Boiler pressure keeps dropping: what it means

Topping the boiler up once after a radiator has been bled is normal. Topping it up every week is not. A sealed heating system is closed, so if it is losing water, that water is going somewhere.

What the pressure should read

On a sealed system the gauge should sit at roughly 1 to 1.5 bar when the heating is cold, and rise to somewhere around 2 bar when everything is hot. Some manufacturers specify slightly different figures, so the boiler manual wins over any general rule.

A small rise when hot and a return to the same cold figure is exactly what a healthy system does. What you are looking for is the cold reading drifting down week after week.

How fast is it dropping

This is the most useful diagnostic you can do yourself, so note it before ringing anyone.

• Weeks or months between top ups: a very small leak, or a slowly failing expansion vessel

• Days: a real leak, and it is losing enough water to find

• Hours, or during every heating cycle: something significant, and the boiler should be off

Where the water actually goes

Radiator valves. The most common leak in any house. Look for a green or white crusty deposit around the valve spindles and unions, which is dried inhibitor and the fingerprint of a slow weep. Run a dry tissue around each union rather than trusting your eyes.

Under floors and in walls. Pipework buried in a screed floor or chased into a wall can leak for a long time before anything shows. Warm patches on the floor, a section of carpet that never quite dries, or a boiler that loses pressure only when the heating runs are the clues.

The pressure relief valve. If the discharge pipe through the outside wall is wet or dripping, the system is pushing water out. That is usually not a valve fault, it is the expansion vessel failing to do its job. See our page on water leaking from a boiler.

The expansion vessel. The classic pattern is pressure that reads correctly cold, climbs sharply past 2.5 bar as the system heats, dumps out of the relief valve, and then reads low again once cold. That cycle repeats until someone recharges the vessel or replaces it.

The filling loop. A loop left even slightly open lets mains water in, which raises pressure rather than dropping it, and it is worth ruling out because it also causes relief valve discharge. Close it fully and disconnect it after topping up.

The auto air vent and the towel rail. Both are frequently overlooked. Towel rails in bathrooms leak quietly into a room that is meant to be damp anyway.

Why you should not just keep topping it up

Every top up puts fresh mains water into the system, and fresh water carries dissolved oxygen. Oxygen plus steel radiators produces magnetite, the black sludge that blocks radiators and heat exchangers. It also dilutes the corrosion inhibitor a little further each time.

So a system topped up weekly for a year is quietly manufacturing the problem that leads to a power flush later, on top of whatever the original leak was.

Finding a leak you cannot see

If the obvious places are dry, the job becomes leak detection rather than guesswork. Pressure testing sections of the system, thermal imaging on solid floors, and isolating circuits one at a time will locate it without pulling up half a floor speculatively.

That is worth paying for. Lifting flooring at random is how a small repair becomes a large one.

Getting it sorted

We find the leak first and then price the repair, because the two are different jobs and pretending otherwise helps nobody.

See boiler repair, burst and leaking pipes and radiator repairs and installation for related work. We cover Manchester, Salford, Stockport and the surrounding towns.

Ring 0161 515 9977 and tell us how quickly it drops. That alone narrows it down.