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My 2,900 square foot house currently has two separate AC systems (one upstairs, one downstairs). They're both very old and need to be replaced.

One of the four HVAC salesman that provided quotes suggested using a single Lennox XC21 system and claimed it can provide 2 cooling zones. They'd merge the supply ductwork and install dampers to allow one or the other or both zones to be cooled.

How well does this actually work? Is the Lennox XC21 a good unit? Based on the other info I've gotten, it seems the XC21 is spec'd out correctly as far as tonnage (5) and SEER (21) go.

On a side note, the salesman said I would NOT be able to use my Nest thermostat with the XC21 because there's some kind of humidity controller in the unit that the Nest won't be able to control. Anyone know if this is really true or if there's a workaround? I've become a bit addicted to my Nest!

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Using a single HVAC unit with zone dampers is pretty well-established and can save the trouble of having multiple separate units. Some things to discuss with your salesman (and if s/he doesn't have detailed answers, ask for someone with detailed knowledge):

  • If your cooling needs will often only require one zone or the other (i.e., daytime downstairs, nighttime upstairs), will the system cool that single zone too rapidly to allow proper dehumidification?
  • will it have the capacity to cool both zones effectively when needed? (balancing this with the first point may require careful sizing of the system)
  • if the system fails, you have no cooling. With two separate systems, you're likely to have some cooling in the house with one system running.
  • this might give you the option for a third zone, if your current ductwork allows (i.e., split one of the zones into two). This could be a cost savings if you are currently cooling space that's not used full-time.

We've been in a house with three zones on one system for 13 years and it's served us quite well.

I can't comment on that specific unit or the Nest.

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  • (I'm the OP) Thanks for the response - very helpful. I think they spec'd it out pretty carefully but I'll ask how well they'll be able to balance the zones. I did some follow up on the Nest and unfortunately it's not compatible because the Lennox thermostat sends more complex signals than just "on" and "off". Maybe the next generation Nest will get more complex and I'll be able to get one. – user249493 May 21 '14 at 23:59

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