How to repressurise your boiler

04th July 2022

Boilers are a mystery to most of us. Like driving a car, you only think about how it works when it doesn’t. Most of us know boilers use gas or oil to heat our homes but what happens when they stop working? The house will get cold, there will be no hot water but is there anything you can safely do to see why they aren’t working and maybe correct?

What is boiler water pressure?

Boiler pressure is a balance of air and water within the boiler and its component parts. Water pressure is an important component of the boiler’s ability to produce heat and hot water. A correctly pressurised system enables the boiler to pump water around the pipes, radiators and valves, producing the necessary hot water and heat. Pressure faults in boilers are relatively common. The boiler water pressure has these common faults:

• No pressure in boiler
• Boiler pressure too high
• Boiler losing pressure

No pressure in your boiler?

A water leak in the central heating system, or shortly after you have bled a radiator are the main reasons for the boiler having very low or no pressure. In most cases, boilers do not have a cut off for low or zero pressure. This means they will often continue to work when there is little or no boiler pressure. If this happens there is an overheating risk, and the boiler may sound louder than it normally does when it has the correct amount of pressure in the system.

If the lack of boiler pressure does not appear to be due to an obvious water leak, or excess water released from the pressure relief pipe, water can be added to the system to re-pressurise it.

Boiler pressure too high?

If the boiler pressure is too high this puts a strain on the boiler and central heating system and if left unchecked will cause it to breakdown. How do you tell if the boiler pressure is too high? Most boilers have a pressure gauge, this may vary in format but usually reads between 0 – 4 bars. Bars are the unit that measures pressure. High pressure is usually in excess of 2.75 bars. Most boilers will cut off when the pressure gauge goes over this reading. Many pressure gauges are marked in red when the pressure is excessively high.

Boiler’s need to return to the normal level of 1-1.5 bars, usually indicated by a green area on the pressure gauge, by removing water from the system. Excess water in the system may result from an earlier re-pressurisation. Water is easily removed from the system by bleeding the radiators. Each radiator should be bled in turn. Insert a radiator key into the valve and turn anti-clockwise. Air will escape. Continue until a drop of water escapes and then check the boiler pressure gauge. If it did not return to the normal level, bleed another radiator and check again. When the boiler pressure is between 1- 1.5 when the boiler is cold, stop.

04th July 2022

Component failure – Pressure release valves and filling loops

Abnormally high boiler pressure maybe because of boiler components like the pressure release valve, and the filling loop not working correctly. The pressure release valve lets water out of the system to release pressure. If this is not working, the water will build up and the boiler pressure will increase.

The filling loop is a metallic flexible pipe usually found underneath the boiler. It has either one or two valves that resemble taps. The taps should be closed. If they are open, then close them. If they are closed but the system is still over pressurised, you need to call your gas safe registered engineer to take a look.

Why your boiler is losing pressure

A boiler losing pressure is something that will need investigating. The main causes are a water leak in the system. Alternatively, a malfunction of the expansion valve, this usually results in damage to the pressure release valve, which may need replacing.

A low pressure boiler is not running efficiently and is likely to produce less hot water and heat, and be more expensive to run. The positive news is that it may be possible to fix a low pressure boiler, without calling in a gas safe engineer. However, if the primary cause is due to a damaged part as described above, this may need replacing, or the low boiler pressure will be ongoing.

Therefore, to return the boiler’s pressure to the normal level of 1-1.5 bars when cold, you need to know how to re-pressurise a boiler.

How to re-pressurise a boiler

First, you need to know how to increase boiler pressure or how to top up the pressure on the boiler. Before you do this, it is important to check your boiler manufacturer’s manual, which may be in hard copy, or accessed online. Check the boiler is one that can be re-pressurised by the householder. The process for re-pressurising the boiler may be in the manual too, but the process below will work for most boilers.

• You need to find the filling loop, which is usually underneath the boiler. It is a flexible, metallic pipe with usually two tap-like valves.
• You need to find the pressure gauge on the boiler. It is important that you can see the pressure gauge when you are increasing or topping up the boiler pressure.
• The boiler and central heating system need to be cold.
• Open both valves on the filling loop. This allows mains cold water to enter the central heating system.
• Let the cold water flow into the filling loop and the boiler until the boiler pressure reaches 1.5 bars. On some boilers, the ideal pressure level will be indicated on the pressure gauge.
• Turn off the valves on the filling loop.
• Check the boiler pressure is steady at 1.5 bars.
• Turn on the boiler and monitor the pressure levels.

If the drop in pressure was not due to an ongoing component fault, the pressure should remain at the normal level. If you find that re-pressurising your boiler isn’t a permanent fix, you will need to contact a gas safe engineer.