You've topped it up. A week later it's back down again. Nobody wants to be doing that every Sunday, and there's a reason it keeps happening.
Here's what you can check yourself, and how to tell which of the two problems you've actually got.
Not sure it's the pressure at all? Start with the six first checks.
Most boilers want somewhere between 1 and 1.5 bar when the system is cold. Under about 0.8 and the boiler will usually refuse to fire and show a fault.
Worth knowing: the pressure rises a little when the heating is hot and drops back as it cools. That's normal. What isn't normal is having to top it up again and again. And if yours is going the other way and climbing to 2.5 or 3 bar every time the heating runs, that's a different fault, and I've covered boiler pressure too high on its own page.
Finding the filling loop is where most people get stuck, and it isn't you being daft, they're all different.
Most are a braided silver hose under the boiler with a small valve at each end. Some are a plastic key you push in and turn. Some are a lever built into the underside. On a few it's tucked behind the bottom panel, and on older systems it can be a separate loop altogether, under the floor or in a cupboard nearby.
Once you've found it, open it slowly, watch the gauge climb to about 1.2 bar, and close it again. Slowly is the important word. It fills faster than you'd think and it's easy to overshoot.
If you can't see anything like any of that, check your model's manual before you start pulling at things.
If you go well past 2 bar, the way to bring it down is to let a little water out of a radiator bleed valve. Only do that if you've bled a radiator before and you're confident with it. Those bleed screws are small, they seize, and they round off. If one won't close again you've turned a pressure problem into a genuine leak, in the house, on the carpet. I've been called out to that more than once.
If you haven't done it before, don't learn on this. Leave the pressure where it is and get someone to bring it down. Same if you can't find the filling loop, or you're not comfortable. It's not worth guessing at.
Pressure only drops if water is leaving the system somewhere. So rather than topping it up forever, it's worth finding out where it's going. There are two possibilities and they're very different jobs.
Walk round the house and check:
You're looking for damp, drips, a stain, a patch of carpet that never quite dries, or green or white crusting on a valve.
If you find water, that's your answer. It's a leak, and it needs fixing rather than topping up.
This is the one almost nobody checks.
There's a small pipe that comes out through an outside wall, usually near the boiler, pointing down. It's a safety discharge pipe. If the system builds too much pressure, water goes out of there.
Go and look at it. If it's dripping, wet, or has a stain running down the wall underneath, that's where your water has been going.
That tells you a lot. It usually means the boiler is building more pressure than it should when it heats up, and pushing the excess outside. Topping it up just gives it more to push out.
Usually that's the expansion vessel. It's a sealed tank, normally inside the boiler, that gives the water somewhere to go when it heats up and swells. When it stops working the pressure spikes every time the heating runs, the safety valve lifts, and the excess goes out of that pipe. By the time you look at the gauge the system is cold again and reading low, so it looks for all the world like a leak.
That's why people top up for months hunting water that was never in the house. Same part, same job, same bill at the end of it.
The other possibility is the safety valve itself. Once one of those has lifted a few times it often won't seat properly again, so it weeps continuously and you lose pressure that way instead. That's a cheaper job, and it's worth knowing which of the two you've got before anyone quotes you.
I've put what both should cost on the boiler pressure too high page.
One more thing worth checking before anyone spends money.
Look at the gauge yourself, and compare it to what the boiler is telling you. If the needle is sitting at a perfectly healthy 1.2 bar but the boiler is still showing a low pressure fault, then the pressure isn't your problem. The boiler is being told the wrong thing by a sensor.
That's more common than you'd expect, particularly on some makes, and it catches people out. They spend weeks hunting a leak that was never there.
Different makes, same problem:
Not every boiler gives you a code. Plenty of older ones just flash a light at you rather than telling you anything useful. If yours isn't showing a code at all, that doesn't mean nothing's wrong.
If yours isn't listed, check the manual, but a pressure-related code will usually be the first one explained in it.
Top it up once and see what happens. That's fair enough and it might hold.
Get someone if:
None of that means sitting in a cold house waiting. If topping it up gets the boiler running again, use it. Just don't let it become the Sunday routine, because it won't fix itself and you're topping up to cover something that's getting worse.
If it's stopped in cold weather rather than losing pressure, that's usually the condensate pipe instead.
Beyond that point there are several things an engineer checks, in a specific order, and getting the order wrong is why people end up paying for parts that didn't need replacing. The most common mistake is replacing the obvious component without checking the thing feeding it first.
I'm Carlo Cennerazzo, twenty-three years in the trade. Point your phone at the boiler and I'll go through it with you, tell you what's wrong, whether you can sort it yourself, and what it should cost if you can't.
£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.
Don't book a call. Leave the property, then ring the National Gas Emergency Service on 0800 111 999. In Northern Ireland it's 0800 002 001.