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  1. #1
    Join Date
    Jul 2009
    Posts
    11

    Who to believe???

    Just a little background. Our house is 2,000sqft in hot/humid NC. The house is 2 story and is 22 years old with the original 3-ton Carrier heatpump. Just one unit for both floors. The temperature difference can be 8-10 degrees different between the 2 floors. Other than the system is very old, the temperature difference is the reason we are replacing. Oh yeah throw in a goverment check, so its time for a new heat pump.

    Two contractors have inspected my house and will be getting me quotes in a few days. They both noticed the small return and supply duct (10") coming from the 2nd floor and saw very little space for a bigger size. The working space is a vertical shaft from the 2nd floor down to the crawl space.

    Contractor (A) recommends a dual zone (2 thermostats) system and Contractor (B) says to do two systems (one unit per floor). In fact Contractor (B) refuses to give me a quote on the dual zone because of the small return/supply from the 2nd floor. He is adamant about the dual zone not being able to send enough cool air up stairs when the system is calling for it because of the smallish 10" duct.

    I have another contractor coming out next week but kinda confused right now.

    Sorry to run long. I described the best I could. Any thoughts on this??

    Thanks.

    FV

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,319
    If you can afford it, you'll likely be happier with one system per floor (two systems total). If there was a higher level of competency among residential installers/service technicians regarding zoning systems, that might be viable option for a one system configuration. In reality, two systems tend to do better in multi-story homes than single systems with no or poorly configured zoning systems do.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    May 2009
    Posts
    196
    A 10" round duct is only good for about 380cfm at normal friction rates. That's enough for 1 ton of heat/cool at most. It will not be good enough for a heat pump system unless the upstairs level is very well insulated and has very good, small, windows.

    If you install a second system it will need to be upstairs (or on the roof) so it isn't stuck behind the 10" bottleneck. If you can't fit a system upstairs, you'll need to either have the duct work expanded or look at a ductless multisplit.

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    If ducts to the second floor can be sized properly,one system ,two zones is the way to go!!!!!!!

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,074
    But if not, a 2nd system is the only way. We added a 2nd system to a zoned house a couple years ago. The duct to upstairs was too small to move enough air to cool that area, even zoned.

  6. #6
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    IMO, i don't see a big difference between either choice

    no matter what you do, you still need some ductwork on that second floor

    but personally, i would have 2 system's, the advantage would be

    if 1 system ever broke down, you could sleep in the other system's area



    .

  7. #7
    Join Date
    Jul 2009
    Posts
    11
    Thanks for the responses.

    There is room for a 2nd air handler in the attic above the 2nd floor. In a 2 unit system, I guess this would eliminate the need for 10" return/supply in the vertical shaft. I assume all you would run up thru is electrical and refrigerant to the 2nd floor attic. This will be a split system of course.

    Leaning torwards the two units but still interested in the zone system with one unit (mainly price), if I can find a way to get more air up there without tearing out walls.

  8. #8
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,842
    Two systems hands down. More control and more efficient.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  9. #9
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    Quote Originally Posted by heaterman View Post
    Two systems hands down. More control and more efficient.
    what are you basing your eficiency numbers on?



    .

  10. #10
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by Airmechanical View Post
    IMO, i don't see a big difference between either choice

    no matter what you do, you still need some ductwork on that second floor

    but personally, i would have 2 system's, the advantage would be

    if 1 system ever broke down, you could sleep in the other system's area



    .
    OTOH, you've double the chances that one of the systems will go down.

  11. #11
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,489
    How many sq. ft is your upper floor? If it's the same as the lower (1000) then the 10" round will not supply enough air to condition the space, even with the zone system. Just use the chase to run lineset and wiring to upper unit, and go with 2 systems. If you can increase size, or add another riser duct, then zoning might work. P.S. If either contractor was the one who put the original system in, don't hire them!

  12. #12
    Join Date
    Jul 2009
    Posts
    11
    The break down of sq ft is like this: (1st Flr 1,118) and (2nd Flr 897) so the upstairs is a bit smaller. If I go with 2 units, should they both be 1.5 ton? You guys are great. Thanks!

  13. #13
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    sounds like you need a heat calc there

    don't forget to factor in global warming


    .

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