Why Does My Boiler Pressure Keep Dropping?
For a sealed central-heating system where the boiler pressure repeatedly falls below its normal range after being topped up.
What this usually means
Boiler pressure that repeatedly drops is usually a sign that water is leaving the sealed heating system or that a pressure-control component is not working correctly. Visible radiator or pipework leaks, water from the pressure-relief discharge pipe, and recent radiator bleeding are useful clues. Repressurising may restore operation temporarily, but pressure that keeps falling needs the underlying cause found.
What may be causing this
A leak somewhere in the heating circuit
Clues: Dampness, staining, corrosion or droplets around radiator valves, visible joints, boiler connections or pipework; hidden pipework can also leak without an obvious puddle.
What to do: Inspect only visible accessible parts. Repairing the leak stops the system losing water; concealed or uncertain leaks need professional investigation.
Pressure-relief valve is discharging
Clues: The external discharge/relief pipe associated with the boiler is wet or dripping when it should be dry.
What to do: Do not work on the internal valve. Arrange professional diagnosis because the valve itself or another pressure-control fault may be responsible.
Pressure fell after radiator bleeding
Clues: The drop happened immediately after one or more radiators were bled and does not continue once the system is correctly repressurised.
What to do: Follow the boiler manual to restore the correct cold pressure if you are competent to use the filling loop.
Expansion vessel or another boiler component is faulty
Clues: No visible system leak is found, pressure swings unusually as the heating warms and cools, or pressure repeatedly falls after normal topping up.
What to do: This requires boiler-system diagnosis by a qualified heating engineer; do not remove the casing.
How to check what is happening
A continuing loss is not solved by repeated topping up; water loss or a system fault needs investigation.
A visible heating-system leak is a likely route for pressure loss.
Worcester Bosch identifies discharge here as a clue that the pressure-relief valve may be faulty.
Abnormal pressure swing can point to a pressure-control/expansion issue that needs professional diagnosis.
Can you fix this yourself?
A homeowner can check the gauge and look for visible leaks without opening the boiler. Repressurising through the approved filling loop can be a user task on some boilers, but only follow the exact appliance manual and do not treat repeated topping-up as a repair.
- Boiler user manual
- Torch
- Dry tissue or cloth for checking accessible joints
What you can safely try
- 1
Record the pressure when the system is cold
Check the manufacturer’s normal range for your exact boiler and note the cold pressure. Avoid relying on a generic number if the appliance manual specifies something different.
- 2
Check visible heating-system joints for leakage
Look around radiator valves, radiator connections and accessible heating pipework for fresh water, rust marks, staining or damp finishes. Do not disturb boiler casing panels.
- 3
Check the external relief/discharge pipe
If you can identify the boiler’s pressure-relief discharge pipe outside, check whether it is wet or dripping. Do not block, cap or alter it.
- 4
Repressurise only as your boiler manual instructs
If the manual permits user repressurising and you are confident using the fitted filling loop, restore the specified cold pressure and fully close the filling loop afterwards. Monitor whether the pressure falls again.
Call a heating engineer if…
- The pressure keeps dropping after one correct repressurisation.
- You find an active leak, especially one that is concealed or near electrical equipment.
- The pressure-relief discharge pipe is dripping or wet.
- Pressure rises close to the boiler's upper limit when hot or swings markedly between hot and cold.
- A boiler fault code persists, the filling loop will not close properly, or you would need to remove the boiler casing.
Get the right heating engineer.
We already know this problem maps primarily to a heating engineer. Choose the closest type of work, add your postcode and continue in the form.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- The heating system is constantly losing pressure – what should I do?Worcester Bosch
- What pressure should my boiler be?Vaillant
- What do I do if my boiler has no pressure?Vaillant
Last reviewed: 31 August 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.