A high reading is rarely an emergency. Every sealed system has a pressure relief valve that opens before the pressure gets anywhere dangerous, and Ideal describe high boiler pressure as unlikely to be dangerous for exactly that reason. It is still worth fixing today, because a system living near its limit dumps water through that valve, and a pressure that rises by itself is telling you something has failed.
How high is too high
Read the gauge with the heating off and the radiators cold. A hot reading is always higher, because heated water expands, and comparing a hot figure against the cold band is the most common reason people think they have a problem when they do not.
| Cold reading | What it means |
|---|
| 1.0 to 1.5 bar | Where Worcester Bosch, Vaillant and Ideal all want it. Nothing to do. |
| 1.5 to 2.5 bar | High, not urgent. Vaillant say no action is required below 2.5 bar at room temperature, though bleeding a radiator or two will bring it down. |
| Above 2.5 bar | Vaillant advise reducing it before you use the boiler. |
| About 2.75 bar or more | The red zone on a Worcester Bosch dial. The relief valve is close to opening. |
Running pressure is different. Around 2 bar while the heating is up to temperature is normal. What is not normal is a rise of more than about 1 bar between cold and hot, which Worcester Bosch treat as a sign the expansion vessel needs attention. More on that below, and the full cold and hot bands are on what boiler pressure should be.
How to reduce boiler pressure yourself
This is a homeowner job. You are letting a small amount of water out of a closed system, and the gauge tells you when to stop.
- Turn the heating off and let it cool. Half an hour is usually enough. Working on a hot system gives you a false reading and scalding water.
- Check the filling loop first. Under most combis it is a braided hose with a valve at each end, or a lever or key on the underside of the boiler. Make sure both valves are fully closed. Ideal name a filling loop left open as the prime suspect for high pressure, and if it is still letting water in, nothing else you do will hold.
- Pick a radiator, ideally downstairs, and put an old towel and a container under the bleed valve at the top corner.
- Open the bleed valve a quarter turn with a radiator key. You will hear air first, then water will spit out. Let it run.
- Watch the boiler gauge, not the radiator. Close the valve when the needle reaches 1 to 1.5 bar. If the boiler is in another room, get someone to call out the reading.
- Restart the heating and check the reading again once the system has cooled the next day.
Bleeding is slow, because the valve is small. Ideal also list opening the filter valve on a magnetic system filter, or the drain-off valve at a low point of the system, with a bucket underneath, closing it at 1 bar. Both drop the pressure faster. Use whichever you can reach, and do not open anything inside the boiler case: Vaillant are explicit that the casing is for engineers. Our guide to bleeding a radiator covers the radiator method in more detail.
If you overshoot and it drops below 1 bar, top it back up with the filling loop, slowly, and close both valves fully afterwards. The walkthrough is on how to repressurise a boiler.
Why the pressure went up in the first place
A sealed system cannot gain water by itself. If the pressure is higher than it was, water has come in from somewhere, or the space that absorbs expansion has gone. Vaillant give three causes, and between them they cover almost every case.
1. Too much water was added
Someone topped up, went a little past the band and closed the valves. Or an engineer refilled after a repair and left it high. This is a one-off: reduce it once, it stays reduced. Nothing is wrong with the system.
2. The filling loop is open, or passing
A valve left slightly open lets mains water creep in all the time, and the gauge rises with the heating off. Closing it fixes that. The harder version is a filling valve that is closed but no longer seals, so mains water still gets past the seat. The test is simple. Bring the pressure down to 1.5 bar cold, make sure both valves are shut, and read the gauge cold each morning. If it keeps climbing with the heating off, water is getting in. On a loop with a hose, disconnecting the hose after filling often stops it. If it does not, or your filling device is built into the boiler, the valve needs replacing.
3. The expansion vessel has lost its charge
This is the one that fools people. The vessel is a tank with a rubber diaphragm and a charge of air or nitrogen behind it, and it soaks up the expansion as the water heats. When the charge escapes or the diaphragm splits, there is nothing soft to push against, so the pressure shoots up every time the heating runs and falls back when it cools. The pattern is a normal cold reading and a very high hot one. Bleeding a radiator to knock the hot peak down only buys one heating cycle here, because the missing cushion is still missing. Ideal say plainly that the vessel is a professional job and not one to inspect yourself, and Worcester Bosch say the same about recharging it. Our page on expansion vessel failure or a leak sets out how to tell the two apart from the gauge alone.
4. An internal fault in the boiler
Vaillant's third cause is a faulty boiler with malfunctioning internal parts. On a combi, the one engineers check first when the pressure rises every time hot water is drawn is the plate heat exchanger, where mains water and heating water pass each other on either side of thin plates. If a plate fails, mains pressure pushes water into the heating side. That is diagnosed with the case off, by a Gas Safe registered engineer, and is not something to confirm at home.
Where high pressure turns into a leak
This is the part that brings people to us. When the pressure peaks past about 3 bar, the pressure relief valve opens and sends water out through a small copper discharge pipe, usually through the wall near the boiler. It is doing its job. But a relief valve that has lifted a few times often does not reseat cleanly, and afterwards it keeps weeping even at normal pressure.
So the sequence runs like this. A vessel fails, the pressure spikes hot, the valve lifts, the valve starts passing, and now the gauge falls every week and you are topping up instead of bleeding down. People who search for high pressure one month often search for boiler pressure keeps dropping the next, and it is the same fault.
Signs that it has got to that stage:
- The discharge pipe outside drips, or there is a stain or a green crust on the wall below it.
- The pressure swings high when hot and low when cold, and you have topped up more than twice this year.
- Water under the boiler, or a wet patch on the ceiling below where it is mounted.
- A pressure fault code that returns after every reset.
A passing relief valve is covered in detail on pressure relief valve passing water. Every top-up after that point also brings fresh oxygenated mains water into the system, which dilutes the inhibitor and speeds up corrosion in radiators and pipework.
What you can do, and when to call
You can safely do all of this: read the gauge cold, close the filling loop, bleed a radiator or open a drain-off valve to bring the pressure down, and keep a week of morning readings. If it was simply over-filled, that is the end of it and you do not need anyone.
Book an engineer when:
- The pressure climbs back up with the heating off and the filling loop closed.
- It jumps by more than about 1 bar every time the heating runs.
- The discharge pipe outside is dripping or has been wet.
- You are now topping up as well as bleeding down.
- Water is appearing under the boiler or on a ceiling.
Anything inside the boiler casing needs a Gas Safe registered engineer. Where the fault has become a leak, whether that is a passing valve, a weeping joint or water that has already reached a floor or ceiling, that is our work. We prove the loss with a pressure test, find where the water is going before anything is opened, repair it, and put right any damage it has caused. See boiler and heating leak repair for how that job runs.
What it costs
Bringing the pressure down yourself costs nothing. If you need us, we charge £150 an hour, agreed before we attend, so the rate is fixed even though the total depends on how long the job takes. If we are called to find a leak and do not find it, you do not pay for the time spent looking. The details are on the pricing page, or tell us what the gauge is doing and we will say whether it needs a visit.
