Usual time for pressure to return to normal after heating?

[Phoebus]

Hello, we had a radiator temporarily removed while builders did some work last month. I'm just not sure if I am being more alert because of this or if I'm misreading normal behaviour for the time for pressure to drop in a combi boiler.

We had some building work and the builders removed then replaced a radiator. Bled it till it was full, topped up the tank, checked for leaks etc so seemingly back to normal. I'd filled the boiler to the gauge was at the top of the green zone (1.5bar) but the other day I noticed it seemed a bit high at 2bar when not firing up. Resolved to bleed to let out pressure today as I was about to head out when I first noticed it.

This morning the pressure was showing as 1bar. So I topped it up to 1.5bar. Heating went on for an hour since then and then gone off and it went back to 2bar and is now steadily dropping along with the water temperature inside the combi boiler.

I could swear that it used to return to its previous pressure straight away after heating. Is this normal behaviour for a combi boiler or am I just looking at it too soon?

Thanks.

 

[flameport]

Pressure will drop as the water in the system cools, and only get to the original starting pressure when the system is completely cold. How long this takes depends on several factors, but it won't ever be immediate.

 

[Phoebus]

Thanks Flameport, should have remembered my physics and that pressure and temperature are related.

 

[Agile]

Typical cold system pressure is about 1.5 bar.

When hot this will increase by about 0.3 bar in a well designed system properly maintained.

Usually takes about an hour for the rads to cool down to room temperature when the pressure should go back to the original cold pressure.

Tony

 

[PullerGas]

Typical cold system pressure is about 1.5 bar.

When hot this will increase by about 0.3 bar in a well designed system properly maintained.

Usually takes about an hour for the rads to cool down to room temperature when the pressure should go back to the original cold pressure.

Tony

Tony is correct as per normal. Many combi boiler expansion vessels are very undersized, so the pressure may go higher as a results of limited space for water to expanded into.