Boiler pressure that keeps dropping means water is leaving the system somewhere. Topping it up is a temporary measure, not a repair.
It is one of the most common heating faults there is, and we see it in homes across Crawley, Horsham, Redhill and the rest of West Sussex and Surrey, most of all in winter when the heating is working hardest. The good news is that most causes are straightforward to identify, and some you can deal with yourself. This guide covers why it happens, how to find the leak, how to top the system up safely, and the point where you need a Gas Safe registered engineer.
What boiler pressure actually is
Boiler pressure is the pressure of the water circulating around your central heating. It is measured in bar and shown on a gauge, usually on the front of the boiler or on the pipework just below it.
This applies to sealed systems, which covers combi and system boilers. If your heating is fed from a small tank in the loft instead, that is an open-vented system. It has no filling loop, and the topping-up steps below do not apply to it.
What the gauge should read
| Reading | Status |
|---|---|
| Below 1 bar | Too low, boiler may not run |
| 1.0 to 1.5 bar cold | Correct |
| Above 2.5 bar | Too high, relief valve may open |
The gauge rises to around 2 bar when the heating is running. That is normal. Always judge the pressure when the system is cold.
Why pressure drops
The causes below run roughly from most common to least common.
Water leaks
By far the most common cause, and even a small leak produces a steady drop. Check radiator valves for damp patches, pipe joints under floors and in ceilings, radiators themselves for corrosion pinholes, the boiler casing, and the pressure relief valve overflow outside.
Older radiators can also split along a seam. Inside the boiler, the pump and heat exchanger can leak too, although that is for an engineer to confirm rather than something to investigate yourself.
Bleeding radiators
Releasing air also releases water. This is normal and expected, and you top up afterwards. Our guide to bleeding a radiator covers the process, including the pressure check at the end.
A faulty pressure relief valve
If the valve is passing it leaks continuously even at normal pressure. The giveaway is water dripping from the overflow pipe outside.
Other signs include:
- A wet patch underneath or near the boiler
- Pressure that drops overnight or after the heating has been on
On most combi and system boilers the relief valve sits inside the casing, so replacing it is a job for a Gas Safe engineer.
Expansion vessel failure
The vessel absorbs the pressure change as water heats and cools. When it fails you get pressure dropping after each heating cycle and the relief valve discharging.
The pattern to look for is a gauge that swings much more than it should: climbing well above 2 bar with the heating on, the relief valve letting water out, then the pressure falling low once the system cools. The vessel needs recharging or replacing, and on most boilers it is inside the casing, so that is an engineer’s job as well.
A filling loop left slightly open
If a loop is not fully shut, water can pass through it in either direction. It can flow back out, which is also a contamination risk to your mains supply, or keep feeding in until the pressure is high enough for the relief valve to dump water. Both valves must be fully closed.
Recent system work
New radiators, pipework or a new boiler will need topping up a few times in the first weeks as air works out. That settles down.
Finding the leak
Work through these checks in order, starting with what you can see.
1. Check the visible radiators and pipes. Damp around valves, drips underneath, wet spots on exposed pipe, dampness at joints. Run your hand around the joints as well as looking, because a slow weep on a hot pipe can dry before it leaves a drip.
2. Check the boiler. Look in the boiler cupboard and underneath the boiler for water, drips from the pump or valves, staining on the casing, and damp on the area directly below it. Do not take the casing off. On most boilers the front panel is part of the sealed combustion chamber, and anything behind it is a Gas Safe engineer’s job.
3. Check the external overflow. Find the pressure relief pipe outside, usually 15mm copper or plastic. Look for dripping, wet ground, or white and green mineral staining that shows it has been dripping in the past.
4. Run an overnight test. Note the pressure before bed with the system cold, use no heating overnight, and read it again in the morning. A significant drop means a leak.
5. Check the hidden places. Ceiling stains, damp patches on walls near radiators, the airing cupboard if you have a hot water cylinder, under floorboards where accessible.
