Guide
Boiler losing pressure? The causes and the fixes
A boiler should read 1 to 1.5 bar when cold. If it keeps falling, water is leaving the system: a leak, a pressure relief valve that is not closing fully, or a failed expansion vessel. Topping up once is fine; every week is a fault.
What the gauge should read
Combi and system boilers run a sealed heating circuit under pressure, and the gauge on the front, or the number on the display, shows it. Manufacturers generally ask for 1 to 1.5 bar with the system cold. It rises as the water heats and expands, and a reading a little under 2 bar with the radiators hot is normal.
| Reading | What it means |
|---|---|
| Below 1 bar cold | Low. The boiler may show a fault code or refuse to fire |
| 1 to 1.5 bar cold | Normal |
| Up to about 2 bar hot | Normal expansion |
| Close to 3 bar hot | Too high. The relief valve is about to open |
Pressure that has dropped once, after radiators were bled or a radiator was changed, is expected. Pressure that falls again within days or weeks of topping up means water is leaving the system, and it has to be going somewhere.
Topping it up yourself
The filling loop is a short braided hose or a built-in lever under the boiler, connecting the mains cold water to the heating circuit. Your boiler manual shows which you have.
- Switch the heating off and let the system cool.
- Open the first valve on the filling loop, then slowly open the second. You will hear water flowing.
- Watch the gauge and close both valves as it reaches 1 to 1.5 bar.
- If the loop is a detachable hose, disconnect it as the instructions say.
- Reset the boiler if it is showing a fault, and turn the heating back on.
If you overshoot, let a little water out of a bleed valve on a radiator to bring it back down.
Where the water goes
There are five usual causes, and they are worth checking in this order, because the first ones cost nothing.
Air let out. If radiators were bled recently, the pressure drop is the space the air used to take up. Top up once and it should hold.
A small leak on a radiator or pipe. Radiator valves are the most frequent source, weeping at the gland nut under the cap or at the joint to the pipe. Wipe each valve with a dry tissue and look for damp, green or white crust. Then look for stains on ceilings and for damp at the foot of walls near pipe runs.
The pressure relief valve passing water. Every sealed system has a safety valve that opens at about 3 bar and discharges through a copper pipe, usually out through the wall near the boiler. A drip from that pipe, or a damp stain beneath it, means the valve is opening or not closing fully.
A failed expansion vessel. The vessel inside the boiler absorbs the water's expansion as it heats. When it loses its air charge, pressure shoots up every time the boiler fires, the relief valve dumps water, and the gauge reads low again once it cools. Big swings between cold and hot readings are the sign to look for.
A leak inside the boiler. Water marks under the casing or in the bottom of the boiler cupboard point to a leak from a component inside. The casing seals the combustion chamber, and only a Gas Safe registered engineer should open it.
A simple drop test
Top up to 1.5 bar cold, write the reading down, and leave the heating off for a day. If the pressure holds cold but falls after a few heating cycles, suspect the expansion vessel and relief valve. If it falls with the heating off, the water is escaping from a leak somewhere on the pipework or radiators. That one observation narrows the search and saves time when an engineer arrives.
What fixing it costs
| Job | Typical price |
|---|---|
| Diagnostic visit, first hour | £70 to £120 |
| Expansion vessel replacement | £180 to £350 |
| Thermostatic valve replacement, each | £60 to £120 |
| Non-invasive leak detection survey | £250 to £600 |
Most firms credit the diagnostic fee against the repair. A leak hidden under a floor is where a detection survey earns its fee, because it finds the leak without lifting boards room by room, and many buildings insurance policies cover that search under trace and access.
Mistakes that make it worse
Leaving the filling loop open is the most common. It lets mains pressure into the circuit, the relief valve opens, and you end up with a dripping discharge pipe on top of the original fault. Topping up every few days for a whole winter is the second: each fill brings fresh oxygenated water into the system, which speeds up corrosion and sludge. And painting over or boxing in a weeping valve hides the evidence a plumber needs.
Common questions
Is low boiler pressure dangerous?
No. A boiler with low pressure protects itself by shutting down or refusing to fire. The risk is a cold house, not a safety one. High pressure is handled by the relief valve, which is why that valve should never be capped or blocked.
Why is water dripping from a pipe outside near my boiler?
If it is the copper pipe from the boiler, that is the pressure relief discharge, and it means the system has gone over pressure or the valve is not sealing. A plastic pipe running to a drain is the condensate pipe, which drips normally while the boiler is running.
How often is it normal to top up boiler pressure?
Once or twice a year, typically after bleeding radiators, is within normal. Monthly or more often means a fault worth tracing.
Will my home insurance pay to find the leak?
Many buildings policies include trace and access cover, which pays to find a leak and open up and make good the floor or wall. Read the policy wording before booking, because some exclude gradual leaks.