Boiler pressure low or high: how to check and top it up
No heating, a fault code on the boiler and the needle on the gauge near zero. Low pressure is one of the most common reasons a sealed heating system stops working, and topping it up is a five-minute job. Working out why it dropped is the part that matters.
Guide · The Honest Fix
Topping up and reading the gauge are DIY. Anything inside the boiler is not.
Tools and materials
- Your boiler manual (most are online by model number)
- The filling loop (braided hose) or the boiler’s built-in filling key or lever
- An old towel and a small container for drips
- A torch for reading the gauge under the boiler
What boiler pressure is and when it matters
Most homes with a combi or system boiler have a sealed (pressurised) heating system. The water in the radiators is kept under a little pressure, shown on a gauge on the front or underneath the boiler. If the pressure drops too low, the boiler protects itself by shutting down.
If you have a regular (heat-only) boiler with a small tank in the loft that feeds the heating, you have an open-vented system. There’s usually no pressure gauge to read and nothing to top up. This guide doesn’t apply.
What the gauge should read
- Cold system (heating off for a while): usually between 1 and 1.5 bar. Your boiler manual has the exact figure.
- Heating running: it’s normal for the pressure to rise a little as the water heats up and expands. Check the manual for your boiler’s upper limit.
- Below about 0.5–1 bar: many boilers will show a low-pressure fault and stop.
- At or above 3 bar: the pressure relief valve opens and dumps water through a pipe to the outside. Too high.
Gas safety: the law
Only a Gas Safe registered engineer may work on a gas boiler. Don’t remove the boiler casing or adjust anything inside it. Topping up and resetting a boiler, as described here, don’t involve opening the boiler.
If you smell gas, or suspect carbon monoxide (headaches, dizziness or nausea that ease when you leave the house, or a yellow or lazy flame), turn the appliance off if it’s safe to do so, open windows, get everyone out and call the National Gas Emergency Service on 0800 111 999. In Northern Ireland, call the NI Gas Emergency Service on 0800 002 001.
How to top up low pressure
- Turn the heating off and let the system cool. You’ll get a misleading reading from a hot system, and it’s easy to overfill.
- Find the filling loop. On most combis it’s a short silver braided hose under the boiler, with a valve at one or both ends. Some boilers have a built-in key, lever or blue tap instead. Check the manual.
- Connect the loop if it’s been removed. Screw it on hand-tight at both ends.
- Open the valves slowly. Open one valve fully, then open the other gradually. You’ll hear water flowing in. Keep your eye on the gauge.
- Stop at around 1–1.5 bar (or whatever the manual says), then close both valves firmly.
- Disconnect the loop if it’s the removable type, with a towel underneath for the drips. Water regulations guidance says the hose should be disconnected when it’s not in use, to stop heating water getting back into the drinking water supply.
- Reset the boiler if it’s showing a fault (usually a reset button, or follow the manual), then turn the heating back on.
- Check the gauge again over the next few days.
How to bring high pressure down
If the pressure is above about 2.5 bar when cold, it’s usually because the filling loop was left slightly open or was overfilled.
- Make sure both filling loop valves are fully closed.
- Turn the heating off and let the radiators cool.
- Bleed a radiator, ideally one downstairs, with a cloth and tray ready. Let a little water out at a time and check the gauge between goes.
- Stop when you’re back in the normal range, then close the bleed valve.
If the pressure creeps back up again with the filling loop closed, the filling valve may be letting by. That’s a job for an engineer.
Stop here: call a pro if…
- You need to top up more than once every few months. Water is escaping somewhere: a leak, a weeping valve, or the pressure relief valve.
- There’s water dripping from the copper pipe that runs from the boiler through the outside wall.
- The pressure shoots up when the heating comes on and drops when it goes off. This often points to the expansion vessel losing its charge.
- You can see water, staining or corrosion around the boiler or on the pipes beneath it.
- The boiler still won’t fire after topping up and resetting, or shows a different fault code.
- Anything involves taking the boiler casing off. That’s Gas Safe territory, no exceptions.
DIY or Hire?
Topping up and bringing pressure down: DIY. It costs nothing and takes minutes. Calling someone out just for this is a waste of money once you know how.
Finding out why it keeps dropping: hire. The usual causes (a hidden leak, a failed pressure relief valve, an expansion vessel that needs recharging or replacing) need an engineer. Some of them involve the boiler itself, which must be done by someone on the Gas Safe Register. Repeatedly topping up also brings fresh oxygenated water into the system, which speeds up corrosion inside your radiators.
Common mistakes
- Topping up with the heating hot, then finding it’s over 3 bar once it cools.
- Opening the filling loop valves fully and walking away.
- Leaving the loop connected and slightly open, so the pressure creeps up.
- Ignoring a pressure relief pipe that’s dripping outside.
- Topping up every week instead of getting the cause found.
- Forgetting to check the pressure after bleeding radiators.
FAQs
Why does my boiler pressure drop after I bleed the radiators?
Bleeding lets water out along with the air, so the pressure drops. Top it back up afterwards.
Is it dangerous if the pressure is too high?
The pressure relief valve is designed to release excess pressure. But a system that keeps hitting 3 bar will wear the valve and needs looking at. Bring it down by bleeding a radiator and get the cause checked.
I can’t find a filling loop. Where is it?
Some boilers have it built in behind a flap, or it may be on the pipework near a hot water cylinder. Look up your model’s manual. If you still can’t find it, ask an engineer to show you at the next service.