Boiler pressure too high: what it means and how to lower it

This guide covers what a high reading means, how to lower it safely and why it can keep rising. If it keeps coming back, you'll need a boiler repair from a Gas Safe engineer. Is your pressure dropping instead? See our guide to a boiler losing pressure. This guide is part of our common boiler problems series.

What boiler pressure is too high?

On most combi and system boilers, the gauge should read about 1 to 1.5 bar when cold, and up to about 2 bar when hot. A reading well over 2 bar is higher than it needs to be. Near 3 bar is too high. Your boiler manual gives the right figure for your model.

"Bar" is the unit the gauge uses to measure pressure. The gauge is on the front of the boiler, or the reading shows on its screen. It's best to check it with the heating off and cool. That's when you get a true reading.

Is 3 bar boiler pressure dangerous?

It isn't usually an emergency, but it shouldn't be left. At around 3 bar, a safety valve opens and lets water out, so the pressure can't keep building. Many boilers also lock out, which means they shut down and show a fault until someone resets them.

High pressure still puts strain on seals, valves and radiators. If a boiler keeps locking out, our guide to a boiler that keeps turning off explains why resetting it again and again won't help.

How do I lower my boiler pressure?

To lower boiler pressure, turn the heating off and let it cool. Check the filling loop is shut, then bleed a radiator slowly while someone watches the gauge. Close the bleed valve once the gauge reads about 1 to 1.5 bar.

The filling loop is the short hose or tap under the boiler that lets mains water in. You'll need a radiator key and an old towel.

1. Turn the heating off and wait until the radiators are cool. Hot water can scald.
2. Check both filling loop taps are fully closed. Your manual shows which way is shut. If one was open, close it.
3. Hold a towel under the bleed valve. That's the small square screw at the top corner of a radiator.
4. Turn the key slowly, about a quarter turn. You may hear air hiss first. Then water will drip out, and it may be dark.
5. Have someone watch the gauge, or check it often. A radiator near the boiler makes this easier.
6. Close the valve once the gauge reads about 1 to 1.5 bar. Don't let it fall below 1 bar.
7. Turn the heating back on. If the boiler locked out, press reset once.

If you let out too much, you'll need to top it back up. Our guide to a boiler losing pressure shows how. For more on the bleed valve, see our guide to cold radiators.

Why does my boiler pressure go up when the heating is on?

Water takes up more room as it heats, so a small rise of about half a bar is normal. A big jump, from about 1.5 bar to near 3 bar, is not. That usually means the expansion vessel has lost its air.

The expansion vessel is a small tank, often inside the boiler case. One half holds air, the other half holds water. The air squashes as the water heats, so it soaks up the extra pressure. Over the years, the air slowly leaks out. Then there's nowhere for the extra water to go, and the pressure shoots up.

This often leads to a cycle. The pressure gets high when hot, and the relief valve lets water out. Then it drops too low once the system cools. People top it up, and it starts again. If that sounds like yours, the expansion vessel is a likely suspect.

Why does my boiler keep gaining pressure?

If the pressure creeps up even with the heating off, water is getting into the system from the mains.The usual cause is a filling loop tap that's open or worn. On a combi, a leak inside the plate heat exchanger can do the same.

• Filling loop left open. After a top-up, one tap may not be fully shut. Mains water then keeps trickling in.
• Filling loop letting by. The tap is shut, but its seal has worn, so water still seeps past. Some manuals say to take the hose off after each top-up. A loop with its hose removed can't let water in.
• Plate heat exchanger leaking (combi boilers). This is the part that heats your tap water using hot water from the heating side. If it leaks inside, mains water pushes into the heating. A clue is the pressure rising after you run a hot tap.
• Expansion vessel. This causes rises when the heating is on, as explained above.
• A faulty gauge. Less often, the gauge itself sticks and shows a wrong reading.

Only the first one is a homeowner check. The others are inside or under the boiler, so they're engineer work.

Why is water dripping from the pipe outside my boiler?

That pipe drains the pressure relief valve. A drip usually means the pressure is rising too high, so the valve keeps opening. The pressure relief valve, or PRV, is a safety valve that opens at around 3 bar.

The pipe is usually copper, and it runs out through the wall near the boiler. After it has opened a few times, the valve may not seal properly, so it can keep dripping. Never cap, block or bend that pipe. Never turn or open the valve yourself. Our guide to a leaking boiler covers other drips.

Do you need a new boiler if the pressure is too high?

Rarely. High pressure is nearly always caused by a small part that can be fixed or swapped. A worn filling loop, an expansion vessel that needs recharging or replacing, and a dripping relief valve are all normal repairs.

Recharging means pumping air back into the expansion vessel to the right level. If its rubber lining has split, it won't hold air, so it's replaced instead. A replacement can be worth weighing up when:

• the plate heat exchanger has failed on an older combi that also has other faults
• the expansion vessel is built deep inside an old boiler, and a new one is hard to get
• the parts for your model have stopped being made

We'll weigh up the repair against the boiler's age with you, and tell you plainly which makes more sense. If it's a new boiler, our boiler installation page shows how it works, from a free survey to a fixed price.

When to call a Gas Safe engineer

Call a Gas Safe engineer if the pressure climbs back up after you've lowered it, or it jumps near 3 bar each time the heating comes on. Both point to a part that is letting water in or has worn.

• the pressure creeps up with the heating off and the filling loop shut
• it jumps a lot when the heating comes on, then drops when cool
• it rises after you run a hot tap
• the pipe outside keeps dripping
• the boiler keeps locking out with a pressure fault

The engineer first runs a thorough check of the boiler and system. That finds the fault and the parts needed. See our boiler repair page for the faults we fix. We repair all makes.

A yearly boiler service helps stop this coming back. The engineer can check the expansion vessel's air charge and the safety valves before they cause trouble.

Other boiler guides

• Boiler losing pressure: what the gauge should read and how to top it up.
• Leaking boiler: what to do first, and where the water is coming from.
• Boiler keeps turning off: lockouts, short cycling and when to reset.
• Cold radiators: what cold at the top or bottom means, and how to bleed a radiator.
• Boiler making noise: what banging, gurgling and kettling mean.
• Boiler won't turn on: the checks to try first, and what a pilot light is.
• Hot water but no heating: thermostat, timer and valve checks.
• Heating works but no hot water: checks to try, and what a diverter valve does.
• Frozen condensate pipe: how to thaw it safely.
• Boiler not working after a power cut: how to reset it, and set the timer clock.
• Boiler tripping the electrics: why it trips the fuse box, and who to call.
• All common boiler problems.

Pressure still too high? Book a boiler repair and we'll find the cause.