We have a 2600 sq/ft house (1500 on main, 1100 on upper) currently serviced with two separate furnaces. One of the two units has a 2 ton A/C to heat & cool the upper floor. No A/C on the main floor via the second system. The main floor ductwork is sized proportionally larger as compared to the upper floor ductwork.
We are evaluating options for replacement of the system with High Efficiency units. The recommendations range in order of the highest cost to lowest as follows:
Option 1: Replace two furnaces with a single unit, sized for the entire house, with new condenser and zone control system. Changes required to merge ductwork are somewhat challenging but not extremely difficult. Contractor has installed similar systems with reportedly good success. The downside is total cost about 40% higher than lowest cost option.
Option 2: Replace two furnaces with two new furnaces and one condenser sized for upper floor only. Essentially replace exactly the same configuration with the downside being that we do not gain A/C on the main floor which is desired (temperature variance between conditioned upper floor and unconditioned main floor is significant). Control over the zones is excellent with the two separate furnaces and thermostats, but A/C only on upper floor.
Option 3: Replace two furnaces with one new furnace and one condenser sized for entire house. No zone control, but consider manual dampers in trunks to adjust for seasonal requirements (more flow to upper during A/C season and more flow to main for Heating season). This is the lowest cost option, but only about 10% less than Option 2 and would not provide any automated zone control.
All three contractors are experienced and highly regarded, but have different opinions on the solution.
Our concerns are with Option 3 providing much by way of zoning efficiencies, but we expect that the fan running 24/7 on low speed will maintain even temperatures throughout the house.
Our concern with Option 2 is not gaining of A/C on the main floor, unless there exists a condenser that may be able to service two independent coils. If such a solution were available then this scenario would be a good cost/benefit solution. Installing separate condensers for each furnace-A/C unit would be impractical for space limitations and would become as costly as Option 1, but would provide absolutely independent and fully programmable zones serviced by their own system and ductwork.
Our concern with Option 1 is the overall cost and the actual benefit of utilizing zoning to save costs during heating season by lowering the temperature of the unused zones at the appropriate hours. Can the extra cost ever pay for itself in dollars or in level of comfort assuming that Option 3 can circulate and maintain even temperatures throughout the building?
I'm interested in comments or concerns with the recommendations.