As we continue to get quotes for replacing our 2-speed 80% AFUE furnaces (15 years old and becoming problematic), we have had 5 different quotes, mainly for Carrier and Lennox systems. Were ready to commit to Carrier 96% AFUE Infinity systems (variable speed) but the latest vendor was wrapping up a quote with the same equipment (Infinity with variable speed) and noticed that although we have 2 furnaces, we have shared return duct work from the first and second floors. He stated that without adding dampers, Carrier does not support using variable speed furnaces with shared return duct work - something about when one unit is on, it reverses the motor on the other because it spins freely, so when the reversing motor does kick on, it literally slams on because it goes from one direction to the other. Apparently the 2-speed motor (non-variable) in the lesser Performance series will not move backward passively in the shared return duct configuration, so that is the way we should go. Although nobody else raised the issue, I wouldn't expect someone to try to sell me less expensive equipment unless this was true. Additionally, he called the Carrier rep who confirmed that the variable speed Carrier product should not be used with shared duct work.
1) Is it correct that with shared return duct work, we cannot go with variable speed without adding dampers to the ductwork (which apparently isn't worth the comfort gain for the cost involved)?
2) Is this a Carrier-specific issue (passive reverse of variable speed motor) or is it the case with all variable speed furnace brands?
Would really appreciate a professional opinion so I put the right equipment in.