High pressure heating only

[ir50]

I have a system boiler that is now plumbed in as a combi using an external PHE. So the flow goes from the boiler into a 3 way valve. One port serves the rads and the other port feeds into the heat exchanger and back to the boiler.

Problem I have is when calling for heating, pressure gets as high as 4 bar. At which point the PRV opens enough to allow a constant drip out the overflow. (Bit odd as PRV is actually 3 bar rated)

Now when hot water is demanded (and valve denies heating), pressure comes back down. Although must admit, still sits at about 2 bar. (At cold boiler pressurized to 1.5 bar).

I've checked rads for air but all were ok. I re-pressurized expansion vessel at week end too to 1 bar. Couldn't tell what had before as first tyre gauge wasn't working properly but air did come out and no water.

Surely must be something on the heating side.

Could the boiler output temperature increase the pressure? When hot water demanded, boiler output probably only gets up to about 60 degs.(Think due to PHE drawing all heat so return temp low) With heating, output get up to 72 which is what set at.

So I'm in situation where I have to re-fill boiler each night

Any ideas?

 

[ir50]

Ah, after reading another thread, wondering if my expansion vessel is not big enough. I've only recently converted it to a "combi" as was feeding a heat bank before. So have had problem since convert as system was not pressurized before.

My reasoning is that it is feeding a house conversion that used to be 2 x 3 bed semis but is now 1 property. So has about 20+ rads in it. Boiler's integrated vessel is 10 litres. Maybe I need to add another 10 litre vessel??

 

[hazetimesfive]

stick an 18 litre on top, another 10 would be borderline imo

 

[ir50]

Cheers. Bought one today so will try this at weekend