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BOILER PRESSURE FAULTS

Pressure Rises When Heating Is On

A noticeable pressure rise during heating usually indicates the expansion vessel is undercharged, waterlogged or unable to absorb thermal expansion.

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Do not keep topping up a boiler that repeatedly loses pressure

Frequent pressure loss or excessive pressure indicates an unresolved fault. Stop if water is leaking, the relief pipe runs continuously or the boiler enters the red zone.

MOST LIKELY CAUSES

Start with these

Expansion vessel undercharged The air cushion may be too weak.
Expansion vessel diaphragm failed Water fills the vessel and leaves no expansion space.
System overfilled when cold There is little spare volume for expansion.
Circulation or overheating issue Excessive temperature rise can increase pressure.
◫ Symptom variations Match the exact pressure pattern and timing. ⌄
  • Cold pressure normal but hot pressure very high
  • Pressure drops again when cool
  • Relief pipe drips after heating
  • Boiler locks out at temperature
  • Gauge swings widely
✓ Quick checks Simple checks before arranging repair. ⌄
  1. Record the pressure when cold.
  2. Record it again when fully hot.
  3. Check visible radiator valves and pipework for leaks.
  4. Inspect the external relief pipe.
  5. Do not repeatedly repressurise.
1 Step 1 — Record cold pressure Allow the boiler and heating system to cool before checks. ⌄

Check the gauge before the heating starts.

2 Step 2 — Record hot pressure Allow the boiler and heating system to cool before checks. ⌄

Check again once the system is fully warm.

3 Step 3 — Stop repeated repressurising Allow the boiler and heating system to cool before checks. ⌄

Frequent topping up can make the cycle worse if the vessel is faulty.

⌁ Common causes explained Why boiler pressure faults develop. ⌄

A noticeable pressure rise during heating usually indicates the expansion vessel is undercharged, waterlogged or unable to absorb thermal expansion.

⌘ Advanced diagnostics Heating-engineer checks and repair work. ⌄
  • Isolate and check vessel pre-charge.
  • Test expansion vessel diaphragm.
  • Check system fill pressure.
  • Inspect PRV for secondary damage.
  • Assess boiler temperature control.
▣ Parts that may need replacing Common components linked to this fault. ⌄
  • Expansion vessel
  • Schrader valve
  • Pressure relief valve
  • Pressure gauge
  • Temperature sensor
! When to call a heating engineer Conditions that should end basic troubleshooting. ⌄
  • Pressure drops repeatedly.
  • Pressure enters the red zone.
  • The relief pipe drips continuously.
  • A leak is hidden or internal.
  • The expansion vessel or PRV may be faulty.
? Frequently asked questions Common boiler pressure questions. ⌄

What pressure should a boiler show?

Many sealed systems sit around 1 to 1.5 bar when cold, but always follow the boiler manual.

Is frequent repressurising normal?

No. It indicates ongoing water loss or a pressure-control fault.