Boiler keeps losing pressure

Symptoms

• Pressure gauge reads low again days or weeks after refilling
• May need repeated topping up to keep heat working
• Possible damp spots near pipes or the boiler itself

Likely causes

• CommonSmall leak at a pipe joint or valveEven a slow drip can gradually drain system pressure over days or weeks.
• CommonLeaking pressure relief valveA worn relief valve can weep small amounts of water, slowly lowering pressure.
• PossibleLeaking radiator or bleed valveOlder radiators can develop pinhole leaks or loose valve seals.
• PossibleFailing expansion tank or vesselA waterlogged expansion tank can't absorb pressure changes properly, leading to apparent loss.
• Less commonAutomatic air vent stuck open and weepingThese small valves release trapped air but can fail and leak water instead.

What to do

1. Check around the boiler base and all visible pipe joints for damp spots, drips, or mineral staining.
2. Check each radiator, especially bleed valves and connections, for moisture or corrosion.
3. Inspect the pressure relief valve's discharge pipe for signs of recent dripping.
4. Top off pressure to the manufacturer's recommended level if it's currently low, then monitor how quickly it drops again.
5. If pressure drops noticeably within a week, treat it as an active leak needing professional repair rather than continuing to top it off.
6. Avoid ignoring a slow leak; it can lead to bigger corrosion or component damage over time.

Tools and parts

• Flashlight

When to call a pro

Call a heating technician to pressure-test the system and find the leak if it recurs after refilling; ongoing leaks can damage the boiler over time.

Safety