Why boilers lose pressure
A slow, occasional drop over weeks or months is often just normal system loss — small amounts of air and water escape from a sealed heating system over time, and topping up now and then is completely routine. It's a different story if the pressure drops quickly, or repeatedly, within days.
- A pinhole leak somewhere in the pipework — sometimes visible as damp patches, sometimes not visible at all if it's under a floor.
- A leaking pressure relief valve — often shows as water dripping from an external pipe outside the property.
- Bled radiators — if you've recently bled air from your radiators, that releases some pressure too.
- A fault inside the boiler itself — a failing expansion vessel or internal seal, which needs a proper inspection to confirm.
What's safe to check yourself
Most boilers have a filling loop — a small braided hose connecting the cold mains to the heating system. If your manual shows one and you're comfortable with it, slowly topping the pressure back up to around 1.2–1.5 bar is safe. If pressure drops back down again within a day or two, that points to a genuine leak rather than normal loss, and it's worth getting looked at rather than repeatedly topping up.
One more reason not to just live with it: every top-up puts fresh, oxygenated water into the system, and fresh water accelerates corrosion inside radiators and the boiler while diluting the inhibitor that's meant to prevent it. Repeated topping up quietly shortens the system's life — finding the leak is cheaper.
Pricing
Diagnosis and first hour: £99. If a part such as a pressure relief valve or expansion vessel needs replacing, that's quoted clearly before it's fitted.
What we test when we arrive
Every diagnosis follows the same discipline as our emergency callouts — our advice article on what we check on every callout covers it in full, but in short: for pressure loss specifically, the engineer checks the pressure relief valve and its discharge pipe, tests the expansion vessel's charge (a failed vessel is one of the most common hidden causes), inspects visible pipework and radiator valves, and works out from the pattern of loss whether the water is leaving the system or the pressure is simply swinging. Electrical and gas-side safety checks are part of every visit as standard.
Likely repairs
The usual fixes are re-pressurising or recharging a failed expansion vessel, replacing a weeping pressure relief valve, tightening or remaking a leaking joint or radiator valve, or repairing a pinhole leak in pipework once it's traced. Parts are quoted before fitting and most repairs carry a 12-month parts warranty. Tracing hidden pipework faults is bread-and-butter work for us — our case studies include a gas leak traced under floorboards in Dudley 2014 the same tracing discipline applies to water.
Not sure pressure is really your fault? The Boiler Problem Finder covers the other possibilities.
Frequently asked questions
How often is it normal to top up boiler pressure?
Occasionally — a top-up every few months as small amounts of air and water leave a sealed system is routine. Needing to top up weekly, or watching the gauge fall within a day or two of refilling, is not normal and points to a real leak or an internal fault.
Where does the water actually go?
Either out of the system — through a pinhole leak in pipework (sometimes invisibly, under floors), a dripping pressure relief valve, or radiator valves — or effectively "absorbed" by a failed expansion vessel, which makes pressure swing rather than hold steady.
Is low boiler pressure dangerous?
No — the boiler shuts itself down as a protection, which is why you lose heating rather than anything worse. The risk with ignoring it is water damage from an undetected leak, not the boiler itself.
What does finding the leak cost?
Diagnosis is £99 including the first hour. If a part such as a pressure relief valve or expansion vessel is needed it's quoted before fitting, and most repairs carry a 12-month parts warranty.