Pressure rises on heating then drops when cool: classic expansion/PRV pattern
Quick answer
If your boiler’s pressure climbs noticeably when the heating is on and then settles low once everything cools down, this pattern often points to the pressure relief valve discharging water during the hot phase — commonly linked to the expansion vessel. Record cold, hot, and cooled-down readings, and check the outside discharge pipe for signs of dripping. If you find evidence of discharge, or pressure keeps ending up too low, call a heating engineer.
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 you may notice
- Pressure rises a lot once the heating has been on for 15–30 minutes — may suggest the expansion vessel isn’t absorbing the water’s expansion as it should.
- Pressure ends up lower than normal once the system has cooled — can be consistent with water having been lost via the pressure relief valve during the hot phase.
- Dripping or staining on the outside pressure relief pipe — supports the idea that the valve has been lifting.
These are possible explanations, not a confirmed diagnosis. An engineer can confirm the exact cause on inspection.
Safe checks to make, 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 you can do safely
- 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.
What to tell the engineer
- Which of the checks above you carried out, and what you found at each step.
- Roughly when the problem started and whether it’s constant, intermittent, or linked to specific conditions (e.g. only when heating runs, only in cold weather).
- Any fault code, unusual noise, or visible sign you noticed.
- Whether the issue affects one fixture/radiator or several.
FAQs
What’s an expansion vessel?
It’s a component inside or near the boiler that absorbs the extra volume as water expands when heated, helping keep pressure stable. If it isn’t working correctly, pressure can swing more than it should.
Should I keep topping up if pressure keeps ending up low?
Repeatedly topping up without addressing the cause isn’t recommended, as it can mask the fault and, over time, may not be good for the system. It’s better to record the pattern and get an engineer to check the expansion vessel.
How do I know if my gauge is just inaccurate rather than there being a real problem?
If you see large swings but no evidence of discharge outside, this is worth mentioning to an engineer, who can verify the reading with their own equipment.
Is this the same issue as boiler pressure too high?
It’s related — both can involve the expansion vessel — but this guide is specifically about the pattern of rising when hot and dropping when cool. See our guide on boiler pressure too high if your pressure stays elevated rather than following this pattern.
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