Combi Boiler pressure raises when CH on

[jonweb]

When the CH is on the boiler pressure gradually rises to the red after about 5 or 10 mins. There is no problem with the hot water from the taps, but I guess that's as they're not on for as long.
The expansion bladder is at the correct presure.

Any ideas why the pressure is building?

Ta.

 

[Bahco]

It is normal to see an increase in pressure of around 10% or so.
Maybe your cold pressure is a little high. Check your man instructions for figure.

 

[jonweb]

The cold presure is a little low, just over 1 instead of 2.5, then in 10 mins or so it's pushing 4.

It's only after the heating hasn't been on all summer, it's like a valve is sticking or something.

 

[billington]

baco is right what he is saying, but the cold pressure on the gauge would normally be set around 1 or 1.5 bar.You can check the correct pressure in your maufactures hand out.