Boiler pressure keeps dropping: what to check next

Start with the heating-system pressure

This guide concerns the pressure in a sealed heating circuit, usually shown on the boiler display or gauge. It is different from the incoming mains-water pressure at a tap. Some older open-vented systems work differently, so identify your boiler and system before following any instructions.

Use your appliance’s user manual for its normal cold operating range and permitted user actions. A reading taken with the heating running cannot be compared directly with a cold reading: pressure normally changes as the water heats and cools.

Useful observations you can make safely

• Record the displayed reading with the system cold, and whether it drops over hours, days or weeks.
• Note any fault code, whether heating or hot water stops, and the date of the last top-up or radiator bleeding.
• Look at accessible radiator valves and visible pipe joints for dampness, staining or water on the floor. Do not lift floors or remove the boiler casing.
• If an outside discharge pipe is safely visible from ground level, note any dripping. Do not touch hot discharge water, block the pipe or confuse it with normal condensate drainage.

A photograph of the gauge and any visible damp area can help when booking. No visible leak does not rule out concealed pipework or an internal appliance problem.

What the pattern may suggest

Persistent loss can involve leakage from the heating circuit or water escaping through a pressure-relief discharge. A large pressure change as the system heats may also prompt an engineer to assess the expansion vessel. These are possibilities to investigate, not a diagnosis from a gauge reading.

For example, a system that rises sharply when hot and then reads low when cold needs a different investigation from one with a steadily dripping radiator valve. Tell the engineer about the complete pattern rather than only the final low reading.

Should you keep topping it up?

Follow only the filling method described in the user manual for your exact appliance, and only if you understand it. Stop and seek advice if the method is unclear, the pressure becomes too high or the system will not hold pressure. Do not use a generic filling-loop instruction for an unfamiliar boiler.

Repeated replenishment is not a repair. Arrange an assessment when loss recurs, especially when the interval between top-ups shortens. If you rent, report the symptoms to your landlord or managing agent before arranging work.

What a diagnostic visit should establish

Share the make, model, fault history and cold/hot readings. The engineer can assess visible and concealed-loss indications, discharge behaviour and the appliance’s expansion arrangements as appropriate. The finding should explain the recommended repair and whether further investigation is needed.

At SwiftHeat UK, an initial diagnostic visit is £85 including VAT. Any further parts or repair work are discussed and quoted separately. A routine boiler service and investigating a recurring fault are different appointments; mention pressure loss when booking.

Pressure loss alone does not establish that a new boiler is needed. We assess the fault and wider system before discussing repair or replacement.

If the assessment identifies a replacement need, our boiler installation guide explains the survey, supporting work and handover to discuss before booking.

Arrange the right next step

Assess First. Recommend Second.

Discuss a boiler diagnostic visit

Discuss these symptoms with our team

Tell us what you have noticed and your postcode. We will explain the appropriate assessment; submitting an enquiry does not confirm an appointment.

Call or WhatsApp 0800 043 3464 · Send us your heating symptoms.

Explore more Heating Advice · Find your service area