What to do if your boiler loses pressure

What is boiler water pressure?

Your boiler heats cold water, then sends that hot water around your home’s circuit of pipes and radiators. For it to work as efficiently as possible, you need stable water pressure within the system.

If your boiler isn’t working properly and you’re not getting hot water as normal, you’ll often find the problem is your boiler pressure is too low, or sometimes too high.

Luckily, it’s usually quite easy to diagnose and fix problems with boiler pressure.

How to check the pressure of a combi boiler

If you have a combi boiler – which is one that gives you instant hot water and has no water tank or cylinder – your boiler pressure is maintained by a ‘filling loop’ that connects your boiler to your cold-water pipe.

You’ll also have a gauge on the front of your boiler that shows your water pressure.

If you’ve got a hydraulic pressure gauge, you’ll see low and high pressure indicated by red sections on the dial. You’ll sometimes have a red indicator needle that shows the ideal pressure. On most digital gauges, you’ll see a flashing pressure reading if the boiler pressure is too high or low.

How to check the pressure of other types of boilers

If you have a water tank and hot water cylinder, you have either a conventional or system boiler. In some cases, you will have a pressure gauge on the boiler itself. But you might find your pressure gauge near your hot water cylinder, usually near a smaller red vessel.

What’s the right boiler pressure?

It’s perfectly normal for your boiler pressure to vary a little. Pressure increases when the water gets hot and decreases when it cools down. But your pressure should always be somewhere between 1 and 2 bar.

If your boiler pressure reads less than 1 bar, you might have lost water from the system, and this will need to be replaced. If it’s above 2.75 bar, that’s a sign of high pressure – and the best way of bringing that down is to bleed a radiator.

How to increase boiler pressure

Pressure loss is the most common boiler pressure problem you’re likely to come across. It might happen a couple of times a year, but it’s very easy to fix.

If you have a system or conventional boiler with a pressure gauge near your hot water cylinder, you’ll have a valve in the pipework – usually near the red vessel and the pressure gauge. Just open this valve a little and you should see the pressure gauge rise. Keep filling until it’s between 1 and 1.5 bar, then close it.

If you have a combi boiler, just follow these step-by-step instructions:

• Switch off your boiler and allow it to cool
• Double check that both ends of the filling loop are securely attached
• Open both valves (or just one, if that’s what your filling loop has) to allow cold mains water into the system. Listen out, you should hear it
• Wait for the pressure gauge to reach 1 to 1.5 bar
• Close both valves, one after the other (or just the single valve, if that’s what you have)
• Switch the boiler back on. If needed, press the reset button
• Undo both ends of the filling loop and remove. Be careful to catch any water spillage and keep the filling loop in a safe place

This advice will help if you have the most common type of filling loop (pictured). If you have a different type or if you’re unsure, please check the user instructions for your boiler. You can usually find these online if you no longer have your printed copy.

How to reduce boiler pressure

Boiler pressure being too high is a more unusual problem. Unless you’ve accidentally overfilled the system yourself, it could be a sign of a more serious issue and it’s worth calling out an engineer to take a look.

One simple thing to check is that the pressure valves under your boiler are closed tightly. If they become loose accidentally, this can cause your boiler pressure to become too high.

Decreasing boiler pressure is relatively straightforward. All you need to do is bleed a radiator to release pressure – and this video shows you how easy it is to do yourself.