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BOILER PRESSURE FAULTS

Boiler Pressure Too Low

Low boiler pressure can follow bleeding radiators, a minor leak, a faulty pressure sensor or gradual water loss from the sealed heating system.

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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

System recently bled Removing air can reduce pressure.
Small water leak Slow leaks may not be immediately obvious.
Filling loop closed or faulty The system cannot be topped up correctly.
Pressure sensor fault The display may show an inaccurate reading.
◫ Symptom variations Match the exact pressure pattern and timing. ⌄
  • Boiler will not fire
  • Low-pressure warning appears
  • Gauge below normal range
  • Pressure drops again after topping up
  • Radiators contain air
✓ 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 — Check the boiler manual Allow the boiler and heating system to cool before checks. ⌄

Confirm the normal cold pressure range for the model.

2 Step 2 — Repressurise carefully Allow the boiler and heating system to cool before checks. ⌄

Use the filling loop only as instructed and stop at the correct pressure.

3 Step 3 — Check whether pressure holds Allow the boiler and heating system to cool before checks. ⌄

Monitor the gauge after the system cools and over the next day.

⌁ Common causes explained Why boiler pressure faults develop. ⌄

Low boiler pressure can follow bleeding radiators, a minor leak, a faulty pressure sensor or gradual water loss from the sealed heating system.

⌘ Advanced diagnostics Heating-engineer checks and repair work. ⌄
  • Check pressure sensor calibration.
  • Inspect filling loop and non-return valve.
  • Pressure-test the system.
  • Check PRV and expansion vessel.
  • Inspect for hidden leaks.
▣ Parts that may need replacing Common components linked to this fault. ⌄
  • Filling loop
  • Pressure sensor
  • Pressure gauge
  • Expansion vessel
  • Pressure relief valve
! 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.