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BOILER PRESSURE FAULTS

Boiler Pressure Keeps Dropping

Boiler pressure that repeatedly falls is usually caused by a water leak, a faulty expansion vessel, a leaking pressure relief valve or air being removed from the 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

Visible system leak Radiator valves, joints or pipework may be losing water.
Expansion vessel fault Pressure may rise when hot and fall when cool.
Pressure relief valve leaking Water may discharge outside through the copper relief pipe.
Recent bleeding or maintenance Removing air can reduce system pressure.
◫ Symptom variations Match the exact pressure pattern and timing. ⌄
  • Pressure falls overnight
  • Needs topping up every few days
  • Relief pipe drips outside
  • Pressure rises sharply when heating runs
  • Damp patches near valves or boiler
✓ 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 for visible leaks Allow the boiler and heating system to cool before checks. ⌄

Inspect radiator valves, exposed pipework and the area beneath the boiler.

2 Step 2 — Check the relief discharge pipe Allow the boiler and heating system to cool before checks. ⌄

Look outside for dripping or staining from the copper discharge pipe.

3 Step 3 — Monitor cold and hot pressure Allow the boiler and heating system to cool before checks. ⌄

Note the pressure when cold and again once the heating is fully warm.

⌁ Common causes explained Why boiler pressure faults develop. ⌄

Boiler pressure that repeatedly falls is usually caused by a water leak, a faulty expansion vessel, a leaking pressure relief valve or air being removed from the system.

⌘ Advanced diagnostics Heating-engineer checks and repair work. ⌄
  • Pressure-test the heating circuit.
  • Check expansion vessel pre-charge.
  • Inspect the PRV seat and discharge.
  • Check hidden pipework and heat exchanger.
  • Assess automatic air vents.
▣ Parts that may need replacing Common components linked to this fault. ⌄
  • Expansion vessel
  • Pressure relief valve
  • Automatic air vent
  • Radiator valve
  • Filling loop
! 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.