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Fix high boiler pressure.
PRV dripping, filling loop left on or expansion vessel gone — drained, fixed and re-pressurised.
✓ Updated May 2026✓ Free quotes✓ Vetted pros
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Four moves that separate a smooth job from a nightmare.
Repeated PRV discharge
— expansion vessel needs attention.
Radiator bleed used to release water
— gradual approach, repeat as pressure rises.
System needs power flushing
— sometimes high pressure linked to dirty system.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Engineer diagnosisHour minimum
£80–£200
Expansion vessel re-pressuriseOften possible
£120–£300
Expansion vessel replacementIf non-rechargeable
£250–£500
External expansion vessel installIf internal fails
£300–£600
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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High boiler pressure — over 3 bar — can trigger the pressure release valve, releasing water externally, and indicates the expansion vessel may be failing. This guide covers what to do when pressure rises, the DIY fix, and when it's a sign of a major repair.
Why Pressure Rises
- Over-filled during top-up (most common).
- Failed expansion vessel — air pre-charge has gone.
- Faulty pressure gauge (reading incorrectly).
- System over-heated and expanded.
DIY Fix (Reduce Pressure)
- Turn off boiler.
- Locate radiator bleed valves.
- Open one slightly with bleed key — water drips out, pressure drops.
- Close when pressure reaches 1.0-1.5 bar.
- Don't release too much — repressurising is hassle.
When to Call an Engineer
- Pressure rises rapidly when boiler heats — failed expansion vessel.
- PRV constantly discharging.
- Hot to touch pipes that shouldn't be.
Typical Costs
| Issue | Low £ | High £ | Notes |
|---|---|---|---|
| Engineer diagnosis | 80 | 200 | Hour minimum |
| Expansion vessel re-pressurise | 120 | 300 | Often possible |
| Expansion vessel replacement | 250 | 500 | If non-rechargeable |
| External expansion vessel install | 300 | 600 | If internal fails |
UK Regulations
- Gas Safe required for boiler repair.
- DIY pressure reduction via radiator bleed is fine.
Common Problems
- Repeated PRV discharge — expansion vessel needs attention.
- Radiator bleed used to release water — gradual approach, repeat as pressure rises.
- System needs power flushing — sometimes high pressure linked to dirty system.
Frequently Asked Questions
What pressure is too high?
Above 3 bar triggers PRV. Above 2.5 bar normally indicates expansion vessel issue.
Why does PRV keep dripping?
Pressure rising above 3 bar — usually failed expansion vessel.
Can I fix expansion vessel myself?
No — requires gas work and system drain. Engineer only.
How long do expansion vessels last?
10-15 years typical. Some last full boiler life; some fail at 5 years.
Is it dangerous?
Not immediately. PRV is the safety device. But repeated high pressure shortens system life.
Will it damage my boiler?
Over time yes — components stressed by repeated high pressure.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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