A boiler loses pressure because water is escaping from the sealed heating system. Most often that’s through a small leak at a radiator valve or pipe joint, a passing pressure relief valve, or a failed expansion vessel. It can also happen simply because you’ve recently bled a radiator, which releases air and takes some of the pressure with it. On most modern boilers the gauge should read between 1 and 1.5 bar when the system is cold, and Worcester Bosch’s guidance is that topping up once or twice a year is normal. If you’re repressurising more often than that, the water is going somewhere it shouldn’t, and the cause needs finding. Topping up is a safe DIY job; a repeat drop is a fault. The Happy Heating Co’s Gas Safe registered engineers (657972) trace and fix pressure loss across West and North West London. See our boiler repairs service.
Low pressure is one of the most common reasons a boiler stops heating, and one of the least alarming once you know what the gauge is telling you. Here’s what normal looks like, what makes pressure drop, how to top it up safely, and the point at which it stops being a DIY job.
What should my boiler pressure be?
When the heating is off and the system is cold, the pressure gauge should sit between 1 and 1.5 bar, usually the green zone on the dial. Pressure rises a little as the system heats up; that’s normal. Two readings matter, according to Worcester Bosch’s boiler pressure guidance:
- Below 0.5 bar: water has been lost from the system and needs replacing. Many boilers lock out and show a low-pressure fault code at this point.
- Topping up once or twice a year is normal. More often than that means the system has a fault worth investigating.
That second point is the one to hold onto. The question isn’t really “why is my pressure low?” It’s “how often am I having to put it back?”
The five common causes of pressure loss
| Cause | Tell-tale sign | What to do |
|---|---|---|
| Small leak in the system | Damp patch, salt-like staining or green crust at a radiator valve or pipe joint; skirting or flooring marks | Note where it is; an engineer can usually fix it quickly |
| Weeping radiator valve | Slow drip or crusty build-up around a valve spindle or nut | Don’t over-tighten it yourself; have it repacked or replaced |
| Pressure relief valve passing | Drips from the copper overflow pipe outside, often after pressure was set too high | Needs an engineer; the valve isn’t resealing |
| Failed expansion vessel | Pressure swings high when hot, then drops right back; frequent lockouts | Engineer job; the vessel needs recharging or replacing |
| Recently bled radiators | Pressure dropped straight after bleeding | Normal; just repressurise back to 1–1.5 bar |
The first four all mean the same thing at heart: water (or the air charge that manages it) is escaping. Bleeding is the only “innocent” cause on the list. Releasing trapped air lowers the pressure, and a quick top-up puts it right.
How to repressurise your boiler
Topping up is designed to be a homeowner job, and on most boilers it takes two minutes. The exact method varies by model, so check your manual or the manufacturer’s site for yours. The typical filling-loop routine is:
- Switch the boiler off and let the system cool.
- Find the filling loop. It’s usually a braided silver hose with a small valve at each end, underneath the boiler. Some models have a built-in filling key instead.
- Open the valve(s) slowly. You’ll hear water flowing in and see the gauge climb.
- Close the valves when the gauge reaches 1–1.5 bar. Don’t overshoot; too much pressure is a problem of its own.
- Switch the boiler back on and reset the fault code if one is showing.
If the boiler works normally afterwards and holds its pressure, you’re done. Keep an occasional eye on the gauge over the next couple of weeks.
When is pressure loss a job for an engineer?
Call in a Gas Safe engineer when:
- The pressure drops again within days or weeks of topping up. The leak or fault won’t fix itself, and system water contains inhibitor you don’t want slowly washing away.
- The gauge falls to zero or the boiler repeatedly locks out on a low-pressure fault.
- You can see water, under the boiler, at the external overflow pipe, or around a radiator valve.
- The pressure swings, climbing high when the heating runs, then sagging back when it cools. That points to the expansion vessel.
- You’ve topped up more than twice this year, per Worcester Bosch’s own threshold.
Our engineers trace pressure loss across West and North West London, from pinhole leaks under floors in Maida Vale flats to tired expansion vessels, and fix the cause, not just the symptom. See boiler repairs or get in touch for a fixed price.
It’s also worth saying: a weeping valve or slowly-failing part is exactly the kind of thing an annual boiler service catches before it becomes a cold-house morning. If your boiler hasn’t been looked at in over a year, that’s the cheapest fix on this page.
Can low pressure damage my boiler?
Running at low pressure won’t usually damage a modern boiler; it protects itself by locking out. The real costs are indirect. There’s no heating or hot water until it’s topped up, and if there’s a leak, fresh water keeps diluting the corrosion inhibitor in your system, which quietly shortens the life of radiators and the boiler’s heat exchanger. That’s another reason repeat drops deserve a proper look rather than a weekly top-up habit.
Frequently asked questions
What pressure should my boiler be at?
Between 1 and 1.5 bar when the system is cold, normally the green zone on the gauge. Pressure rises slightly as the heating warms up, which is normal. Below 0.5 bar, water has been lost and needs replacing before the boiler will run properly.
Why does my boiler keep losing pressure?
Because water is escaping somewhere: a small system leak, a weeping radiator valve, a passing pressure relief valve, or a failed expansion vessel. Bleeding radiators also drops pressure, but only once. Worcester Bosch’s guidance is that topping up more than once or twice a year signals a fault.
Is it safe to repressurise a boiler myself?
Yes. Topping up via the filling loop is a designed-in homeowner job and takes a couple of minutes. Follow your boiler manual, fill slowly to 1–1.5 bar, and don’t overshoot. What you shouldn’t do yourself is open up the boiler or work on gas: that’s Gas Safe territory.
My boiler pressure is at 0, what do I do?
Repressurise via the filling loop back to 1–1.5 bar and reset the boiler. If it drops to zero again quickly, stop topping up and have it looked at. A fall that fast usually means a real leak or a failed component that needs a Gas Safe engineer.
Does bleeding radiators lower boiler pressure?
Yes. Bleeding releases trapped air from the system, and the pressure drops with it. That’s expected, not a fault. Check the gauge after bleeding and top the system back up to 1–1.5 bar. If pressure keeps falling afterwards without bleeding, something else is going on.
Boiler losing pressure again and again?
Send us a message and we’ll find where the water’s going and give you a clear fixed price to put it right.
- WhatsApp us — Book in 60 seconds
- Call 020 8051 2124. Our line is answered 24/7 and we’ll get your details to an engineer
- See the service: boiler repairs
We’re Gas Safe registered (657972) and cover West and North West London. Heating that just works, no drama.
About the author Adam Said is a Gas Safe registered domestic gas engineer (657972) at The Happy Heating Co, repairing boilers and tracing pressure faults in homes across West and North West London. This guide reflects what we see on real jobs every week.
Related reading
Pressure figures reflect Worcester Bosch’s boiler pressure guidance (verified July 2026); always follow your own boiler’s manual. If in doubt, ask a Gas Safe registered engineer.
