What Should Boiler Pressure Be When Heating Is On?

It’s a common worry.

Your boiler isn’t performing quite right or you’re not sure if you’re reading your boiler gauge correctly, but it seems that your boiler pressure is too high or too low.

Getting it just right, with some boilers can feel like an impossible balance.

But it shouldn’t be.

So whether it’s a Vaillant Ecotec Pro 28 with pressure that’s too high or a Worcester 28cdi with boiler pressure that’s too low, or indeed any other kind of combi boiler that has pressure problems, there is one excellent piece of news.

You’re asking the question and looking for a solution rather than ignoring it.

Read on to learn how you can save yourself money and learn a key skill in pressure and boiler upkeep.

Central Heating Pressure – What’s Happened?

When you’re installing a new boiler or just doing a bit of preventative maintenance and upkeep on an older boiler, you may notice a pressure change.

If your system has completely cut out or failed, it’s not too late, and the place all good engineers will check first:

the boiler’s pressure gauge

If your central heating has gone off, this is probably for the best. Your system has probably noticed something isn’t right and is protecting your home from further damage.

There are numerous reasons behind pressure fluctuations in boilers.

While high pressure puts a strain on your system, low pressure can cause your central heating to cut out – both are equally troublesome and its good practice to know how to repressurise your particular model.

Recommended reading: Balancing Radiators: A Detailed, Step-by-Step How-to Guide

The Basics: What Pressure Should my Boiler Be?

There are a few things you need clear in your mind to tackle any boiler pressure problems.

1. Understand that boiler pressure is pressure levels at which hot water is running within your sealed central heating system. This is different from water pressure which is the pressure that you get coming out of your taps. You’d be surprised by how many people get that confused!
2. Next is knowing the optimal pressure for your boiler. The exact pressure for your combi boiler can be measured on the boiler pressure gauge which is normally on the front of your boiler – some are more advanced LCD/digital control panels, while others are a simple metre or gauge with needles. You’re looking for something that on a normal day, will read 1 to 2 bars.
3. As we’ve already said, it’s good you’re thinking about doing something and not ignoring the warning signs.

Boiler Pressure Too Low – Boiler Losing Pressure

There are 2 most likely causes if you’re experiencing a drop in pressure.

1. Have you checked for water leaks?

If water is constantly leaking out somewhere in your system – however small the leak is – you will continually see your boiler pressure drop.

In some cases, a leak can be easy to detect: look out for damp patches and check joints in obvious places like by your radiators.

If your boiler itself appears to be leaking – this is not a job for you. Gas Safe Engineers are always the only people who should work on the inside of your boiler.

Some systems, such as underfloor heating can be a little more complex to diagnose.

If you think the culprit is the boiler, or you’re regularly topping up your system with no obvious sign of a leak – give a PHS engineer a call today and we can help you diagnose the problem.

2. Bleeding Radiators

Have you bled your radiators recently? If so, great! That will explain a dip in pressure. It’s not a problem but if you’ve released a bit of air from the system, the pressure is quite likely to drop.

Recommended reading: Bleeding Radiators Sequence and How To Bleed Underfloor Heating

If your systems pressure appears to be too high, by that we mean is exceeding 3 bars, you need to check what’s happening before it damages your system/home.

In most cases boiler pressure increases due to water expansion which occurs naturally in the heating process. To check if you think this is the case, try some of our processes below.

Boiler Pressure Too High – How to Reduce Boiler Pressure

High boiler pressure is indicative of too much water in the system and it’s trying to find a way to escape.

Recommended reading: How to Drain a Central Heating System

If you have a draining point which is easily accessible (check beneath a radiator) you can open it up with an Isle of Man key or another tool (plier/spanner) and let the water out.

Top tip: do this with someone else present if you can’t do it close to the boiler. You’ll need to keep an eye on the pressure gauge as you go to prevent going too far the other way.

Another common way we can react when pressure is too high on combi boiler systems is to bleed a radiator.

This is definitely something you can do yourself. You just need a radiator key which you probably have lying around somewhere (if not, pop to your local DIY store).

Turn the release valve on the radiator (normally at the top). Excess water and air bubbles will come out of the system – so have a towel/bucket on the floor in case!

Top tip: never perform either of these processes when the heating has been on. Let it entirely cool first, or you will risk coming into contact with scalding water.

Recommended reading: Bleeding Radiators Sequence and How To Bleed Underfloor Heating

Boiler Pressure Top Up – How to Repressurise a Boiler

The most common way to top up the pressure in your boiler is to use the filling loop.

You’ll need to know your boiler’s model and age in order to be sure where to look for the filling loop.

If you know your particular model check your manual or even the boiler manufacturer’s website as they often have helpful tips.

However, if you are struggling with this, check for these 3 varieties in their common homes.

1. External filling loop/link – It’s usually beneath the boiler or to the side and is not within any housing.
2. Keyed filling loop/link – this will be in housing and probably beneath the boiler. You’ll need a key, as the name suggests.
3. Keyless filling loop/link – surprise surprise – there’s no key here. This one opens via a lever. Again most likely situated beneath the boiler

When you turn the valve you will begin to hear water flowing. This is the system bringing more water in and increasing that pressure.

You’ll need to watch the gauge carefully to make sure you don’t cause more damage by overfilling it.

Stop when the gauge gets to 1 bar, try to keep it under 1.5 bar so that you never go too far in error.

How Often Should I Check my Boiler Pressure

If you’re finding yourself checking the boiler pressure a lot and subsequently having to top it up more than once or twice a year, there may be another problem that needs looking into – such as a leak.

With increased boiler pressure, corrosion can sometimes cause this to build up quickly and again could indicate deeper issues.

In a healthy boiler, the pressure should remain stable so unless you recently bled your radiators you shouldn’t be expecting a sudden pressure drop/increase.

But do not ignore your boiler pressure gauge – it’s there for a reason!

If you check it regularly (once a month or so) you will notice if there are any problems and you can catch them before it goes too far.

When do I Need a Gas Safe Engineer?

It’s time to call PHS for a Gas Safe Engineer if:

• You are struggling repeatedly with boiler pressure problems
• You can’t diagnose the issue yourself
• You think your boiler is leaking/dripping
• Have a complex system or are unsure about anything you’re doing

With gas appliances, it is always better to be Gas Safe than sorry and at PHS whether you’re in London, Essex, Suffolk, Hertfordshire or Kent, we aim to be with you within 24 hours so there’s no need to let boiler pressure (whether it’s too low or high) upset the day-to-day running of your home.