Boilers & heating
Why does my boiler keep losing pressure?
Topping up the boiler gets the heating working, then a few days later the pressure is down again. Sound familiar? Repeated pressure loss needs a diagnosis, but it doesn’t automatically mean you need a new boiler.
First, check which pressure you’re looking at
The gauge on a sealed heating system measures the pressure in that circuit. It isn’t a reading of the water pressure at your kitchen tap. Open-vented heating systems work differently, so this advice won’t apply to every installation.
Read the gauge when the heating is cold and compare it with your boiler’s user instructions. The correct range and filling procedure depend on the appliance and installation. Don’t use a generic online number as a substitute for your own manual.
Did it happen once or does it keep happening?
Pressure can fall after radiators have been bled or after work on the system. A recurring drop is different. Note how quickly it falls and whether the problem happens after the heating has run. A photo of the gauge when cold and another when hot can be useful for the engineer.
Look for visible signs of water loss
Without removing any covers, look around accessible radiator valves and visible joints for dampness, staining or corrosion. Check for marks on ceilings below pipework too. Small leaks don’t always leave a puddle, particularly around warm fittings.
If you can safely see an outside discharge pipe dripping, mention it. Don’t block it or assume every pipe beneath the boiler has the same purpose: condensate drainage is different from the pressure-relief discharge.
What might the engineer find?
Possible causes include a leak on the heating circuit, a pressure-relief valve passing water or an expansion-vessel problem. A large pressure rise during heating followed by a low reading once cold is a useful clue, not a diagnosis on its own. Hidden pipework or a fault within the boiler may also need investigation.
Changing a part without establishing why the pressure changed can leave the original problem unresolved.
Should you keep topping it up?
Only follow the appliance’s user instructions for any permitted topping-up procedure. If it repeatedly needs doing, book an investigation rather than making it part of your weekly routine. Don’t remove the boiler casing, adjust safety components or cap a discharge pipe.
What to send us
Tell SMC the make and model, the cold pressure, how quickly it drops and whether you’ve noticed leaks or fault codes. We’ll investigate whether the cause is in the boiler or elsewhere in the system and explain the repair options.