Boiler losing pressure? Here's what's actually happening
A sealed heating system shouldn't lose pressure. If yours needs topping up more than once a month, water is escaping — either from a visible leak or from an internal boiler component.
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The likely causes, in order
- PRV (pressure relief valve) discharge — Look for a small copper pipe outside your house near the boiler. If it's dripping, the PRV is releasing pressure — usually caused by a failed expansion vessel.
- Failed expansion vessel — The rubber diaphragm inside the expansion vessel splits with age. Fix is either re-pressurising the vessel or replacing it — often the cheapest permanent solution.
- Radiator or pipe leak — Look under every radiator valve and follow visible pipework. Small pinhole leaks under floorboards are common in 1970s copper.
- Heat exchanger corrosion — On boilers 8+ years old — internal corrosion of the heat exchanger. Diagnosed with borescope inspection; often the point at which replacement makes more sense than repair.
What you can safely try first
- Top up via the filling loop to 1.2 bar cold
- Check under every radiator with kitchen roll for damp
- Check the PRV pipe outside for any dripping
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Frequently asked questions
How often is 'too often' to top up?▾
Once every few months is normal. More than monthly, and you have a leak somewhere that needs finding.
Can I keep topping up indefinitely?▾
You can, but every top-up dilutes the corrosion inhibitor in the system, which shortens the life of your radiators and boiler.
Related pages you might want
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