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Thread: 2 furnaces and 2 A/Cs

  1. #1

    Default 2 furnaces and 2 A/Cs

    Hello all. We have finally bought our dream house in Colorado. A 6 year old, 3000 sq ft, 2 story home. We have 2 50 gal water heaters, 2 furnaces, and 2 a/cs. We do not need 100 gallons of hot water, so I plan to isolate one of the water heaters with ball valves.

    As far as the furnaces go, how am I supposed to set the thermostats with a dual system, so that they work together? One is in the attic for the second floor (2nd), the other is in the basement for the first floor (1st). I installed programmable thermostats for each system.

    I keep the temps down during the day, as well as during the night (64 deg). For the mornings I have the 2nd furnace warm up for about an hour while we are getting ready for work (69 deg), but leave the 1st furnace at the same temp (64 deg). The 2nd thermostat is in the master bedroom, so with the doors shut, shower steam, and a space heater, it doesn't take the thermostat long to reach that temp in the room. We are looking for an aesthetically pleasing way to hang a curtain in the opening between the bath and bedroom. It is wide open now. That should replace the want for running the temp up on the 2nd in the mornings.

    For the evenings I have the 1st furnace warm up to 69 deg, and leave the 2nd at 64 deg. I assume the heat will rise so that we do not need to run the 2nd in the evenings.

    As far as efficiency, are these settings a total waste? Should I be running both furnaces at the same temps all of the time? Or, should I not worry about the 2nd furnace, and run only the 1st? How about the A/Cs? Should I run them the same way?

    Thanks for your input.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Some of it depends on just how open the floor plan is. Any setback helps, but more is typical. Some of the thermostats are smarter than others. Most of the Honeywells (and maybe some others), you set the time you want the house a certain temp, and it learns how fast it cooled off and how fast it can warm up. Then, it may turn on 15-minutes early on a mild day, or an hour early on a nasty one.

    It also depends somewhat on the size of the furnaces...from a comfort standpoint, running one continouosly makes for the most comfort. I'd think about that, and use the second as a backup if it is needed on a cold day, but you'll have heat distribution issues since the second story isn't being fed by the duct system, and the coverage may be uneven, especially if you close doors.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014


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