shower pump installation problem. vent branch separation.

[hazetimesfive]

I am converting a large house to pumped hot and cold supplies.. but the vent branch is not in the cylinder cupboard, its a very old house and it will be impossible to rip it apart to find out where it separates, i wouldn't be surprised if it feeds the whole house and then continues up to the tank, i cannot put a pump on that as it will continue to pump through the vent... forever...

any thoughts on a solution? running new pipes seem to be the only thing I can think of, but that's soooo not gonna happen in a house like this...