Boiler pressure: too low, too high, and
why it will not stay put

If the pressure is low the boiler will usually shut itself down, and on a system boiler that means you lose the heating and the stored hot water together. Topping it up is often something you can do yourself. Where it gets less straightforward is when it will not stay topped up, or when it climbs too high, and those two point at different things.

What the gauge should read

We aim for roughly 1.5 bar, and we tell customers anywhere between 1.5 and 2 is fine. We would not want it above 2.

That is a slightly different answer from the one you will find most places, which is to fill to 1 bar. We do not like 1 bar. If the pressure drops slightly, and it will move a little with a change in the weather or as the system settles, you are back with the same problem within weeks. Starting at 1.5 gives the system somewhere to go.

Read the gauge with the boiler off and settled for a few minutes, because the pump affects the reading while it is running.

Where the filling loop actually is

This is the part that stops people, and it is not their fault.

On a combi the filling loop is usually underneath the boiler. On a system boiler with an unvented cylinder it very often is not, and it will be by the cylinder instead. We had exactly this a few weeks ago on a video call with a customer whose pressure had dropped to 0.3. She was standing at the boiler looking for something that was never going to be there. Once we knew it was a system boiler with an unvented cylinder we asked her to show us the cylinder, and the loop was right there.

If you have been looking under your boiler and finding nothing, go and look at the cylinder.

Topping it up

The loop has two valves on it. Open them one at a time and watch the gauge, then close them both again when you get where you want to be. Then go back to the boiler and check the reading there.

On that call the first fill brought it to just over 1, which is where a lot of people would stop. We asked her to go again and bring it up to 1.5.

One thing worth saying, because we have been called out for it more than once. If you are reaching about under the boiler looking for the filling loop, be careful what you close. There is often a gas lever down there as well, and turning that off by mistake will leave you with a boiler that will not light at all.

When it will not stay there

Topping up a boiler that keeps losing pressure is treating the symptom. If you are filling it every few weeks something is letting water out, and the usual places are a leak on the pipework or radiators, a pressure relief valve passing, or an expansion vessel that has lost its charge.

The one that catches people is a leak you will never see. If the heating pipework is leaking under the floorboards, and there are often deep foundations under there, nothing shows up in the house at all. No damp patch, no stain on a ceiling, nothing. The gauge is the only thing telling you about it.

We have also found the pressure going out through the condensate, where there is a pinhole in the main heat exchanger and the system water is quietly draining away with everything else. Again, nothing to find on the floor.

None of these get better on their own, and only one of them is anything like a visible problem.

When it is too high

Pressure that climbs above 2, or that rises when the heating comes on and does not come back down, usually points at the expansion vessel.

If the gauge is filling up on its own without anybody opening the filling loop, there are two things behind that. The common one is the filling loop itself not having closed properly, so the system keeps quietly taking on mains water, and that one costs nothing to check. The other is a pinhole in the plate heat exchanger on a combi, which lets cold mains water cross into the heating side and pushes the pressure up the same way.

When it is not a job to do yourself

Topping up the pressure is fine. Finding out why it will not hold is not, and neither is anything involving the expansion vessel, the relief valve or the gas side of the boiler.

If you are topping it up more than once or twice a year, it is worth having somebody look at why.

Quick answers