Short answer: If your boiler pressure keeps dropping, water is escaping from the sealed heating system or the boiler is pushing it out. Common causes are a slow leak at a radiator valve or under the floor, a pressure relief valve letting by, or an expansion vessel that has lost its charge. Topping up helps short term, but the cause needs finding.
Most London homes with a combi or system boiler have a sealed heating system. That means the water in the radiators and pipes is held at pressure, shown on a small gauge on the boiler. If the pressure falls, the boiler may stop and show a low pressure fault. Topping up gets it going again. If it keeps falling, water is going somewhere.
How a sealed system holds pressure
A sealed system is a closed loop of pipes and radiators. Nothing should leave it. Three parts keep it stable:
- The filling loop. A hose or valve that lets you add mains water to raise the pressure.
- The expansion vessel. A tank with an air cushion inside that takes up the extra volume when the water gets hot.
- The pressure relief valve. A safety valve that lets water out through a pipe to the outside if pressure goes too high.
If any of these fail, or there is a leak in the pipes, the pressure drops.
The common causes
| Cause | What you might notice |
|---|---|
| Weeping radiator valve | Damp or green crust at the valve, tide marks on the floor |
| Leak under floorboards | Pressure falls but no drips anywhere you can see |
| Pressure relief valve letting by | Drips from a small copper pipe outside the wall |
| Expansion vessel lost its charge | Pressure jumps high when hot then falls when cold |
| Leak inside the boiler | Water under the boiler or staining on the casing |
| Recent radiator bleed | Pressure drops after you let air out |
The last one is not a fault. Bleeding radiators lets out air, and pressure falls a bit. Top up once and it should hold.
Why converted Victorian houses are prone to it
Stoke Newington is full of Victorian terraces, and many have been split into two or three flats. That history shows in the heating. Pipework is often run under old floorboards, through joists and between flats. Systems have been extended, cut back and joined over the years. Older joints and valves dry out and start to weep.
In a split house the flat below may be the first to notice. A brown mark on their ceiling under your radiator is a classic sign. If you rent, tell your landlord early. HSE says landlords are responsible for repairs from general wear and tear on gas appliances and flues they provide.
Our Stoke Newington area page has more on the N16 housing.
Hard water makes a small leak worse
Every top up brings fresh mains water into the system. In N16, that water is hard. Thames Water’s 2025 report for the Highbury zone shows an average of 272 mg/l, and Stamford Hill averages 260 mg/l. Both are in Thames Water’s hard band. Fresh water also dilutes the inhibitor, the chemical that protects the system from corrosion. A slow leak topped up for months can leave a system full of scale and sludge. That can lead to noise, which we cover in why boilers kettle.
Safe checks you can do
You should never open the boiler casing. These checks are fine:
- Write down the pressure when the heating is cold, morning and evening, for a few days. This shows how fast it is falling.
- Look at every radiator valve for damp, drips or crust.
- Look outside for the end of the pressure relief pipe, usually a short copper pipe near the boiler. Drips or a wet patch below it are a clue.
- Check ceilings and walls below pipe runs and radiators.
- Top up as your manual says using the filling loop, and close it fully after.
If the pressure shoots up very high when the heating runs, stop topping up and book a check. That points to the expansion vessel.
What an engineer does
A boiler repair visit looks for where the water goes. A Gas Safe registered engineer will check the expansion vessel charge, test the pressure relief valve, inspect the boiler for internal leaks and look at the visible pipework. If the leak is under floors, they can isolate parts of the system to narrow it down.
Fixes range from replacing a radiator valve to recharging or replacing the expansion vessel or the relief valve. Hidden leaks under floors take longer to find. The price is confirmed after a look at the job.
Once the leak is fixed, ask about restoring the inhibitor level. A system that has had lots of fresh water may also need a clean.
When to act fast
Book a repair soon if:
- you are topping up more than once a week
- there is water on a ceiling or wall
- the boiler locks out even after topping up
- the pressure relief pipe is dripping all the time
If water is coming through a ceiling near light fittings, turn off the electrics to that area at the fuse box if safe and keep clear.
Quick questions
What should my boiler pressure be?
Your boiler manual gives the right figure. Many sealed systems run at around 1 to 1.5 bar when cold, but follow your own manual. Pressure rises a little when the heating is hot.
Is it safe to keep topping up the pressure?
Topping up through the filling loop as the manual describes is a normal user task. Doing it every few days is a sign of a leak. Each top up also brings in fresh hard water, which adds scale.
Can I find the leak myself?
You can look for drips and damp at radiator valves, under the boiler and on ceilings below pipe runs. Do not open the boiler casing. Leaks inside the boiler or under floors need an engineer.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.