BoilerFaultCodesBoiler pressure too high? You can usually bring it down yourself.
High pressure is far less alarming than low pressure, and the fix is often two minutes with a radiator key. The important part is knowing what the gauge should say, because a lot of people top up to a number that was never right in the first place.
What the gauge should read
| Reading (cold) | What it means |
|---|---|
| Below 0.5 bar | Too low. Many boilers lock out and stop firing. |
| 1 to 1.5 bar | Where it should sit with the heating off and cold. |
| 2 to 2.5 bar | High. Normal briefly when hot, not when cold. |
| Near 3 bar | The relief valve is about to dump water outside. Act now. |
Manufacturers agree closely on the cold figure. Worcester tells installers to "fill the system to a pressure up to 1.5 bar". Ideal specifies "with boiler off & system cold fill System to 1.0 Bar" on the Vogue Max, and for the Logic Heat says to check the pressure is "between 1 & 1.5bar on the system pressure gauge". Your own manual is the final word, so check it if you have it.
One thing that catches people out: pressure rises when the system heats up. Water expands. Seeing 1.2 bar cold and 2 bar with the heating running for an hour is normal and needs no action. Judge the reading cold.
How to lower it
Bleeding a radiator lets a little water out and drops the pressure. It is the same job as bleeding out trapped air, and it is safe to do yourself.
Turn the heating off and let the system cool. Do not do this with hot water in the pipes.
Make sure the filling loop is fully closed. If it was left slightly open after a top-up, that is your cause, and nothing else you do will hold.
Put a cloth and a small container under the bleed valve at the top corner of a radiator.
Open the valve slowly with a radiator key, no more than a quarter turn. Water will trickle out.
Watch the boiler gauge. When it reaches about 1.5 bar, close the valve.
Recheck an hour later once everything has settled.
Go slowly. Letting out too much in one go leaves you repressurising instead, and you will be back and forth between the radiator and the filling loop. A quarter turn is plenty.
Why it went high in the first place
Overfilled when topping up
Safe to fixEasily the most common cause. The filling loop moves water fast, and a couple of seconds too long takes you past 2 bar. Bleed a little back out and you are done.
The filling loop was left open
Safe to fixIf the loop is not fully closed, mains pressure keeps feeding the system and the gauge climbs no matter how often you bleed it. Check both valves on the loop are shut, and disconnect the flexible hose if yours is the removable type.
A failed expansion vessel
Engineer jobThe expansion vessel absorbs the extra volume when water heats up. When it loses its air charge it cannot do that, so pressure swings dramatically between cold and hot and the relief valve starts discharging. The giveaway is pressure that shoots up as soon as the heating runs and drops back when cold. Recharging or replacing it is an engineer job.
A passing filling loop or faulty relief valve
Engineer jobWorn valves can let water through even when shut. If pressure keeps climbing with the loop definitely closed and a healthy expansion vessel, this is usually why.
Water dripping outside the house?
A sealed heating system has a pressure relief valve that opens if pressure gets too high and sends water out through a copper pipe on the outside wall. Its rating is printed on the valve body, and 3 bar is the usual figure on UK systems. Water running from that pipe means the valve has done its job, and it often will not reseat properly afterwards, so it can keep weeping. Bring the pressure down, then get the cause looked at.
Safety
Bleeding a radiator is safe once the system is cool. Anything inside the boiler casing, on the expansion vessel or on the relief valve is for a Gas Safe registered engineer. If you smell gas, leave the house and ring 0800 111 999.
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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