Is it 1 bar, 2 bar, chocolate bar? What even is a bar? Jokes aside, boiler pressure is probably the most important thing to keep an eye on. Keeping it at the ideal pressure is crucial.
- First, What is a Combi Boiler and Why Does it Have Pressure?
- What Should the Ideal Pressure Be on a Combi Boiler?
- What Happens If My Boiler Pressure Is Too Low?
- What Happens If My Boiler Pressure Is Too High?
- Why Does My Boiler Keep Losing or Gaining Pressure?
- Final Thoughts: Re-Pressurise it, or Replace it
If your boiler pressure is all over the place, you're probably wondering whether you should be REALLY worried, or if you should just straight up ignore it… (spoiler, you shouldn’t, LOL).
To put it simply, boilers work best within a specific pressure range, and if your combi boiler pressure is too high or too low, you could end up with cold showers, weird noises, and eventually, a very expensive repair bill.
So, let’s break it down: what should your ideal boiler pressure be, what happens when it’s wrong, and most importantly, how to bleeding fix it.
First, What is a Combi Boiler and Why Does it Have Pressure?
Before we get into the nitty-gritty of ideal boiler pressure, let's clear up what a combi boiler actually is, because not all boilers have pressure gauges.
A combi (a.k.a combination) boiler is a type of high-efficiency boiler that provides both heating and hot water from a single unit, without the need for a separate hot water cylinder or storage tank. Unlike traditional system boilers, a combi boiler heats water on demand, meaning there’s no waiting around for a storage tank to fill up.
Want to bore yourself to death with the differences? Here’s a long guide on the different types of boilers.
How Does This Relate to Boiler Pressure?
Unlike regular boilers, which don’t have a pressure gauge because they rely on gravity-fed water systems, combi boilers operate in a sealed pressurised system. This means that the water inside is under pressure and needs to be within a certain range to function properly.
If your combi boiler’s pressure is too low, your heating won’t work. If it’s too high, you risk damaging costly components. Urgh.
If your boiler is busted beyond repair, and you couldn’t care less whether it’s under pressure or not, here’s how to choose the best combi boiler (which won’t break).
What Should the Ideal Pressure Be on a Combi Boiler?
For most combi boilers, the ideal boiler pressure is between 1.0 and 1.5 bar when the system is cold. When your heating is on and running, the pressure may rise slightly, but it shouldn’t go over 2.5 bar.
Boiler State | Ideal Pressure |
Cold (Off) | 1.0 - 1.5 bar |
Heating On | 1.5 - 2.0 bar |
Too High | Above 2.5 bar |
Too Low | Below 1.0 bar |
If your pressure is outside this range, your boiler could be struggling, or about to cause problems.
If your boiler pressure is dropping frequently (or, even daily), check out our guide on how fix our guide on why your boiler is losing pressure.
What Happens If My Boiler Pressure Is Too Low?
If your boiler pressure drops below 1.0 bar, you might notice:
A complete loss of heating/hot water
A low (or, no) pressure boiler can’t circulate water properly around your heating system, sometimes causing a total loss of your home’s cosiness.Radiators not heating up properly
Especially at the top floors of your house, or in radiators which are located the furthest away from your boiler.Weird gurgling noises
Your boiler may make odd noises, like a hungry toddlers tummy. That’s because there’s too much air in the system, and a not-so-ideal pressure.
How to Fix Low Boiler Pressure
First, check for leaks
If your system is losing pressure regularly, you might have a leak in a radiator, pipe, or even the boiler itself. So, before we worry about finding a fix, let’s find the root of the cause (leak, roots.. get it?)Re-pressurise the system
Most boilers have a filling loop (a small silver hose under the boiler). Turn the valves slightly until the pressure reaches 1.5 bar. You’ll usually open the left one first, then the right one.Bleed your radiators
If there’s trapped air, it could cause pressure fluctuations. Here’s a guide on bleeding your bleeding radiators.
What Happens If My Boiler Pressure Is Too High?
If your boiler pressure is above 2.5 bar, you might experience:
A leaking pressure relief valve
If the pressure is too high, it’ll try release the pressure through it’s PRV. You may see excess water leaving via your condensate pipe (as they’re typically connected together)Loud banging noises
Like a horny neighbour, you’ll hear banging noises from your boiler, also similar to a “hammering” sound. A good hammering.Boiler shutting down
Many modern combi boilers will automatically cut out if the pressure is too high, in the hope of protecting it’s precious components.
How to Reduce High Boiler Pressure
Turn off the heating
Let your boiler cool down before doing anything.Bleed the radiators
This releases excess water and lowers system pressure.Check the filling loop
Make sure it’s fully closed. A slightly open valve can continuously add pressure.
DEEP READ: If your boiler pressure keeps rising, here’s a big tutorial on how to fix high boiler pressure.
Why Does My Boiler Keep Losing or Gaining Pressure?
If your boiler pressure is constantly dropping or rising, you might have a bigger problem:
Leaks in the system
Even a small drip can cause pressure loss over time.Faulty expansion vessel
This component controls pressure inside the boiler. If it’s broken, pressure will fluctuate.A dodgy pressure relief valve
If it’s not sealing properly, your system might keep losing pressure.
If you’re constantly having to top up your boiler, it’s a sign that something’s wrong. You might need a new expansion vessel, pressure relief valve, or even a new boiler altogether (NoooOOOoooOoooo).
Final Thoughts: Re-Pressurise it, or Replace it
Your combi boiler should ideally sit at 1.0 - 1.5 bar when cold and rise slightly when heating is on. If the pressure keeps dropping or skyrocketing, it’s time to take action before bigger problems develop.
Still having pressure problems? Bite the bullet and buy a new boiler, that’s what everyone else does.
