open vent back boiler pressure

Hi. A combi boiler usually has 1 to 1.5 bar in the rads.

What is the pressure at the rads for a baxi back boiler system ?

Its open vent there is a tank in the loft etc.

The pressure is detemined by the head of water eg water level in tank down to boiler. 1 bar is 33 ft head so if a bungalow it would be around .2 of a bar and a three story house up to 1 bar.

Ok. im concerned that all the crap plumbing in this house will leak when we change to a combi.

Generally it doesn't leak apart from maybe some rad valves weeping. But there was a post earlier regarding Servowarm rads that balloon out as they were only suitable for low pressure. as long as system is in copper and in reasonable condition it shouldn't be a problem.
I had a 70+ year old curved steel rad work ok under a sealed system. Had to test it before we filled with water and it held ok.

You may have more problems with the hot water side as its going to be under more pressure than it even had. Check connections to taps and any showers. Make sure there is not a booster pump on the shower.