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  1. #1
    We are about to replace the two old force air, gas furnaces that heat our multi-story home with two new, high efficency furnaces. One furnace serves the first two levels, while the other one serves the top floor. We have been very happy with a two furnace set-up for many years and also like the redundancy of a second furance.

    My question...the two furnaces sit about two feet from each other in the basement and share one large cold air return duct. The cold air return system is also shared among the various levels of the house. In other words, there are NOT segregated cold air returns that come together near the furances. Rather, they feed together on the upper floors and come to the furnaces in one duct. We would like to go with a variable speed blower in the new furnace that handles most of the heater, and a single or 2-speed blower in the second furnace. One contractor says that a variable speed blower set-up won't work because of the shared cold air return, and the closeness of the two furnaces. Something about static pressure problems causing the variable speed blower to go nuts. One other contractor says he would split the cold air return for about six feet from each furnace and install a separate 5" filter box on each furnace. He believes this would enable us to use a variable speed blower in the furnace. What do the experts here think? I'd like to get these furnaces bought before it gets real cold here in Wisconsin. Thanks, John

  2. #2
    gto boy Guest
    If you install 2 variable speed furnaces and they share any return they will fight each other.

  3. #3
    Does anyone else have any ideas on this topic? Would one variable speed furnance and one single speed furance be any better a combination? Want to buy soon, but am still uncertain. Thanks, John in Wisconsin

  4. #4
    Join Date
    Jan 2002
    Location
    Ft.Worth,Tx
    Posts
    4,584
    With the type of duct system I suggest that you might want to maybe use (2) stage furnace with multi-speed blower. As for a variable drive with this type of return, he is right you will have problems.

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    You already have a problem with the standard motors,in that the static of the return duct chages ,depending on weather one or two are running,so you air delivery changes with the change in static.Which means you can't have the required correct air flow ,all of the time

    I be concerned about using VS ,but I not sure it will be a major problem a the VS will or may(not really sure) adjust ,but as one adjusts the other will also,I don't know if they will eventually settle in,or keep adjusting Throughout the cycle.So I guess I woudn't try it.

  6. #6
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,912
    I would probably go with a 2, 2 stg. 90s.
    " Kill a Commie for Mommy! "

    - Colonel David Hackworth (1930-2005), Korean War Vet

  7. #7
    Join Date
    Apr 2004
    Posts
    7,680
    sharing returns is just asking for trouble.

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