or showing an overheat code
• Anything at all that needs the casing off

Past that point it's testing rather than looking. What an engineer should do is drop the system to nothing and put a gauge on the vessel, and that single test decides whether it's a recharge or a replacement, which is the difference between an hour and most of a morning. Worth knowing that before anyone gives you a price, because plenty of people only ever get quoted for the replacement.

There are three outcomes here and they're priced very differently, so it's worth knowing which one you're being quoted for.

A recharge, where the diaphragm is intact and it just needs pumping back up. The cheapest of the three, but it's still a callout and it's still someone's time, so expect to pay for it. Approximately £90 to £150.

An external vessel fitted on the heating return, approximately £150 to £200. The quicker job, and it's accessible forever afterwards.

The vessel inside the boiler replaced, approximately £250 to £300. That means stripping the case, so most of the cost is time rather than parts. The vessel itself is only £80 to £120.

Expect the top of that if the pressure relief valve is being done at the same time, which it often is. Once that valve has been lifting for months it frequently won't seat properly again afterwards, so doing both in one visit is sensible rather than anyone padding the bill.

Ask which of the three you're being quoted for. Plenty of people are only ever offered the internal replacement and never told the other two exist.

One thing to watch for. A failed expansion vessel is not a sign that your boiler is finished, and this is exactly the point where people get talked into a new one. Get the vessel sorted, see where you are, then decide about the boiler separately if you need to.

I'm Carlo Cennerazzo, twenty-three years in the trade. Take the reading cold and hot like I've described, point your phone at the gauge and the boiler, and I'll tell you which of the two faults you've got and what it should cost to put right.

£20, no callout charge, same day when there's a slot free. A written report follows afterwards, so you've got it in black and white rather than trying to remember.

And if you've already had someone out and you're holding a quote you're not sure about, that's worth a call too. I've no interest in who does the work, so I'll tell you straight whether the price is fair and whether it's even the right job.