Variable Furnace vs. Two Zones
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  1. #1
    Join Date
    Aug 2005
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    Question

    Our 2-story 3600sf home has only one zone, with a twenty-year old 5-ton HVAC system. We have the classic problems with a single-zone house -- the second floor is always 10 degrees hotter/cooler than the first floor.

    We're thinking of converting to two zones, but the additional cost is daunting. A number of contractors have suggested that a variable speed furnace (such as the Trane XV90) might help reduce the need for two zones by continuously moving air through the house.

    I would love for this to be the case, but am unsure. Does this strike you as a good approach?

    Thank you for your thoughts.



  2. #2
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    May 2000
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    Indianapolis, IN, USA
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    33,833
    I don't think that will make any difference. Variable speed furnace on constant fan moves little air.

    Is the upstairs OK in heating just too hot in cooling? What we are doing more of it putting small cooling systems in the attic just to cool the upstairs.

  3. #3
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    Mar 2004
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    That will help with stratofied air but the only way to solve your temperature problem between the upstairs and main floor is to zone the system or have a seperate system.
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  4. #4
    Join Date
    Aug 2005
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    Originally posted by BaldLoonie
    I don't think that will make any difference. Variable speed furnace on constant fan moves little air.

    Is the upstairs OK in heating just too hot in cooling? What we are doing more of it putting small cooling systems in the attic just to cool the upstairs.

    The problem is less pronounced with heating, but it is still an issue. Of course, my memory could be playing tricks. It's late August and been 95 degrees every single day, so heating is only a dim concept. Right now though, in order to get the second floor cool enough for sleeping, we have to turn the first floor into a meat locker.

  5. #5
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    Aug 2005
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    Originally posted by coolwhip
    That will help with stratofied air but the only way to solve your temperature problem between the upstairs and main floor is to zone the system or have a seperate system.
    Hi,
    Sorry to be dense, but what do you mean by stratofied air?
    Thanks.

  6. #6
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    Dec 2004
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    Derby City
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    Philsoc8, my question to you would be do want to do something that 'might' work, or are you interested in fixing the problem permanently? If the installation of a variable speed furnace 'might' help the problem, that wouldn't really make me warm and fuzzy. On the other hand, if an independent system for the 2f satisfies the heating and cooling for that space independently of the 1f system, and maintains a desirable temperature, then that would be my course of action.

    Agree that if the problem is only in the cooling, then a cooling only system would be appropriate. Otherwise, think about a heat pump for the second floor. This will take care of the cooling load AND provide a certain amount of heat without the need for auxiliary heat strips.

    I would offer that the contractor offering the single system is trying to satisfy the 'cost' factor for the system. I would strongly recommend that you consider the zoning of two systems.
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  7. #7
    Join Date
    Apr 2004
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    7,680
    Well, I'd like to chime in on this one. Variable speed for the most part will not resolve his problem as we understand it unless his current airflow is way to low all over and that would need to be known, not simply a matter of how it blows the curtains.

    Next, while zoning may help it is certainly not the only solution. What can someone do? The system can be balanced to provide the right air if the ductwork is satisfactory. In this case, a pair of balancing dampers would be used, one in the downstairs, and one in the upstairs trunk. These dampers would be set in cooling mode to provide the proper airflow to the specific floors in the correct volume.

    In winter, with the airflow being different, the balancing dampers would need to be adjusted seasonally, once in fall, and once in spring. The house can be tuned for pretty much even temps. That said you would not have seperate control like you would a zoning system. There in lies the difference in cost and value. You would certainly pay more for zoning but would have more control, you could pay less to have it properly balanced and shoot for similar temps in each zone.

  8. #8
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    Aug 2005
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    Thanks to everyone for their feedback. I don't think the contractor is trying to sell fancy equipment, I just think he's trying to keep us from getting freaked out by the cost of implementing two zones.

    That being said, adding a second cooling-only zone to the second floor may be the best way to go.

  9. #9
    Join Date
    Jun 2004
    Location
    Western MA.
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    505
    Next, while zoning may help it is certainly not the only solution. What can someone do? The system can be balanced to provide the right air if the ductwork is satisfactory. In this case, a pair of balancing dampers would be used, one in the downstairs, and one in the upstairs trunk. These dampers would be set in cooling mode to provide the proper airflow to the specific floors in the correct volume.

    In winter, with the airflow being different, the balancing dampers would need to be adjusted seasonally, once in fall, and once in spring. The house can be tuned for pretty much even temps. That said you would not have seperate control like you would a zoning system. There in lies the difference in cost and value. You would certainly pay more for zoning but would have more control, you could pay less to have it properly balanced and shoot for similar temps in each zone. [/B][/QUOTE]

    That`s assuming that the system has two separate trunks for upstairs and down.

    i`ve seen a number of systems where the 1st and 2nd floor supply runouts come off the same trunk line and the second floor supplys are run through risers in interior and exterior walls.


  10. #10
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    Jan 2004
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    Lancaster PA
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    Originally posted by sirjames
    That`s assuming that the system has two separate trunks for upstairs and down.

    i`ve seen a number of systems where the 1st and 2nd floor supply runouts come off the same trunk line and the second floor supplys are run through risers in interior and exterior walls.

    [/B][/QUOTE]

    Not really.
    I've done more then one system with the zone dampers installed in the individual supply runs. It about the same cost as running seperate supply mains, and works as just as good.
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  11. #11
    Join Date
    Aug 2005
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    8
    Originally posted by John Lloyd
    Philsoc8, my question to you would be do want to do something that 'might' work, or are you interested in fixing the problem permanently? If the installation of a variable speed furnace 'might' help the problem, that wouldn't really make me warm and fuzzy. On the other hand, if an independent system for the 2f satisfies the heating and cooling for that space independently of the 1f system, and maintains a desirable temperature, then that would be my course of action.

    Agree that if the problem is only in the cooling, then a cooling only system would be appropriate. Otherwise, think about a heat pump for the second floor. This will take care of the cooling load AND provide a certain amount of heat without the need for auxiliary heat strips.

    I would offer that the contractor offering the single system is trying to satisfy the 'cost' factor for the system. I would strongly recommend that you consider the zoning of two systems.
    John,

    You really hit the nail on the head (including the part about the contractor not wanting to scare us with the price of doing it right).

    We're going to be cutting off the second floor ducts and add a new a/c and heat pump in the attic to cover the second floor. The reason for the heat pump as opposed to the furnace is that as a result of an unusual number of picture windows on the first floor (and therefore a high load), the expecation is that the furnace covering the first floor will be on a great deal of the time in the winter, and a lot of rising heat will make its way to the second floor so a full furnace for that zone is not needed.

    Please let me know if you can think of any other considerations for us to take into account.


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