How to Reduce Boiler Pressure in an Older Boiler

What You'll Need
Bleed key
Old cloth

Boiler pressure can be a tricky thing. With an older boiler, in particular, it’s vital to know how to reduce the boiler pressure. Luckily, it’s not a complex procedure. You don’t have to be a qualified heating engineer to do it. It’s also quick, and should only take a few minutes.

Step 1 - Pressure

Boiler pressure builds up in the system. The way to reduce it is to bleed the system by letting air escape. The boiler feeds a closed circulation system. By expressing air from the system you reduce the pressure in the system. You can then adjust the pressure level later by turning on the feed of cold water to the system.

When bleeding the system, the heating from the boiler should be off. If it’s on, the risk is of actually drawing more air into the system when you bleed it.

Step 2 - Radiator

When reducing boiler pressure, begin by bleeding a radiator. Choose the radiator that’s highest in the house, for instance in an upstairs bedroom, as this will be most effective, although you can bleed any of the radiators in the house. Switch off the heating while the water in the system is warm.

Find the bleed valve on the radiator. This will vary, but with many systems it’s on the top rear of the radiator, although with some radiators it might be at the bottom.

Step 3 - Bleeding

You should have a bleed key for your radiators. If you don’t, go to the hardware store. You’ll need to know the make of the boiler and describe the radiator, as well as making a sketch of the bleed vale opening.

Fit the bleed key on the bleed valve of the radiator. Place a bucket under the valve and then wrap the rag around the key. Turn the key counter clockwise about ½ turn. You’ll hear air being expelled. Wait until water begins to come out of the valve and then tighten the valve again, going ½ turn clockwise. It’s important not to tighten the valve too much. Where you have a sealed system be prepared to add cold water to the system to reach the optimum pressure for the system.

Step 4 - Circulation System

Switch off the heating when the water in the radiator is warm. At the boiler, check the pressure release. Place a bucket under the pressure release valve. Very gently, use your wrench to open the valve, making sure you stand clear of it. You should only need to open the valve about ½ turn.

Wait until water starts to drip from the valve and then close the valve again. You don’t need much water coming from the valve. Don’t over tighten the valve when you close it. Bleeding the system will immediately reduce the pressure. You need to be aware that it can leave the pressure too low in the system. Know where the pressure should be and add cold water from the mains line.