Combi boiler pressure fluctuation

[PerranOak]

The pressure in my combi boiler (oil fired) seems to vary hugely.

When the heating is on it is at about 1 bar.

When the heating is off it can be as low a zero but then rises to 1 bar after a few minutes after the heating comes on.

Other than this everything seems to be fine; both the heating and hot water are working well.

Should I top-up the pressure?

Cheers.

 

[terryplumb]

If it works ,it doesn't need fixing !!

 

[PerranOak]

Ah. So I can ignore the pressure reading?

 

[terryplumb]

As long as the boiler continues to work ,yes. Cold system pressure would typically be 1 bar ,and would rise when central heating is on as hot water expands and increases system pressure. You may have an expansion vessel that needs attention at next service. In the meantime leave it as is

 

[PerranOak]

But the system pressure when cold is zero.

 

[Harry Bloomfield]

But the system pressure when cold is zero.

It could be the gauge which is a little out, if it were really zero the boiler should be prevented from firing. Try bleeding a radiator on same level as gauge, if there were really no pressure, no water would escape.

 

[terryplumb]

Zero pressure reading on a gauge doesn't mean there is no water in the system. You can release a cup full of water from a sealed central heating system and drop the pressure to zero ,and still have dozens of litres of water still in the system.

 

[PerranOak]

Cheers both. Yes water does come out of the rad pretty sharpish!

I'll leave well alone.

 

[vxxbcks]

It could also be low on water, meaning that there’s no water in the expansion vessel when cold. When heated the water expands a bit into the vessel, which should have between 1 and 2 bar in it, and hence it serves to stabilise the pressure accordingly.