I have a home HVAC system that is zoned into two zones. One zone runs the first floor and basement and the other runs the second floor. I am replacing both the furnace and the a/c. The salesman tells me that a two-speed furnace is more appropriate for zoned systems than a variable speed. Is that correct? I'm also looking at replacing both thermostats with digital programmable units. I know I need to buy a two-stage compatible thermostat, but will I run into any other problems considering this is a two-zoned system? Thanks much!