Replace Furnace or Stick with Dual Zone Setup

hvac

I recently moved in to a new home that is setup with a single stage Goodman furnace that is divided in to two zones (upper/main). When we first moved in to the home, the upstairs was always cold. I have since had the furnace serviced, and installed new thermostats and upstairs now gets to temperature. I am currently working out of an unfinished basement and I have been noticing the furnace is running a lot (and the NESTs are now telling me the furnace is running ~16 hours a day).

After some digging, I am seeing that the furnace is regularly erroring with a "primary limit circuit open" LED error code. I switched out to a low resistance (almost no resistance) air filter to confirm the filter was not restricting air flow too much. I recently had the wiring reviewed as well since I needed to add a common wire for the NEST thermostats. After talking with the technician again, they have indicated that single stage furnaces do not work well with multi-zone and I should disable the zone control and second thermostat and run everything as a "regular setup". The internet seems to confirm that single stage furnace + multiple zones is indeed "bad design".

I am the type of person who likes to fix problems rather than work around them. I plan on adding AC to the setup in the spring and I was planning on getting a two stage AC. Would I be crazy to replace the furnace in order to have a two stage furnace and keep the zone control? or should I really just disable the zone control as suggested?

Edit

The zone control is handled by an Aprilaire temperature zone controller connected to 4 electronic dampers. Image below showing one side of the setup.

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Best Answer

It's not clear to me how the second zone turns on and off.

Normally I would expect it to work by controlling the air flow. So one zone (the one that gets chronically cold) is heated whenever the furnace comes on. The other is heated when a damper to that zone is opened.

The problem with this:

Forced air furnaces are designed to output a certain volume of air. Restricting the air flow results in a higher output air temp and a loss of efficiency. In addition metal vent grates can be painful to touch. (But do a nice job of drying wet mittens.)

So a better way would be to keep the air flow constant, but split the flow between the two zones. Visualize a 1x2 foot duct that splits into two adjacent parallel 1x1 ducts, and has a 1 foot square plate that can slide to any position from covering all of duct 1 to all of duct 2. I've seen devices like this in commercial HVAC, but not in residential systems.

If you are doing a major refit to add AC anyway, you may want to take advantage of future differential prices of electricity:

  • Use the AC hardware at night when power is cheaper to chill a large tank of water to near freezing.
  • Circulate that chilled water though a fan coil to provide cold air for the house.

In winter the AC is turned off, and a small high efficiency gas water heater is used to heat the tank of water. Since the tank has no circuity or fittings, it has very long life and is fairly cheap. Insulate it well.

Potentially this also allows you to add warmer zones to the house. E.g. hot water under the bathroom floors. Hot water towel warmers. (Note that the requirements for embedded hot water, and radiant hot water take much different temperatures.) This however makes the system more complex.

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