Replacing Both Furnaces

  Our house is about 2400 sq ft on two levels and was originally built in 1990 with two furnaces. Both furnaces are located in the basement side by side. It is getting close to the time to replace both units with higher efficiency systems. Is it better to go with two smaller units or one larger unit?

You are fortunate to have a choice. Most homes have one unit in the attic and the other in the basement. In your home, both the upstairs and downstairs ducts are in the same location making it possible to serve both with a single unit.

What is growing in popularity is a single high efficiency 90%+ furnace with a two-stage setup with a variable speed blower. It has a "smart" 2-stage timer that attempts to adjust to bring on the high stage a little sooner if it thinks it'll be needed, or a little later if it thinks it won't be needed, based on recent history.

The zoning is done via a separate box (though I believe some furnaces have the function built in). It's a computer board that the thermostats feed into, and it controls the furnace and AC. It also has outputs to operate the electric dampers to control the amount of conditioned air to each floor.
The zoning is really pretty simple. If either zone calls for heat, the furnace fires, and the dampers divert the air to one zone, the other, or both, as indicated by the thermostats. The furnace still decides on its own when to go to high & kick up the fan.
AC is done essentially the same - the zone computer turns on the AC and tells the furnace fan to come on, then selects the zones.

Some people think one furnace with a zoned system is probably the best way to go.  You will save money over two separate units and probably on energy costs through the years also.  If you have someone do accurate heating and cooling load calculations, you may be amazed how small the furnace can actually be.  Many people oversize furnaces "to be safe."

Other people feel two separate units are the only way to go. The installation cost may be a little higher, but you do not have to rely on a damper system. Also, if one unit goes down, you always have the other one to supplement.

The ductwork is as important as the furnace. Most ducts are not sealed properly, and flex ducts may be kinked which will reduce air flow. Also have the duct system tested and balanced by a certified NCI duct balancing company. Industry testing revealed the average home loses 43% of its rated capacity due to substandard ductwork. Whatever you decide, I suggest you get factory authorized contractors to discuss these options in person with you.

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Barbara & Ausker Morris
AT Morris Realty Group, Inc.
3162 Johnson Ferry Road
Marietta, Georgia 30062

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