Boiler Losing Pressure? 5 Things to Check Before You Call a Gas Engineer
If your boiler keeps losing pressure, the water in your heating system is escaping somewhere. That usually means one of three things: a leak (often a small one you can’t see), a pressure relief valve that’s letting water out, or an expansion vessel that has lost its charge.
The first two can often be spotted with a few safe checks at home. The third needs an engineer.
Most combi and system boilers run at roughly 1 to 1.5 bar when the heating is off and cold. Check the manual for your model, because the exact figure varies. If the pressure drops a bit once in a while, topping it up is normal. If you find yourself topping it up every week or every few days, something is wrong.
Which boilers this applies to
Pressure loss only happens on sealed systems. Combi boilers and system boilers have a pressure gauge on the front or in the display, and they’re filled from the mains through a filling loop.
If you’ve got a regular (heat-only) boiler with a feed and expansion tank in the loft, the system is “open-vented”. It doesn’t run under pressure in the same way. There won’t be a pressure gauge to watch, and the problems it gets are different.
1. Read the gauge properly
Check the pressure when the heating has been off for a while and the system has cooled down. Pressure goes up as water heats and expands, so if you read it with the heating on, you’ll get the wrong idea.
| Reading (system cold) | What it usually means |
|---|---|
| Below about 0.5 bar | Too low. Many boilers will lock out and show a low-pressure fault code |
| Around 1–1.5 bar | Normal for most boilers (check your manual) |
| Rising towards 2–2.5 bar when hot | Normal expansion as the water heats up |
| At or near 3 bar | Too high. The pressure relief valve is likely to open |
Write down the reading and the date. A simple log showing how fast the pressure drops is really useful to an engineer.
2. Think back: have you bled any radiators recently?
Bleeding radiators lets air out, and the pressure drops as water moves in to take its place. If you’ve recently bled one or more radiators, a one-off drop is expected. Top the system back up once and keep an eye on it.
If it holds steady after that, you’ve probably found the cause.
3. Look for visible leaks
Walk round the house with a torch and some dry kitchen roll. Check:
- radiator valves, where the pipe goes into the valve and where the valve joins the radiator
- towel rails and any radiators that were moved or added recently
- visible pipework under the boiler and in airing cupboards
- the bottom of the boiler casing, looking from the outside only
Watch for small signs as well as drips: green or white crusting on joints, rust marks, stained floorboards or damp patches on ceilings under upstairs rooms.
Don’t take the boiler casing off. On a gas boiler, anything inside the casing is work for a Gas Safe registered engineer. If water is coming from inside the boiler, turn it off and get it looked at.
4. Check the pressure relief valve pipe outside
Every sealed system has a pressure relief valve (PRV). It’s a safety device that lets water out if the pressure gets too high. It usually drains through a small copper pipe that runs through the outside wall near the boiler, often pointing down towards the ground.
Look at the end of that pipe, ideally just after the heating has been running. Drips, wet brickwork below it, or a white stain are all signs the valve has been letting water out.
A PRV can let water out for two reasons:
- The pressure is going too high. This often happens when the system has been overfilled, or when the expansion vessel isn’t doing its job (see below).
- The valve itself is worn and doesn’t seal properly any more, often after it has opened a few times.
Either way, this needs an engineer to diagnose it.
5. Top up correctly, then watch what happens
Most combi and system boilers have a filling loop. This is either a braided hose with two taps underneath the boiler, or a built-in key or lever. Follow the manufacturer’s instructions for your model. The general idea is:
- Turn the heating off and let the system cool.
- Open the filling loop valves slowly and watch the gauge.
- Close them once the gauge reaches the recommended pressure (often around 1–1.5 bar).
- Make sure both valves are fully closed. If you have an external hose, disconnect it if the instructions say to.
- Reset the boiler if it’s showing a fault.
Overfilling is common, so take it slowly. If the pressure goes too high, the PRV may open, and then you’ve got a new problem.
After topping up, keep an eye on the gauge for a few days. How quickly the pressure falls tells you a lot.
Why the pressure keeps dropping after you top it up
If the pressure keeps falling and there’s no visible leak or dripping PRV pipe, the most likely causes are:
- A hidden leak under floors, inside walls or in a boxed-in section of pipe. Microbore pipework in older homes and pipes under concrete floors are common culprits.
- A leak inside the boiler, for example from a pump seal, heat exchanger or internal pipe.
- An expansion vessel that has lost its air charge. The vessel absorbs the extra volume as water heats up. When it stops working, the pressure jumps up when the system is hot and falls when it’s cold. That often makes the PRV open, so water is lost each time. A typical sign is pressure that swings a long way between hot and cold.
Recharging or replacing an expansion vessel is a job for an engineer. On most combi boilers the vessel sits inside the casing.
Why topping up every week is a bad idea
Topping up keeps the boiler running for now, but it isn’t a fix. Every time you add fresh water, you also add oxygen and dilute the corrosion inhibitor in the system. Over time that speeds up rust and sludge building up inside radiators, pipework and the boiler’s heat exchanger. A small leak left for months can end up causing much more expensive problems.
If you’re topping up more than once a month or so, book an engineer.
When to stop and call an engineer straight away
- There’s water coming from inside the boiler casing.
- Water is dripping near electrics, or through a ceiling.
- The pressure drops to zero again within hours of topping up.
- The boiler keeps locking out and resetting it doesn’t help.
- You can smell gas. This isn’t a pressure problem, but the rule applies whatever else is going on: leave the property, don’t use electrical switches, and call the National Gas Emergency number on 0800 111 999.
When you book, it helps to tell the engineer the boiler make and model, the fault code if there is one, how quickly the pressure drops, and whether the PRV pipe outside is dripping.
FAQs
Is low boiler pressure dangerous?
Not usually. Low pressure is more a nuisance than a hazard. Modern boilers are designed to shut down safely when the pressure is too low. The risk is from the cause: a leak can damage the property, and repeated topping up speeds up corrosion inside the system.
Can I recharge the expansion vessel myself?
It’s not recommended. On most combi boilers the expansion vessel is inside the casing, and on a gas appliance that’s Gas Safe engineer work. Even where the vessel is outside the boiler, the system needs draining down properly and the correct charge setting to get it right.
Why does my boiler pressure go up when the heating is on and then drop again?
A small rise when hot is normal, often up to around 2 bar. A big swing, for example from 1 bar cold to near 3 bar hot, often points to an expansion vessel that has lost its charge.
Does a boiler service catch pressure problems?
A service should include checking the system pressure and, on many boilers, the expansion vessel charge. It won’t necessarily find a hidden leak somewhere else in the house.
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