Common Boiler Problems & How To Fix Them

Whether you’ve recently installed a new combi boiler, just wanting to ensure that your older one is in the best working order and well maintained, or if you have suffered with boiler problems in the past, you need to know:

  • What the correct pressure should be
  • Why it might be too low or too high
  • What you can do to correct the pressure levels

As a rule of thumb, the boiler pressure should be in between 1 and 2 bar. Of course, while this would be a standard guideline, you should always check the manual for your specific model, or check with the manufacturer or heating engineer, to be certain.

Water Pressure v Boiler Pressure

One thing to note, and to avoid confusion, is that boiler pressure and water pressure are not the same thing.

Whereas water pressure refers to the bar that water runs through your taps, the pressure of your boiler is referring to the bar that hot water is running through your sealed central heating system.

Why Boiler Pressure is Important

The pressure of the boiler is essential for ensuring that it works properly, safely and efficiently, keeping your home warm, your hot water supply consistent and your energy bills low.

The majority of central heating systems used in the UK are sealed, meaning they have no vents in place to deal with the water’s expansion and contraction as it fluctuates from hot to cold. Instead, the systems rely on pressurisation to accommodate the changes, meaning the correct pressure level is essential for the system to work correctly.

Depending on the make and age of your combi boiler, it’s quite possible that it will have an in-built sensor that will cause it to switch off if it detects that the pressure is too low. Which can be a particular nuisance as it will mean your home is going to be without hot water.

How to Check the Bar

You need to find the pressure gauge to check if you have the correct bar on your boiler.

On older boilers, the gauge will generally have a needle and will quite likely have a colour code system with green denoting the ‘safe’ zone, red the ‘danger’ zone.

If all is well and the bar is correct, then the needle will most likely be somewhere between 1 and 2 bar. Anything either side of this and you are very likely to have a problem.

You may find that you pressure gauge has two needles, one a fixed marker pointing the bar that it should be running at, the other pointing to the bar that it’s actually running at. A well pressured boiler should see both needles in alignment.

Newer boilers are likely to have a digitised control panel, with the pressure displayed on an LCD screen.

Low Pressure in Your Combi Boiler Equals No Heating

If the pressure of the boiler drops too low (below 1 on the gauge), then the likelihood is that the boiler will recognise it and cut out.

If it doesn’t cut out, it will still, nevertheless, fail to have the capacity to let the hot water to adequately circulate around your home.

Either way, your system is not going to be working. Which means inadequate supply of hot water coming from the tap, and not enough pressure to supply your home with central heating.

Which can be a particularly inconvenient problem for your boiler and for you; especially during the colder winter months.

Dealing with the Problem

Low pressure means that the system needs to be topped up. Before doing so, you should first switch off the system and allow time for it to cool down.

Topping up the pressure is, in most cases, achieved by using the filling link or loop.

Filling links will vary from boiler type, age and brand. However, you’ll find that conform to one of three general varieties.

  • External Link – A filling loop located on the outside of the unit (hence the external) which you will usually find on the bottom or side.
  • Keyed Filling Link – Located (in most cases) inside the unit and operated by a specific key, which is supplied with the boiler.
  • Keyless Filling Link – A lever at the bottom of the unit is used to operate this.

Whichever variety you have, once you’ve found the valve, start to turn or twist it. Make sure you’re in a position to keep watch of the gauge. As you turn you should hear the water starting to flow as the system starts to correct itself and the pressure will start to rise.

When you can see the needle point between 1 and 2, you know the system has been successfully topped up and you can stop turning the valve.

Your system should again be in good working order and something that you would typically only need to do once or twice a year maximum. If you find that you need to do this more frequently, then this suggests a more serious fault or leak in your water supply.

Obviously, the exact task for doing this will depend on the make and type of boiler that you have.

So, you need to refer the manufacturers manual, or call in an expert heating engineer.

What About When Your Boiler Has High Pressure?

It’s not just low pressure that’s a cause for concern with your boiler.

High boiler pressure, anything over 2 bar, and especially if it’s running around 3 or higher, can be a cause for real concern. It’s a condition with, if not addressed, be a cause of serious malfunction of the system, which could potentially lead to costly and potentially dangerous damage at home.

You can often get visual evidence of a boiler with pressure that’s too high, if you happen to notice leaking dripping from in and around the pressure valve.

If this leaking has been going on for some time, you might find that the pipework has suffered from corrosion. And, failure to deal with the problem is likely to require the replacement of the entire boiler.

How To Solve This

When you have a high pressure problem (having a bar in excess of 2) then you need to release this pressure.

This can be achieved by bleeding one or more of the radiators located throughout your home.

As with the low pressure resolution, you need to turn the system off first, and let it cool, then follow this simple procedure.

  • Place a bowl under the top valve of the radiator to catch any water that leaks out
  • If you have a radiator key in the house, take it and put it into the valve. If you don’t have a key, a flathead screwdriver will usually work.
  • Use the key or screwdriver and turn the valve in an anticlockwise direction
  • There will be an initial rush of air escaping, followed by a trickle of water
  • Check the gauge on the boiler
  • Once the bar is back within the 1 to 2 range close the valve

How Often Should You Check the Pressure Gauge?

If your boiler is in good working order and regularly maintained then you can be relatively confident that the pressure is going to remain consistent and within the safe zone.

Nevertheless, it’s good habit to periodically check the gauge to ensure that it’s OK, and provide you with peace of mind.

This might be no more than a quick glance once a month, or every few weeks.

As the autumn months start to draw in and the temperatures start to give thought to using the central heating once again, then it’s certainly worth checking that the pressure is right before you start firing up the heat.

You don’t want it to fail as the temperatures start to drop.


Whether you have a combi or a more conventional system, your boiler is an integral and extremely important aspect of your home or property. It’s the source of your hot water supply and the reason you can stay warm and comfortable when the weather turns for the colder in the winter months.

You simply don’t want it failing on you when you need it most.

Key to this is the need to ensure your boiler is operating at the right pressure. Too low and it’ll stop the system working. Too high and you jeopardise the system breaking altogether.

A well-maintained system will help ensure that the pressure remains stable, but it’s certainly good practice to routinely monitor the system so you know it’s in the right ‘zone’.

Follow the simple procedures for when you discover the pressure that’s too low or too high to quickly and relatively easily resolve the problem before it escalates to a major issue.

And if the problem persists, or you find you are having to take action to reduce or increase the boiler pressure frequently, then it suggests an underlying boiler problem that will require the services of a qualified heating engineer.

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