None of us like being under too much pressure, but without any at all, things can stop moving, the same rules apply to boilers. Too much, big problemo. Too little, big problemo.
The right amount of pressure is crucial when it comes to your boiler functioning.
Too much, or too little and your boiler may just stop working.
You can find out the pressure on your boiler via the pressure gauge, this is typically just underneath the boiler.
So, what should your boiler pressure be when the heating is on?
In this guide, we’ll explain exactly what your pressure should be and what you can do if it’s too high or too low.
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What should the pressure be when the heating is on?
When your boiler is turned on and the heating is in operation, the pressure on your boiler should be between 1.5 and 2.0 bar.
Between 1.5 and 2.0 bar
On the majority of boilers, you’ll discover a pressure gauge, this looks like a watch face and includes a dial and the pressure levels, typically ranging from 0 to 4.0.
The dial should indicate what the boiler pressure is at, and the normal range of pressure is usually marked as reassuringly green.
If the pressure is too low (below 1.5) or too high (above 2.0) the needle will be within either of these critical zones, which is usually marked as red to indicate that the pressure is out of the normal (green) range.
If the dial on the gauge is in either of the critical zones, it indicates that there is a potential issue with the boiler or central heating system.
What should the pressure be when the heating is off?
When your boiler is no longer on and the heating is off, the pressure should be between 1.0 and 1.5 bar.
Between 1.0 and 1.5 bar
This means that the dial on the pressure gauge should remain within the green area, although slightly lower than the operating pressure.
The exact amount of pressure can vary between different boiler makes and models, but typically when not turned on, the pressure should be at 1.30 bar.
However, to be on the safe side and to avoid making incorrect assumptions, it’s always recommended that you check the boiler manual to ensure the pressure of your boiler is operating at the optimum.
Why is your boiler pressure higher when the heating is turned on?
When water is heated it expands, so when your boiler is turned on, it starts heating the water up, in turn the water expands and increases the pressure within the entire central heating system.
Therefore, when the heating is on you should be able to observe the pressure gauge dial on your boiler slightly increase, however, this should not be above 2.0 bar.
Boilers are designed to quickly respond to changes in pressure and can release excess pressure via the pressure release valve.
This key design feature means that you can safely use your central heating and hot water without worrying about any major and/or dangerous changes in pressure.
When you are using your heating, you should not observe your boiler pressure increase by 1.0 bar above the optimal working range. If it does, this may indicate that there is a problem with your boiler.
What if the pressure is too high or too low?
If your boiler pressure isn’t within the normal range e.g., the dial on the gauge indicates it’s in the critical red zones, there may be a fault with your boiler.
Boiler pressure too low
Typically, if your boiler pressure is too low, the boiler will “lockout” i.e., turn off and display the relevant error code on the interface.
You can easily top up the pressure yourself and restart the boiler, however, this isn’t usually advised until you know what caused the pressure to drop in the first place.
If you’re asking, “what should your pressure be when your heating is on?”, low pressure is unlikely to be your issue, since the boiler will have most likely switched off by now, often referred to as “locking out”.
A common cause of low boiler pressure is a leak within your central heating system, potentially within a boiler component, a radiator or pipework. These pin holes are commonly found around joints and/or caused by corrosion to major components.
Unfortunately, you will need the help of a Gas Safe Engineer to fix a leak, as it will usually require them to reseal or potentially re-pipe your system where necessary.
If there is no obvious leak, you can always try topping up your boiler pressure yourself, this is a relatively easy thing to do. However, if it keeps losing pressure, then it’s likely caused by a leak somewhere that will need to be repaired in order for the issue to be resolved.
You can read our full guide on low boiler pressure as well as how to top it up here.
Boiler pressure too high
If your boiler pressure is too high when your heating is on, then this will be indicated on the gauge, and you will see the dial in the red zone above 2.0 bars.
The most common cause of high boiler pressure is a high demand for hot water and the pressure being pushed towards 2.5 bars is usually nothing to be worried about.
However, if your boiler is consistently displaying a very high pressure e.g., between 3.5 and 4.0 bars, it’s certainly something you should get checked out by a registered Gas Safe Engineer.
Since boilers are designed to deal with high pressure, this signals a potential fault with a component that has a central role in boiler pressure modulation, such as the expansion vessel or the pressure release valve.
