Heating & hot water
Boiler losing pressure? What it means and how to fix it
A boiler that keeps sagging below 1 bar is one of the most common heating problems in UK homes, and one of the few you can usually fix yourself in five minutes. Here is what the gauge is telling you, how to top it up, and when a dropping needle stops being normal.
By the Tickover team · Published 2 March 2026 · 4 min read
How urgent is this?
Fix it this week, not this minute
Low pressure will not damage anything overnight. The boiler locks out to protect itself. Repeated drops are the thing to take seriously.
What the pressure gauge is telling you
Sealed central heating systems, which is almost every combi and most modern system boilers, run on a closed loop of water. That loop needs to sit at roughly 1 to 1.5 bar when cold. Drop much below 1 bar and the boiler assumes something is wrong and shuts down, often showing a low-pressure error code on the display.
Pressure falls for two honest reasons: water has left the system, or air has been let out of it. If you have recently bled your radiators, a pressure drop is completely normal, because the air you released was helping hold the needle up. If nothing has been bled and the pressure still sags, the water is escaping somewhere.
Top it up in five minutes
Repressurising is a homeowner job, no engineer needed. You are looking for the filling loop: usually a braided silver hose beneath the boiler with a small valve at each end, though some boilers have a built-in filling key or lever instead. Your manual (or a search for your model) will confirm which.
Repressurise a sealed system
Switch the boiler off
And let it cool for a few minutes. You want a cold reading, and no pump running while you fill.
Find the filling loop
The braided hose under the boiler. Check both ends are hand-tight before you start.
Open the valves slowly
Turn one valve, then ease the second open a quarter turn. You should hear water flowing.
Watch the gauge to 1.2 bar
Close the valves the moment the needle reaches about 1.2. Overshooting past 2.5 means bleeding a radiator to let some back out.
Close both valves firmly
Hand-tight only. A weeping filling loop is itself a classic cause of pressure creep.
Restart the boiler
Clear the error code if one is showing, then let it run a heating cycle and check the needle holds.
Losing pressure again and again?
Topping up once or twice a year is nothing to worry about. Topping up every week means the water is going somewhere, and there are only two exits: a leak in the system, or a fault inside the boiler. For the wider map of boiler misbehaviour, the complete boiler guide covers every branch of it.
Where is the water going?
Slow leaks love hiding at radiator valves, under floors and inside the boiler itself, where a failing expansion vessel or a pinholed heat exchanger can dump water down the condensate pipe without a single visible drip. A tell-tale sign of an expansion vessel problem is pressure that swings high when hot and collapses when cold. If water is appearing somewhere it should not be, treat it like any other water leak and isolate it early.
What it costs to put right
Typical costs when it is not DIY
Filling loop or valve fix
Trace and fix a system leak
More if floors come up
Expansion vessel replacement
Typical UK ranges, mid-2026, as a guide only. Your home, your area and the job itself all move the number — always compare real quotes.
Pressure rules of thumb
Do
- Check the gauge monthly, and always before winter
- Top up to about 1.2 bar cold, no higher
- Note the date every time you repressurise
- Call a Gas Safe registered engineer for anything inside the boiler casing
Don’t
- Do not keep silently topping up every week and hoping
- Do not open the boiler casing, ever. That is engineer-only territory
- Do not run the pressure at 3 bar because it keeps dropping
- Do not ignore water dripping from the boiler or its pipes
Not sure what you’re dealing with?
Answer a few questions about what your boiler is doing and Tickover will tell you how urgent it is, what is likely wrong, and line up a vetted Gas Safe engineer if you need one.
Quick answers
What pressure should my boiler be at?
Around 1 to 1.5 bar when the system is cold, which is usually the green zone on the gauge. It will rise a little as the heating warms up; that is normal. Below about 0.8 bar most boilers lock out, and above 2.5 bar cold is too high.
How often is it normal to top up boiler pressure?
Once or twice a year is nothing to worry about, and always after bleeding radiators. If you are topping up weekly or even monthly, water is leaving the system somewhere and it is worth getting the leak traced before winter.
Can I repressurise a boiler myself?
Yes. Using the external filling loop or filling key is a normal homeowner task and does not require an engineer. What you should never do is open the boiler casing itself; any work inside the boiler must be done by a Gas Safe registered engineer.
Why does my boiler lose pressure only when the heating is on?
Pressure swinging up when hot and collapsing when cold often points to the expansion vessel, which cushions the system as water expands. Its diaphragm can fail or lose its air charge. An engineer can recharge or replace it, typically for £150 to £400.
This guide is general information for UK homes, not advice about your specific property. Anything involving gas must be done by a Gas Safe registered engineer, and notifiable electrical work by a qualified electrician. If you smell gas, call the National Gas Emergency line on 0800 111 999. If you’re ever unsure, stop and get a professional in — that’s exactly what Tickover is for.
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