First of all welcome.
This would probably be better answered in the residential forum, but it sounds like to me simple OAT High & Low Lockouts to open the circuit to the Venmar 4100 and the IAQ could still be used. I haven't worked with the Venmar unit described, I am just surmising.
This is not a DIY site, but by the detailed explanation something tells me you should post a bit more about your background and also your general climate location so this can be answered better. I tried checking your profile, but there is really no information there either. I am requesting this thread be moved to residential.