Boiler pressure dropping is one of the most common faults we're called out to — and in most cases it's easily resolved. Here's what causes it and what to do.
What should boiler pressure be?
A sealed central heating system should be pressurised to between 1.0 and 1.5 bar when cold (i.e., the heating has been off for an hour or more). Most boilers will lock out if pressure drops below 0.5 bar or rises above 3 bar.
Common causes of pressure loss
1. Small leak in the system
The most common cause. Check all visible pipework, radiator valves, and the boiler itself for signs of moisture or dripping. Even a tiny drip at a radiator valve joint can lose enough water to drop the pressure over weeks.
2. Bleeding radiators recently
Every time you bleed a radiator, you release a small amount of water from the system, which drops the pressure. This is normal — simply repressurise via the filling loop.
3. Faulty pressure relief valve
If the pressure relief valve (usually a copper pipe ending outside the property) is weeping water, the valve is faulty and needs replacing. This is a job for a Gas Safe engineer.
4. Faulty expansion vessel
The expansion vessel absorbs pressure changes as water heats up and cools down. If it fails, pressure fluctuates wildly or drops consistently. A Gas Safe engineer can recharge or replace the vessel.
How to repressurise your boiler
Locate the filling loop (a flexible hose with two valves, usually under the boiler) and slowly open both valves to let mains water into the system. Watch the pressure gauge and close the valves once it reads 1.2–1.5 bar.