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Why is my boiler losing pressure?
If you're topping up more than once a month, water is getting out somewhere. Here are the usual causes, what you can check yourself, and when it's a job for us.
- 1.2 to 1.5bar: what to top up to, cold
- 1top-up a month or more means a fault
- £120repair visit, parts priced first
£120repair visit
A heating system is a sealed loop of water. The pressure only drops if water leaves it or air is let out. So when the gauge keeps falling, the question is where it's going.
Sometimes it's simple, like having just bled the radiators. Sometimes it's a small leak or a worn part. Work through the checks below before booking an engineer.
The usual causes
Most pressure loss comes down to one of four things.
You've bled the radiators
Bleeding lets air out, and the pressure drops with it. That's normal. Top it up once and it should hold.
A small leak
A weeping radiator valve or pipe joint can lose enough water to drop the pressure over days or weeks. Older pipework, especially under floors, is where these start.
The pressure relief valve
If this safety valve is worn or has dirt in its seat, it lets water out through the copper pipe on the outside wall.
The expansion vessel
It soaks up the rise in pressure as the water heats. Once it loses its air charge, the pressure shoots up when the heating runs, the relief valve lets water out, and then the pressure falls when it cools. This is common on boilers over about 10 years old.
Valves and joints
What you can check yourself
Five minutes with a torch.
- Radiator valves and pipe joints: look for drips, damp or crusty green or white deposits
- Ceilings and floors under radiators for stains
- The copper pipe on the outside wall near the boiler: is it dripping, or is the wall wet below it?
- Under the boiler for drips or a wet shelf
- Whether anyone has bled the radiators recently
Watch the gauge too. If it jumps well above 2 bar every time the heating comes on and then falls back, the expansion vessel is the likely cause.
Expansion vessel
On a combi
The expansion vessel is usually inside the boiler casing, so checking or recharging it is an engineer job.
How to top up safely
Using the filling loop under the boiler. Some Worcester boilers have a built-in filling key instead; the manual shows where.
Heating off
Switch off and let the system cool, so you get a true reading.
Find the loop
Find the silver braided hose with a valve at each end under the boiler.
Open slowly
Open the valves slowly and watch the gauge rise.
Close fully
Close both valves at 1.2 to 1.5 bar, then switch back on and check it fires.
Once is fine. Every few weeks isn't.
If you need to top up more than once a month, there's a fault. Topping up over and over also brings fresh oxygen into the system, which speeds up corrosion inside it.
When to call us
And what we'll do when we get there.
- You're topping up more than once a month
- The pressure drops within hours or days of topping up
- Water is dripping from the pipe outside, or there's damp under a radiator
- The boiler shows a fault code again after topping up
Our Gas Safe registered engineers check the boiler, the expansion vessel, the relief valve and the pipework to find where the pressure is going. Typical fixes are recharging or replacing the expansion vessel, a new relief valve, a new filling loop or repairing a leak.
A repair visit is £120 inc VAT. If we can fix it on the visit, that's all you pay. If a part is needed, we tell you the price before fitting it.
Finding the cause
Which one sounds like you?
- Just bled the radiatorsTop up once
It should then hold.
- Drops every few weeksFind the leak
Book a repair visit.
- Shoots up, then dropsExpansion vessel
Usually a recharge or a new vessel.
- Water from the pipe outsideRelief valve
Often the valve, sometimes the vessel.
Common questions
How often is it normal to top up boiler pressure?
Rarely. Once after bleeding the radiators is normal. If you're doing it more than once a month, something needs fixing.
Can I use the boiler while the pressure keeps dropping?
Yes, as long as you keep it topped up and there's no water coming from the boiler itself. Get it looked at soon, as the cause usually gets worse.
Why is water dripping from the pipe outside?
That's the relief valve pipe. Water comes out when the pressure gets too high, often because the expansion vessel has lost its charge, or because the valve itself is worn.
Can I recharge the expansion vessel myself?
On most combis it's inside the boiler casing, so no. Only a Gas Safe registered engineer should open the boiler.
Is it a big job to fix?
Usually not. Most pressure faults are a recharge, a valve or a small leak. We tell you what's wrong and what it costs before doing any work.
How much is a repair visit?
£120 inc VAT. If we can fix it on the visit, that's all you pay. Any parts are priced before we fit them.
Related pages
Not sure what you need? Give us a shout.
Book online in a couple of minutes or give the office a call, Mon to Fri, 9am to 5pm. We'll tell you what we think, and if the answer is to leave things as they are, we'll say that too.
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