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Boiler Losing Pressure
You top the gauge up to 1.5 bar and days or hours later it has dropped again.
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What's happening?
A sealed central heating system holds a fixed volume of water under pressure, typically around 1 to 1.5 bar when cold, and it should stay there for months. Pressure that falls means water has left the system — there is no other way for it to go. Sometimes it leaves visibly, from a radiator valve or a pipe joint. Sometimes it leaves invisibly, out of the pressure relief valve to a discharge pipe outside, which is what happens when the expansion vessel has lost its air charge and the system has nowhere to expand into. A slow fall over months can be normal ageing; a fall you can watch over a day is a leak or a failed vessel, and topping up repeatedly just puts fresh oxygenated water into the system and accelerates corrosion.
What usually causes it?
- A leaking radiator valve gland, compression joint or towel-rail connection
- An expansion vessel that has lost its air charge, so the system dumps through the pressure relief valve when it heats
- A failed or weeping pressure relief valve passing water to the outside discharge pipe
- A pinhole leak in buried pipework under a floor, showing only as a warm patch or a stain
- Air bled out of radiators repeatedly, each bleed taking pressure with it
- A leaking auto air vent or a corroded pump head gasket inside the boiler casing
- A filling loop left slightly open, which raises pressure rather than lowers it — but hides a genuine loss
Is it dangerous?
Low. The boiler is designed to lock out safely rather than run dry, and the pressure relief valve exists to release excess safely outdoors. The real damage is slower: repeated refilling introduces fresh oxygen, which corrodes radiators from the inside and builds the sludge that later blocks heat exchangers. A leak into a floor void can also do serious damage to joists and ceilings long before anyone sees water.
Can I fix it myself?
Topping up is a homeowner job on most boilers and worth knowing. With the boiler off and cool, find the filling loop — usually a braided silver hose with a valve at each end, or a filling key on some models — open it slowly and watch the gauge until it reaches the level marked for your boiler, generally about 1 to 1.5 bar cold, then close both valves fully and check the loop is not passing. What you should also do is look: check under every radiator valve, feel along skirting for damp, and go outside to see whether the small copper discharge pipe near the boiler is wet or dripping. Do not top up more than occasionally without finding out why.
When should you call a heating engineer?
Call a heating engineer if you are topping up more than about once a month, if you can see water anywhere, or if the outside discharge pipe drips when the heating runs — that last one is the expansion vessel or the relief valve and will not fix itself. Call promptly if the pressure drops to zero and the boiler locks out, and if pressure falls suddenly rather than gradually, treat it as a leak worth finding before it soaks a ceiling.
What will the heating engineer actually do?
- Pressure-test the system and isolate it in sections to prove where water is going
- Check the expansion vessel charge pressure against the system and recharge or replace it
- Test and replace the pressure relief valve where it is passing
- Trace concealed leaks with thermal imaging or acoustic equipment before lifting floors
- Repair the failed joint, valve or section of pipework and refill with inhibitor
- Re-pressurise, vent the system and confirm it holds over a heat cycle
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