Fault guide
Boiler losing pressure: causes and what to do
Most combi and system boilers need about 1 to 1.5 bar when the heating is cold. If the pressure drops below about 1 bar, the boiler may show a fault code and stop working.
Phones answered 7am–8pm, 7 days a week. Gas Safe registered 943380.
Further work and parts are always priced and agreed before we carry on.
What it means
Pressure loss means water is leaving the sealed heating system or the system can’t cope with expansion. Topping up gets you going again, but if you have to top up more than a couple of times a year, something needs fixing.
Common causes
- A small leak on a radiator valve, joint or pipe (look for damp or green/white crusting)
- The pressure relief valve letting by: check the copper pipe outside for drips
- A flat expansion vessel: pressure shoots up when the heating is hot, then drops
- Water lost after bleeding radiators
- A leak inside the boiler
Safe checks you can do
- Check the pressure gauge on the front of the boiler. With the heating cold, it should usually read between 1 and 1.5 bar.
- If it is low, top it up using the filling loop. This is usually a silver braided hose or a tap or lever under the boiler. Your user manual shows how. Open slowly, watch the gauge reach about 1.2 bar, then close it fully.
- Check radiator valves, visible pipework and the outside discharge pipe for drips.
- If the pressure drops again within days, book a repair. Constant topping up brings in fresh oxygen and causes corrosion inside the system.
When to call an engineer
If the checks above don’t clear it, or the fault keeps coming back, the boiler needs a Gas Safe engineer to diagnose it. Our diagnostic visit is £144 including VAT, covering attendance and up to the first hour. We tell you the repair price before doing any further work.
When you call, tell us the boiler make and model, the exact code on the display and what you have already tried.
Other guides
Need a repair? Boiler repair in Portsmouth · Southampton · Guildford