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Quick answer
A sealed heating system should hold its pressure for months. If yours drops over days or weeks and you cannot see any water, the most common explanations are a pressure relief valve discharging outside, an expansion vessel that has lost its charge, a small leak hidden under floors or behind walls, weeping radiator valves that evaporate before you notice, or an automatic air vent letting water out. Repeatedly topping up without finding the cause adds fresh oxygen to the system, which increases corrosion. The cause needs finding.
Key takeaways
- A healthy sealed system should hold pressure; repeated drops mean water is escaping.
- Check the copper relief valve pipe outside for drips or staining.
- An expansion vessel without its air charge causes pressure swings and relief valve discharges.
- Small hidden leaks under floors are common and can cause damage over time.
- Topping up repeatedly introduces oxygen and makes corrosion worse.
The relief valve outside
Start outside. Find the copper pipe that leaves the wall near the boiler, usually turned back towards the wall. Look for drips, staining on the wall or wet ground below. If it has been discharging, the system has been reaching high pressure, often because of the expansion vessel. See pressure relief valve dripping outside.
The expansion vessel
If pressure rises sharply when the heating is on and then falls below normal when it is off, the expansion vessel is likely to have lost its charge. The cycle goes: pressure rises, relief valve lets out water, system cools, pressure drops. An engineer can recharge or replace it. See boiler pressure too high.
Hidden leaks
Pipes under floors, in walls and in ceiling voids can develop pinhole leaks. Look for:
- Damp patches on ceilings or skirting.
- Warm spots on floors.
- Musty smells.
- Staining on downstairs ceilings below radiators.
Our guide to signs of a hidden water leak covers the general principles.
Radiators and valves
Radiator valves weeping slightly can lose water that evaporates quickly from warm surfaces. Look for crusty deposits or green staining around valve nuts. See radiator valve leaking.
Automatic air vents
Automatic air vents on boilers and pumps release air, but if their seals fail they can release water too, often inside the boiler casing where you will not see it.
Inside the boiler
Seals on pumps, diverter valves and heat exchangers can weep inside the casing. An engineer will look inside for staining or corrosion. See water leaking from a boiler.
Cold weather
If pressure drops mainly in cold snaps, that can be normal contraction. See boiler pressure drops in cold weather.
Book with Gasmaster
Tired of topping up? Gasmaster Plumbing & Heating, Gas Safe registered (220932), traces pressure loss and repairs boilers across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service.
Frequently asked questions
How often should I have to top up my boiler pressure?
Rarely. Topping up more than a couple of times a year suggests a fault.
Is losing pressure dangerous?
Not directly, but the boiler will shut down on low pressure, and leaks can cause damage.
Can a boiler lose pressure without a leak?
Pressure changes with temperature, but repeated drops almost always mean water is leaving the system somewhere.
Sources
- Manufacturer user guides for sealed-system boilers
- BS 7593, water treatment for heating systems (bsigroup.com)
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
