Why Is My Boiler Losing Pressure?
Low or dropping boiler pressure is one of the most common heating problems. Here's what causes it, how to repressurise, and when to call an engineer.
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Upfront cost — no obligation If your boiler pressure keeps dropping or the gauge is in the red low zone, you’re not alone — it’s one of the most common boiler problems we’re called about. Low pressure stops the boiler working properly and is often easy to top up, but if it keeps falling there’s usually an underlying cause that needs fixing. This guide explains why it happens and what you can safely do.
What boiler pressure should be
Most boilers should sit at around 1 to 1.5 bar when cold, rising a little when the heating is on. If your gauge reads below 1 bar, the boiler may lock out or fail to heat. A pressure that drops slowly over weeks can be normal topping-up territory; a pressure that drops quickly or repeatedly points to a fault.
Most boilers should sit at around 1 to 1.5 bar when cold, rising a little when the heating is on.
How to repressurise your boiler
Most boilers can be repressurised using the filling loop — a flexible braided hose or built-in valve beneath the boiler. With the boiler off, open the valve(s) slowly to let water in, watch the gauge rise to around 1.5 bar, then close the valve(s). Your boiler manual shows the exact method for your model. If you’re not comfortable doing this, an engineer can show you.
Most boilers can be repressurised using the filling loop — a flexible braided hose or built-in valve beneath the boiler.
Why pressure keeps dropping
If you’re repressurising often, something is wrong. The usual causes are a leak somewhere in the system (radiators, pipework or the boiler itself), a faulty pressure relief valve releasing water outside, or a waterlogged expansion vessel that can no longer absorb pressure changes. These all need a Gas Safe engineer to diagnose and repair — topping up repeatedly only masks the problem.
If you’re repressurising often, something is wrong.
We fix pressure faults across East London
If your boiler won’t hold pressure, Gas First can find and fix the cause across East London and Essex — locating leaks, replacing failed valves, or recharging the expansion vessel. Call us if you’re topping up more than occasionally.
If your boiler won’t hold pressure, Gas First can find and fix the cause across East London and Essex — locating leaks, replacing failed valves, or…
Frequently Asked Questions
What pressure should my boiler be at?
Around 1 to 1.5 bar when cold for most boilers, rising slightly when heating is on. Below 1 bar usually causes problems.
How do I repressurise my boiler?
Use the filling loop beneath the boiler: with the boiler off, slowly open the valve(s) to let water in until the gauge reaches about 1.5 bar, then close them. Check your manual for the exact method.
Why does my boiler keep losing pressure?
Repeated loss usually means a leak in the system, a faulty pressure relief valve, or a waterlogged expansion vessel — all needing an engineer to fix.
Is low boiler pressure dangerous?
It's not usually dangerous — the boiler typically just stops working. But the underlying cause, such as a leak, should be addressed.
How often should I need to top up the pressure?
Only occasionally. If you're repressurising every week or two, there's a fault that needs investigating.
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