What to do if your boiler pressure is too low

What is boiler pressure?

To understand low boiler pressure, it’s important to first understand what boiler pressure is. Boiler pressure is essentially the amount of water in the heating system at any time. Too much water and the pressure will show as too high, but too little water and it will show as low. Boilers have a pressure gauge located on the front that will tell you the current pressure of the system.

Check out our guide to find out more about how to read boiler pressure gauges.

Signs of low boiler pressure

There are various signs to watch out for that indicate your boiler pressure is too low. For example, you might find your radiators don’t warm up properly, or you may even have no heating or hot water. If you notice issues like these, check the pressure gauge on your boiler. The clearest indication of low pressure is that the dial on your pressure gauge has fallen.

Why is the boiler pressure too low?

The two main causes of low boiler pressure are:

Leaks in the system

When you have a leak, even if it’s a slow leak, the amount of water in the heating system is reduced, and this causes the pressure to drop. It’s important to find the source of the leak so you can get it under control, and then you can use the filling loop to add more water to the system. 

The most obvious sign of a heating system leak is often dripping around the boiler itself. So, if you suspect you may have a leak, start by checking the appliance for any signs of water escape and damage. Look out for puddles underneath the boiler, or water coming from the pipework. If you spot evidence like this, you should call a Gas Safe engineer to investigate the cause and fix the problem. 

It is sometimes hard to spot very slow leaks. So if you don’t see any obvious signs of water escape, you may want to place a sheet of paper underneath the boiler and wait to see if over time drops of water fall on it. 

Faulty radiator valves can also be a source of leaks, so this is another area to check. These valves are usually located at the bottom right or left of the radiator. If there is no immediate sign of water escaping, you can make sure by wrapping tissue or cloth around the valve and checking it after a day or so for evidence of dampness. If water is escaping, you might need to replace the faulty valve. 

You can check the radiators themselves for signs of leaks too. These central heating components can corrode over time and develop pinhole leaks that are tricky to spot. If you see any rust on your radiators, this can be a possible cause of a leak. Again, place a tissue or cloth over the affected area and leave it for a while to see if water is escaping. If your radiators have developed leaks, you will need to get replacements.

Sometimes, it is the pipes carrying water around your system that develop faults. The best way to check for this is to carefully inspect the walls and floors in your home where your central heating pipes are located, looking for evidence of dampness or mould.

You recently bled your radiators

If you have recently bled your radiators, the pressure will drop slightly because you’ve released the trapped air in the system. This air needs to be replaced with water to get the pressure back up again.

How to fix low boiler pressure

If the boiler’s pressure gauge is showing low pressure, as above, you can fix the issue by adding more water to the system. Before you do this, it would be a good idea to check for leaks to ensure that isn’t the reason for the low pressure.

If you have done this, you can follow the steps below to increase the pressure of your boiler. If you’re unsure of anything or can’t find the filling loop, contact the manufacturer or check their website for guides on your specific model of boiler.

1. Turn the boiler off and wait at least an hour for the water to cool down.
2. Attach the filling loop at both ends (one end to the boiler and one to the mains water valve)*.
3. Open both valves of the loop to allow fresh water to enter the system.
4. Keep an eye on the pressure gauge and continue to let water in until it reaches 1.5 bar.
5. Close both valves, one first and then the other.
6. Turn the boiler back on. Some may require you to reset them by pressing the ‘reset’ button.
7. Undo the filling loop from both ends, using a towel to catch any excess water that may drip out.
8. Keep the filling loop in a safe place until you need it again.
   
   

*If your boiler has an internal filling loop, you don’t need to attach it yourself. Instead, you can flick the switch on the valve to open it. 

When should I call a Gas-Safe engineer?

You should call a Gas Safe engineer to check over your boiler if you’ve repressurised the system but the pressure has dropped again or if you are continually increasing the pressure. It could be a sign that there’s a leak elsewhere that you haven’t noticed or it could be an issue with your boiler. The engineer may be able to provide some advice or fix the problem for you.

Should you notice a leak in your system, don’t try to increase the low pressure yourself. Instead, call a reputable heating engineer to look at your heating system for you.

For more information on this topic, read our guide on leaking boilers.

Summary

• Check the pressure gauge on the front of the boiler to see if it has low pressure.
• Causes of low pressure include leaks and recently bled radiators.
• The boiler’s pressure should sit between 1 and 2 bar
• You can increase the pressure by putting more water into the system via a filling loop.
• Call a Gas Safe engineer if the problem isn’t resolved.