If the pressure drops and you can see no water anywhere, the leak is concealed. That needs proper leak detection rather than more guessing. Leaks can hide inside walls and under floors, and finding them without lifting half the house takes the right equipment. A leak inside the boiler itself is a job for a Gas Safe engineer.
How to repressurise safely
Only do this once you are satisfied there is no obvious leak. Topping up a system that is pouring water somewhere just feeds the leak.
Before you start, make sure the heating is off and the system is cold, have towels ready, and know where your stopcock is. Check the boiler manual too, because some models use an internal filling key or a different procedure.
1. Find the filling loop. Usually under or near the boiler, a flexible metal hose between two valves. Some modern boilers have an internal filling key instead. If yours has one, the manual shows where it goes and which way to turn it.
2. Note the current pressure. You are aiming for 1.0 to 1.5 bar.
3. Open both valves slowly, turning anticlockwise. You will hear water flowing. Go slowly, because it fills faster than you expect.
4. Watch the gauge. The moment it reaches 1.0 to 1.5 bar, close both valves. Do not go past 2 bar. If you overshoot, you can bring it back down by bleeding a radiator, with the system still cold, until the gauge is back in range.
5. Check both valves are fully closed and not dripping. Some filling loops are meant to be disconnected after use. The manual will say, and if yours is one of them, take it off.
6. Turn the heating back on. The pressure will rise to around 2 bar as it heats. That is correct. Check the boiler fires and the radiators warm up.
Monitor it over the following days. If it drops again quickly, you have a leak.
It helps to note the reading and the date each time you top up. If you do need an engineer, knowing how fast the system is losing water helps narrow down where it is going.
When to stop and call someone
You need a Gas Safe registered engineer if:
- You are topping up more than once a month, which means there is a leak to find and fix
- You cannot find the leak, since hidden leaks need detection equipment to locate
- The leak is inside the boiler casing, which is never a DIY job
- Pressure regularly exceeds 2.5 bar, suggesting the expansion vessel or relief valve has failed
- The boiler is showing pressure-related error codes, which point to an internal fault
- You are not confident about any step, because it is safer and may well cost less than putting a mistake right
For anything inside the boiler, including the pump, the expansion vessel and the relief valve, you need a boiler repair from a Gas Safe registered engineer.
Preventing it
- Annual boiler service, which includes checking for exactly this. Ours is from £105.
- Check the gauge monthly, so you notice a slow drop early
- Make sure the filling loop is fully closed after topping up
- Bleed the radiators before winter, then check the pressure and top up
- Look over radiators, valves and visible pipework now and then for drips
- Fix small drips before they become leaks
- Keep the boiler manual somewhere you can find it, so you know your own system’s procedure
- Keep the system protected with inhibitor and a magnetic filter
- If the system water is dirty, consider a system clean, because sludge drives the corrosion that eventually causes leaks
Frequently asked
How often should I need to top up boiler pressure?
In a healthy system, rarely. Once or twice a year after bleeding radiators is normal. If you are topping up monthly or more often, you have a leak that needs finding rather than a topping-up habit.
Will low pressure damage my boiler?
Low pressure itself will not damage it, and most modern boilers cut out below about 0.5 bar to protect themselves. The problem is the cause. A leak left unfixed will do damage, just not to the boiler first.
Why does my boiler lose pressure overnight?
That usually means a slow leak. Check the pressure relief valve overflow pipe outside. If the ground beneath it is wet in the morning, the valve is passing and that is your answer.
Can I overfill my boiler?
Yes. Above about 2.5 bar the pressure relief valve opens and dumps water to protect the system, which wastes water and can lock the boiler out. Stop at 1.5 bar when the system is cold.
What if my boiler has no filling loop?
Some modern boilers have an internal filling key or an automatic filling system instead. The manual explains the procedure for your model. If you are unsure, ask a Gas Safe engineer rather than guessing, because getting it wrong can damage the system.
Why does the pressure go up when the heating is on?
Water expands as it heats, so a rise to around 2 bar with the heating running is normal. If it climbs past 2.5 bar and the relief valve starts letting water out, the expansion vessel has probably lost its charge and needs a Gas Safe engineer.