A boiler that keeps losing pressure nearly always has one of four problems: water escaping through a small leak, air let out when radiators were bled, a pressure relief valve that’s letting water by, or an expansion vessel that has lost its air charge. Low pressure isn’t dangerous, but it will eventually stop your boiler working, so it’s worth finding the cause rather than topping up every week.
We’re Clever Energy Boilers, a Gas Safe registered (no. 554138) installer based in Bingley and accredited by Worcester Bosch. We’ve fitted more than 25,000 heating systems across Yorkshire, the North West and the Midlands over the last 15 years, so pressure problems are something we see all the time. This guide covers what’s normal, what causes the drop, what you can safely check yourself and when it makes more sense to replace the boiler.
What counts as low boiler pressure?
Anything under 1 bar with the heating off is low. Most combi and system boilers like to sit between 1 and 1.5 bar when cold, and they’ll creep up towards 2 bar once the radiators are hot.
You’ll see the reading on a dial gauge (usually with a green zone) or on the boiler’s digital display. If you’re not sure what your boiler should read, our guide on what your boiler pressure should be has the full breakdown.
| Gauge reading (heating off) | What it means | What to do |
|---|---|---|
| Below 0.5 bar | Very low. The boiler has probably locked out | Top up, then look for the cause |
| 0.5 to 1 bar | Low. Heating may cut out | Top up to around 1.2 bar |
| 1 to 1.5 bar | Normal | Nothing |
| Above 2.5 bar | Too high | See our guide to high boiler pressure |
Is low boiler pressure dangerous?
No. Low pressure is a performance problem, not a safety one. Your boiler has a sensor that shuts it down when pressure falls too far, which is why you might suddenly lose heating and hot water and see a fault code on the display.
The bigger worry is what’s causing it. A slow leak can quietly damage ceilings, floors and the boiler’s electrics if it’s left for long enough.
Why does my boiler keep losing pressure?
There’s always a reason water is leaving a sealed system. These are the ones we come across most.
Have you bled your radiators recently?
This is the most common cause and the easiest to fix. Every time you bleed a radiator you let out air, and the pressure drops with it. Topping the boiler back up once afterwards is normal. If you’ve just worked through our radiator bleeding guide, this is probably all it is.
Is there a leak somewhere in the system?
Small leaks are behind most repeat pressure loss. They’re often tiny weeps that evaporate before you notice a puddle. Look for:
- damp or rusty marks around radiator valves and pipe joints
- drips or white crusty deposits under the boiler
- stains on ceilings below pipe runs or an upstairs bathroom
- green or brown marks on copper pipework
If the leak is coming from inside the boiler casing, don’t try to find it yourself. That needs a Gas Safe engineer. Our post on why boilers leak explains the usual suspects.
Is the pressure relief valve letting water out?
Your boiler has a pressure relief valve (PRV) that dumps water outside through a copper pipe if pressure gets too high. Once a PRV has opened a few times it can fail to seal properly and keep dribbling. Check the end of the copper pipe on your outside wall. If it’s wet or dripping, that’s where your pressure is going. We cover this in more detail in our guide to a dripping boiler overflow pipe.
Has the expansion vessel lost its charge?
The expansion vessel is a small tank inside or next to the boiler with air on one side of a rubber diaphragm. It soaks up the extra volume when water heats up. If it loses its air charge, pressure shoots up when the heating’s on, the PRV opens to let some out, and then pressure falls too low once everything cools. If you notice big swings between hot and cold, this is the likely culprit. Re-charging or replacing it is an engineer’s job.
Could the filling loop be faulty?
Occasionally a filling loop valve doesn’t close fully. That usually pushes pressure up rather than down, but a loop that’s been left connected and is weeping at its joints can lose water too.
Why does my boiler pressure keep dropping if there’s no visible leak?
Because the water is going somewhere you can’t see. The most common hidden culprits are a pressure relief valve letting water out through the pipe on your outside wall, a weep from pipework under floorboards or boxed in behind walls, and a flat expansion vessel that makes the PRV open every time the heating runs. An automatic air vent inside the boiler can also let small amounts of water escape.
It’s also worth knowing that boiler pressure drops a little when the heating switches off and the water cools. A fall of a few tenths of a bar between hot and cold is normal. A drop that carries on day after day isn’t.
What can I check if my boiler is losing pressure?
You can do quite a bit before calling anyone in. Work through these in order:
- Write down the pressure reading with the heating off.
- Walk round every radiator and look closely at the valves and the pipes underneath.
- Look under the boiler for drips or damp patches.
- Go outside and check the copper PRV pipe for water.
- Top the pressure up to around 1.2 bar and note the reading again 24 hours later.
If it holds, it was probably air from bleeding. If it drops again within a few days, there’s a fault that needs finding.
How do I top up my boiler pressure?
With the boiler off and cool, you open the filling loop slowly until the gauge reaches about 1.2 to 1.5 bar, then close it again. It takes a few minutes. Our step-by-step guide on how to repressurise a boiler walks through it, including where to find the filling loop on Worcester, Baxi, Ideal and Vaillant models.
How quickly should a boiler lose pressure?
A healthy sealed system shouldn’t noticeably lose pressure at all. A drop of a fraction of a bar over several months can happen, but needing to top up every week or two means water is escaping somewhere.
When should I call a Gas Safe engineer?
Get an engineer in if pressure drops again within a few days of topping up, if water is coming from inside the boiler, or if the PRV pipe keeps dripping once pressure is back to normal. Expansion vessels, PRVs and internal leaks all mean opening the boiler, and that’s not a DIY job.
Is it time to replace your boiler?
If your boiler is over 8 years old and losing pressure keeps coming back, replacement is usually the better use of your money. Worn seals, tired expansion vessels and corroded heat exchangers tend to fail one after another on older boilers, and the call-out bills add up quickly.
A modern A-rated boiler will also use noticeably less gas. We offer boiler replacement with fixed prices and Gas Safe engineers, and every new boiler can be protected by a long, market-leading warranty through Clever Energy. If you’d rather spread the cost, have a look at our boiler finance options.
Low pressure on Worcester, Ideal, Vaillant and Baxi boilers
Worcester Bosch
Most Worcester Bosch boilers show a CE 207 fault when water pressure drops too low. As accredited Worcester Bosch installers, we can tell you whether an older Greenstar is worth keeping. Our Worcester error codes guide lists what each code means.
Ideal
Ideal Logic and Vogue boilers typically display an F1 fault when pressure drops too low. Our Ideal F1 guide explains it, and you can see the current range on our Ideal boiler installation page.
Vaillant
Vaillant ecoTEC boilers usually show F22 for low water pressure. Read our Vaillant F22 guide or look at Vaillant boiler installation if yours is getting on.
Baxi
Baxi boilers commonly report low pressure with an E119 or similar code, depending on the model. If you have an older Baxi that keeps dropping pressure, our Baxi boiler installation page covers the newer models.
Boiler losing pressure FAQs
Why does my boiler lose pressure overnight?
Usually a small leak or a weeping PRV. Pressure also naturally drops a little as the system cools, so check the reading at the same time each day before you decide.
Can low boiler pressure cause no hot water?
Yes. Once pressure falls below the boiler’s minimum, it locks out and stops supplying both heating and hot water until you top it up.
Is it normal to top up boiler pressure every week?
No. Once or twice a year, or after bleeding radiators, is normal. Weekly top-ups mean there’s a leak or a faulty part.
Will a boiler service find the cause of pressure loss?
Often, yes. An annual service includes checking the expansion vessel and PRV. Our Clever Care servicing plans include a yearly service with heating cover in one monthly payment.
