Pressure rises on heating then drops when cool: classic expansion/PRV pattern
Quick summary
If pressure climbs when the heating is on and ends up low when cool, the system may be discharging via the PRV during heat-up.
Safety first
- Do not keep topping up daily — it can mask the real fault and accelerate corrosion.
- If you see water discharging outside, don’t ignore it.
- Smell gas or suspect a leak: ventilate, leave the area, and call 0800 111 999.
- CO alarm sounding or anyone unwell: get fresh air and get urgent help.
Full checklist: Safety guidance
What to check (in order)
- Record pressure cold, then again at full operating temperature (after 15–30 mins).
- After the system cools fully, record pressure again.
- Check PRV discharge pipe outside for any dripping/white staining.
- Check the filling loop is fully closed and not passing.
- Note whether the boiler cuts out on overheat or shows pressure-related faults.
What the result means
- Big rise on heat-up + discharge evidence: PRV lifting on expansion is likely.
- Ends low after cool-down: consistent with water lost via PRV.
- No discharge evidence but large swings: gauge may be inaccurate or discharge is intermittent/out of sight.
What you can safely do
- Record the cold pressure before the heating comes on.
- Watch how high the pressure goes when hot.
- Check the outside discharge pipe for signs of PRV lifting.
- Avoid repeatedly topping up the system while you are trying to confirm the pattern.
When to call a professional
- Any PRV discharge evidence.
- Pressure regularly ends up too low to run the boiler.
- You suspect expansion vessel issues or repeated pressure faults.
Engineer notes
Document cold fill, hot peak, and cooled pressure. Classic failure: vessel waterlogged/flat → pressure rises rapidly → PRV lifts → cool ends low. Confirm vessel charge, connection integrity, and PRV seat. Verify actual pressure with a known gauge if needed.
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