There are many other potential causes of high boiler pressure e.g., too much water in your system or a faulty boiler part. You can read our full guide on high boiler pressure here.
How often should I check my boiler pressure?
It’s a good idea to routinely check your boiler’s pressure gauge, ideally once every few weeks, to make sure it’s not too high or too low.
This will help ensure you spot any signs of a problem early and they are fixed at the earliest opportunity. Often, if left to fester, problems can deteriorate and end up costing even more to repair.
If you have just bled your radiators, it’s also a good idea to check your boiler pressure hasn’t dropped too low, as you may need to top it up.
How can you increase boiler pressure?
If your boiler has dropped too low (below the 1.0 bar) you will need to repressurise it, which is easy to do in next to no time at all.
Before you begin, make sure your boiler is turned off and the pipework has cooled down.
Step 1 - Find your filling loop
Just underneath your gas boiler should be some pipework, amongst which will be your filling loop.
The filling loop is the silver Flexi pipe with either one or two black (usually) taps on. A silvery snake.
Step 2 - Increase the pressure
To increase the pressure, you’ll need to open your one (or two) taps, full until they lock at 45 degrees. As you’re doing this, you should see the pressure rise on the boilers pressure gauge to around 1 bar. Ta-da, your boiler has been repressurised.
Step 3 - When you're in the green, close the taps
Once it’s back in that oh so reassuring green zone – close the taps again – ensuring they’re fully closed back to their original position.
Please note: As mentioned previously, if your boiler pressure is consistently dropping, even after being topped up, it’s likely that there is a leak within the system somewhere. If this is the case, consult the help of a registered Gas Safe Engineer to help you diagnose and repair the issue.
How can you decrease your boiler pressure?
The most common cause of boiler pressure being too high is having too much water in your system. This is common just after topping up boiler pressure and accidentally adding too much.
Check your pressure release valve
Have you recently topped up your boiler pressure? Before deciding to remove any water, make sure you haven’t mistakenly left the pressure release valve slightly open.
When air goes in, the pressure goes up. So, make sure to check that they are tightly closed before you start to lower the pressure further.
Once you’ve done this and the pressure still needs reducing, you will need to bleed your radiators.
Bleeding your radiators
In order to reduce the pressure, you will need to release some water from the system, which you can do by bleeding your radiators.
To bleed your radiator (don’t worry, they won’t be in pain) you will need a radiator key, these are very easy to source from local DIY stores.
Once you’ve summoned this tool and located the radiator valve – very slowly twist the valve anti-clockwise. You should first hear air escaping before water starts to leak from the valve. Let out a small amount of water from each radiator – checking the pressure after each one.
Continue to do so until your pressure gauge is back in the glorious green zone.
What should boiler pressure be when heating is on - key takeaways:
Here is a summary of the key things to bear in mind regarding what your boiler pressure should be:
When your boiler and heating is on pressure should be between 1.0 to 2.0 bar.
When the heating is off, boiler pressure should be between 1 and 1.5 bar.
If you need to top up the pressure - add water via the filling loop.
If you need to reduce pressure, consider bleeding your radiators.
If you suspect a leak or a faulty boiler in need of repairs, always consult the assistance of a registered Gas Safe Engineer.
Related reading:
How to know when it’s time to replace your boiler?
Boilers don’t last forever. In fact, on average they last between 10 and 15 years.
You can try to keep your boiler on life support even when it’s struggling to hang on for dear life – but this will likely cost you, even more, paying for constant repairs and potentially losing lots of heat due to poor efficiency and performance.
So, if you're not sure read our quick guide on deciding whether to repair or replace your boiler.
In the meantime, here's what else you can do to prepare:
Check out reviews on your current boiler
Get quotes for the necessary repairs and consider the likelihood of that – or another issue – occurring again. If it’s over 10-years old, you’re on thin financial ice
Put some leg work in and read up on the best boiler to buy (to make sure you don't buy a wrong'un)
Check out which boiler brands are best to make sure you are buying from a reliable brand
Weigh it up against the cost of a new boiler, covered for repairs under a 10-year warranty. 0% monthly options & Buy Now Pay Later available
If your boiler can be fixed easily and cheaply with little chance of a future breakdown, a repair is your best bet.
An old boiler, with an expensive repair required, is certainly worth taking the leap to a new boiler.
