Quick answer
A boiler usually loses pressure because water is escaping from the sealed heating system: a leak on pipework, a radiator valve or inside the boiler, a faulty pressure relief valve, a failed expansion vessel or bled radiators. Most systems should read about 1 to 1.5 bar when cold. If pressure keeps dropping, have a Gas Safe engineer find the cause.
What pressure should a boiler be?
Combi and system boilers run on a sealed heating circuit with a pressure gauge on the front or underside of the boiler. Many manufacturers recommend around 1 to 1.5 bar when the system is cold, rising a little when the heating is running. Always check your boiler's manual, as figures vary by model. Below roughly 0.5 bar, many boilers lock out to protect themselves; above about 2.5 to 3 bar, the pressure relief valve may open.
Common causes of pressure loss
1. A leak somewhere in the system
Look for drips, damp patches or staining under radiators, around valves and towel rails, along visible pipework and below the boiler. Small leaks often evaporate before you see a puddle, so look for white or green crusting on joints.
2. Bleeding radiators
Releasing air from radiators also releases a little pressure. It is normal to need to top up after bleeding.
3. Pressure relief valve (PRV) letting by
The PRV releases water if pressure gets too high, usually through a copper pipe that runs outside. If that pipe drips or there is a wet patch below it, the valve may be faulty, or the system may be over-pressurising because of another fault.
4. Expansion vessel needs recharging or has failed
The expansion vessel absorbs the increase in water volume as the system heats up. If its air charge is low, pressure rises sharply when hot, the PRV discharges, and the pressure then drops when the system cools. A pattern of high pressure when hot and low when cold points to this.
5. An internal boiler leak
Seals, pumps and heat exchangers inside the boiler can leak. Do not remove the boiler case yourself; only a Gas Safe registered engineer should work inside a gas boiler.
How to repressurise a combi boiler
Most combi boilers have a filling loop, either a braided hose with two valves or a built-in key or lever. With the boiler off and the system cold, follow the manufacturer's instructions: open the valves slowly, watch the gauge rise to the recommended pressure, then close them fully. If you have a removable hose, disconnect it afterwards. If you are unsure, stop and ask an engineer.
When to call an engineer
- You need to top up more than occasionally (for example, every few weeks).
- The pressure climbs very high when the heating is on.
- Water is dripping from the PRV pipe outside.
- You can see water inside or under the boiler.
- The boiler keeps locking out with a low-pressure fault code.
An engineer can pressure test the system, check the expansion vessel charge, test the PRV and trace leaks. Finding the cause early can prevent damage to floors and ceilings and to the boiler itself.
Can low pressure be dangerous?
Low pressure on its own is not a gas safety risk; the boiler is designed to shut down. But any water leak inside a boiler can damage electrical and gas components, so it should be checked. If you also smell gas, follow our gas emergency steps and call 0800 111 999.
CombiCert in Telford
CombiCert Ltd is a Gas Safe registered (621146) gas and heating company based at The Old School House, Park Lane, Telford TF3 4TG, working across Telford and Shropshire for homes and businesses. We are open Monday to Saturday, 08:00 to 19:45 (closed Sunday).
Boiler repair from £90 + VAT (£108 inc VAT). Parts quoted separately.
See Boiler repairs for details, or call or WhatsApp 07393 220694, or email hello@combicert.co.uk.
Frequently asked questions
Is it normal for boiler pressure to drop?
A small, occasional drop is normal, especially after bleeding radiators. Needing to top up regularly, such as every few weeks, is not normal and usually means water is escaping through a leak, the pressure relief valve or a faulty expansion vessel. Have a Gas Safe registered engineer find the cause before damage occurs.
What should my boiler pressure be?
Most sealed combi and system boilers should read around 1 to 1.5 bar when the system is cold, rising slightly when the heating runs. Check the manufacturer's manual for your model. Many boilers lock out below roughly 0.5 bar, and the pressure relief valve may open if pressure goes much above 2.5 bar.
Why does my boiler pressure go up then down?
Pressure that rises sharply when the heating is on, then falls when it cools, usually points to an expansion vessel that has lost its air charge. Excess pressure is released through the pressure relief valve, so less water remains when the system cools. An engineer can recharge or replace the vessel.
Can I fix boiler pressure loss myself?
You can safely repressurise using the filling loop by following the manufacturer's instructions, and look for visible leaks on radiators and pipework. Do not remove the boiler case or attempt internal repairs. Work inside a gas boiler must be carried out by a Gas Safe registered engineer by law.
Will low boiler pressure stop my hot water?
Often, yes. Many combi boilers lock out when pressure falls too low, stopping both heating and hot water until the pressure is restored and the boiler is reset. Repressurising usually brings it back, but if the pressure drops again, there is an underlying fault that needs investigating.
This guide is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.