What pressure should my boiler be?

Another hot topic for discussion is the boiler pressure. This is a very misunderstood area and comes down to a couple of factors.

•The boiler brand/ model

•How the system gets its water for the radiators.

A common gauge found on most boiler models, 1 bar is the normal setting, but this isn’t always the case

Boiler Brand/ Model

Most gas boiler brands require pressure to operate,  it’s as simple as that.

They either have a pressure switch or in most modern boilers the main circuit board is programmed to be pressure dependant.

However, some brands can operate on little or no pressure and these boilers are a personal preference of mine, for that very reason.

The filling point for my radiators.

Most boiler manuals or instruction videos will direct you to the boiler for the fill point.

However, in most cases the more likely position is going to be the hot press.

This is simply because all pipework is accessible there, where the boiler is usually in a cramped kitchen press or similar unit.

In the hot press you’ll likely find what’s called a filling loop. The filling loop is a special design, that only allows water to flow in one direction.

This is a safety measure to prevent water from your radiators mixing with domestic water you’ll use to wash with or drink.

An example of a filling loop with valves in the closed position.

Pressure or no pressure

If you have a pressure dependant boiler then this will require the filling loop to be directly connected to your incoming water mains.

In this case the filling valve is opened,  the system is charged to 1 bar and then the valve is CLOSED.

Closing the valve is vitally important, failure to do this can be detrimental to a heating system.

This is a direct result of a failed mains pressurised system, the brown stain is a result of the filling loop not been closed by the homeowner

What causes the damage?

By leaving the mains fill valve on, the pressure will go way past the recommended 1 bar.

If the pressure goes above 3 bar,  the safety valve will open and water from the system will disperse out side.

This is a catastrophic cycle as fresh water is constantly passing into the heating system, fresh water is rich in oxygen, this in turn causes rapid corrosion, hence the brown stains.

This is just the beginning,  the high pressure will eventually cause the expansion vessel to burst.

Expansion vessel

The expansion vessel is the main component of a system gas boiler, but can also be found on the heating system external to the boiler.

As water heats it expands, like steam would emit from a kettle.

To accommodate this in your radiators we use an expansion vessel.

As the water expands, a bladder inside the vessel expands and contracts as the water cools.

Over pressurising a system can cause the bladders to burst and when this happens the safety valve comes into play again.

The pressure from the expansion now finds its way through the safety valve, to the outside.

This is noticed by the Home owner who in turn opens the filling valve to let fresh water back in.  Eventually the home owner, fed up refilling, just leaves the valve on.

It’s only a matter of time before the boilers main heat exchange becomes damaged beyond repair, due to corrosion.

Why I prefer low pressure boilers

A low pressure boiler doesn’t need to be pressure dependant, which means it can get a low pressure supply from storage tanks.

Because of this the filling valve can be left open as the tank pressure will never get anywhere near the 3 bar required to lift the safety valve, and the high pressures that can burst the vessel.


Mains pressurising a system is absolutely sound practise and in some cases has to be done,  ie an attic conversion where the radiators are higher than the storage tanks.

A mains regulating valve can also be used, these are preset to not allow anything above 1.5 bar into a system.

However, they are expensive and can be prone to failure.

The main problem is the homeowner not closing after bleeding rads or topping up a system.

If you’re losing pressure check for a tell tail brown stain on the outside,  this is almost certainly a burst vessel and will need immediate attention.




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