HVAC pressure


I'm working on a HVAC control system for a house that was built in 1937. The ducts were obviously not designed for zoned control, and I don't want to mess with them because there is some asbestos insulation. So I'm planning on adding servos to control the individual vents.

I'm concerned about having too many vents closed, causing too much backpressure and damaging the vents and/or the furnace. I'm going to add a pressure sensor to the duct right after the furnace to monitor the pressure and I'll have the system open up extra vents to keep it from getting too high. My question is, how high is "too high"? How many PSI, kPa, inches H20, mm Mercury (pick your favorite pressure unit) is reasonable?