We are about to replace the two old force air, gas furnaces that heat our multi-story home with two new, high efficency furnaces. One furnace serves the first two levels, while the other one serves the top floor. We have been very happy with a two furnace set-up for many years and also like the redundancy of a second furance.
My question...the two furnaces sit about two feet from each other in the basement and share one large cold air return duct. The cold air return system is also shared among the various levels of the house. In other words, there are NOT segregated cold air returns that come together near the furances. Rather, they feed together on the upper floors and come to the furnaces in one duct. We would like to go with a variable speed blower in the new furnace that handles most of the heater, and a single or 2-speed blower in the second furnace. One contractor says that a variable speed blower set-up won't work because of the shared cold air return, and the closeness of the two furnaces. Something about static pressure problems causing the variable speed blower to go nuts. One other contractor says he would split the cold air return for about six feet from each furnace and install a separate 5" filter box on each furnace. He believes this would enable us to use a variable speed blower in the furnace. What do the experts here think? I'd like to get these furnaces bought before it gets real cold here in Wisconsin. Thanks, John