Boiler Pressure Keeps Dropping? Causes & Fixes UK 2026
If your boiler pressure keeps dropping, it usually means water is leaving the system or pressure isn’t being managed correctly. In many UK homes this is caused by a leak, a faulty pressure relief valve, or an expansion vessel issue. This guide explains the most common causes, safe checks you can do yourself, and when you need an engineer.
What Boiler Pressure Should Be
Most sealed systems sit around 1–1.5 bar when cold (check your manual). Pressure rises when the heating is on. If it regularly drops below the boiler’s minimum, the boiler may shut down or lock out.
Common Cause 1: A Leak Somewhere in the System
Even small leaks at radiator valves, pipe joints, or under floors can reduce pressure over time. Look for damp patches, corrosion marks, staining around valves, or frequent need to top up.
Common Cause 2: Pressure Relief Valve (PRV) Discharge
If the system pressure gets too high, the PRV releases water via an external pipe. A dripping discharge pipe (often outside) can slowly drop your pressure. PRVs can also weep after they’ve opened and may need replacing.
Common Cause 3: Expansion Vessel Problems
The expansion vessel absorbs pressure changes as water heats and expands. If it’s failed or incorrectly charged, pressure can swing high when hot and then fall low when cold, often triggering PRV discharge.
Safe Checks You Can Do
- Check visible leaks: radiators, valves, and accessible pipework.
- Check the discharge pipe: look for dripping outside when the heating is on.
- Note pressure pattern: does it rise a lot when hot and drop when cold?
Avoid repeatedly topping up without addressing the cause—fresh water introduces oxygen and increases corrosion risk.
When to Call an Engineer
If you can’t find a leak, the PRV is discharging, or pressure swings wildly, book a heating engineer. Expansion vessel and PRV issues usually require professional tools and safe procedures.
Considering a Replacement Boiler?
If your boiler is old and you’re dealing with repeated faults, replacement can be more cost-effective. Use our calculator to estimate the right kW output range for your home.
Use the Boiler Size CalculatorFrequently Asked Questions
How quickly should boiler pressure drop?
It shouldn’t drop quickly on a healthy sealed system. Slow drops over days/weeks can still indicate a leak or PRV weep.
Is it okay to keep topping up the boiler pressure?
Occasional topping up may be fine, but frequent topping up suggests a fault. It can increase corrosion risk over time.
Could bleeding radiators cause pressure to drop?
Yes. Bleeding releases air and a small amount of water, which can reduce pressure on sealed systems.
What if my pressure rises a lot when heating is on?
Large hot-pressure rises often point to an expansion vessel issue and can lead to PRV discharge.
When should I call a professional?
If pressure keeps dropping after topping up, you see PRV discharge, or you can’t find a leak, book an engineer.