Boiler keeps losing pressure? Here is why, and how to fix it
Why your boiler keeps dropping pressure, the quick checks you can do yourself, and when it is time to call a Gas Safe engineer.
If your boiler pressure keeps dropping and you are getting tired of topping it up, you are not alone. It is one of the most common boiler niggles in UK homes. Here is what is actually going on and what you can do about it.
What the pressure gauge should read
Check the gauge on the front of your boiler. With the heating off, most boilers are happy between 1.0 and 1.5 bar. If it keeps falling below 1.0, pressure is escaping somewhere.
The usual suspects
- A small leak. The most common cause. Check around radiator valves, pipe joints and under the boiler for damp patches. A pinhole leak can drop your pressure slowly over days.
- Bleeding radiators. If you have recently bled your radiators to get rid of cold spots, that releases pressure. Just top the boiler back up (see below).
- A faulty pressure relief valve. This valve can weep water outside through a small pipe on an external wall. If you see dripping there, the valve likely needs replacing.
- A failing expansion vessel. Common on older boilers. This one is an engineer job.
What you can safely check and do yourself
You can top the pressure back up using the filling loop, usually a small silver braided hose under the boiler. Your manufacturer has a short guide for your exact model, for example Worcester Bosch and Vaillant both publish simple how-to videos. Open the valves slowly until the gauge reads about 1.2 bar, then close them.
What you should not do is open the boiler casing or touch internal components. Only a Gas Safe registered engineer may legally work on the internals of a gas boiler, and you can check any engineer's registration on that same site.
When to call an engineer
If you are re-pressurising more than once a week, or you can see water leaking, book an engineer. If your boiler is over 10 years old and this is one of several niggles, it may be worth reading our guide on the signs you need a new boiler before spending money on repairs. Independent reviewers like Which? are also a useful sanity check on which models last.
The Norvera way
If a repair is the smart move, we will say so. If your boiler is on its last legs, we will get you a fixed written price for a replacement, often with next-day installation. Get your fixed price in a couple of minutes.
Frequently asked questions
What pressure should my boiler be at?+
Most boilers sit happily between 1.0 and 1.5 bar when the heating is off. If the needle keeps dropping below 1.0, something is letting pressure escape and it is worth investigating.
Is it safe to keep re-pressurising my boiler?+
Topping up now and then is fine. Doing it every few days is a red flag, it usually means a leak or a failing part, and it is worth having a Gas Safe engineer look before it gets worse.
Can low pressure break my boiler?+
Low pressure alone will usually just stop the boiler firing for safety. But the underlying cause, like a slow leak, can cause damage over time, so it is best not to ignore it.
Related guides
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