Leaks in the heating system
The most common cause is a small leak — often on a radiator valve, a pipe joint under floorboards, or the boiler itself. Look for damp patches near radiators, water marks on ceilings below pipework, or corrosion on visible fittings. Even a pinhole leak will drop pressure over a few days.
Faulty pressure relief valve (PRV)
The PRV vents excess pressure through a copper pipe usually exiting an outside wall. If the valve is passing, water dribbles outside and system pressure falls. Check the discharge pipe outside — if it's wet or dripping, the PRV needs replacing.
Failing expansion vessel
Inside the boiler, the expansion vessel absorbs pressure changes as water heats up. Once the internal bladder fails, pressure swings wildly — high when hot, low when cold — and the PRV often opens. Recharging or replacing the vessel is a common Gas Safe job across KT postcodes.
Air in the system after bleeding radiators
Bleeding radiators releases air but also lowers pressure. Top up via the filling loop to bring the gauge back to 1.2 bar cold. If it stays there, the drop was normal.
Boiler pressure dropping repeatedly? Book a diagnostic visit across KT & TW.
Call 07932 898534Frequently asked questions
Is it safe to keep topping up a boiler that loses pressure?
Topping up occasionally is fine, but repeatedly refilling adds fresh oxygenated water that corrodes radiators from the inside. If you're filling more than once a month, get the underlying leak or PRV fault fixed.
How do I repressurise my boiler?
Locate the filling loop (usually a silver braided hose under the boiler), open both valves slowly until the gauge reads 1.2 bar, then close both valves tightly. Check the boiler manual for your model's exact procedure.
Can low pressure damage my boiler?
Running a boiler at very low pressure can cause it to lock out on a low-pressure fault code, and if ignored it can strain the pump. Modern boilers will refuse to fire below 0.5 bar to protect themselves.
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