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  1. #1
    Join Date
    May 2009
    Posts
    13

    Need advice on system, please.

    I live in Memphis, TN in a 3400-sf brick house with 2100 sf and 9-foot ceilings down, 1300 sf and 8-foot ceilings up with an entry way that's open to the second floor. The first-floor master bath also has a raised ceiling.

    I need to replace the existing 18-year-old dual systems which are currently 4-ton A/C down and 2.5 tons up. Furnaces are 78%-efficient gas. The current downstairs system, even after a Freon charge, does not sufficiently cool the southern half of the downstairs when the outside temp exceeds the low 80s. Now the condenser motor on the big unit needs replacing and the attic flooded three months ago, necessitating replacing the decking underneath the furnaces, so I'm taking advantage of the cool weather to just replace everything.

    I've gotten three similar bids for the following. Going from the current 8.5 SEER units to 13 SEER units should, of course, result in significant electricity savings. However, all three contractors have stated that going from 13 SEER to 16 SEER or higher will not result in enough extra savings to recoup the difference in equipment cost over the life of the units. The 95% furnaces will qualify for the $1500 tax credit, so the SEER ratings of the A/C units are not an issue. All have ECM VS blowers.

    1. 4-ton and 2.5-ton Amana GSX13 13-SEER units w/ GMV95 100,000 BTU and 70,000 BTU 95% two-stage gas furnaces.

    2. 4-ton and 2.5-ton Trane 13-SEER units w/ 95% gas furnaces. (Bid didn't specify models or furnace sizes.)

    3. 5-ton and 3-ton Coleman 13-SEER TCGD series units w/ LX series 120,000 BTU and 80,000 BTU 95% two-stage gas furnaces.

    All seem knowledgeable and own reputable companies. No. 3's reasoning behind suggesting the larger units was that with our ceiling height and the layout of the first floor, the 1 ton/500 sf rule doesn't necessarily apply and the fact that our current 4-ton unit has never effectively cooled much of the downstairs. I realize you can have too much tonnage, too, but he's convinced these are the optimum sizes for this house. Prices are all close with #3 being slightly better.

    Is it really worth the extra $ to get the 16-SEER units? The charts (you guys love your charts!) indicate we'd expect to save, on average, only about $50 a year by going from 13 to 16 SEER.

    What is your opinion of "balancing" by partially shutting off vents on the "cold" side of the house to try to cool off the "hot" side of the house versus installing air duct dampers and/or increasing the size of some ductwork? That's what one guy suggested, and that option is cheaper, but does it put any extra "strain" on the system to do this? In a way it seems logical to me it could, and another guy said it could potentially cause the coil to freeze up. There is also the question of whether the existing downstairs ductwork is adequate for a 5-ton system.

    I'm just trying to wade through all the conflicting information. Any advice would be appreciated.
    Last edited by topcat57; 10-01-2009 at 04:03 PM. Reason: added models, furnace capacities

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,876
    Best to balance using dampers in the duct. Less noise in the room.

    You might want to do your own load calc HVAC Calc.

    Its worth the fee.

    If your systems are over sized. The correct size will help to correct some of your air balancing problem.
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  3. #3
    Join Date
    May 2009
    Posts
    13
    That makes sense about the dampers. Can partially shutting the vents on one side of the house off potentially damage the system?

    Neither of the current systems is oversized. What I was concerned about was whether the proposed 5-ton/3-ton combo would be oversized when two other people only recommended the 4-ton/2.5-ton. I'd opt for a 4.5-ton, but they don't make one.

    I ran the HVAC Calc a long time ago but wasn't being pushed into making a decision then and have since forgotten what the numbers were. I'll run it again. Thanks.

  4. #4
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    3,371
    Could be the capacity you have now is fine, but the ductwork supporting it isn't--causing uneven temperatures and less cooling/heating delivered.

  5. #5
    Join Date
    May 2009
    Posts
    13
    That seems to be what the guy who suggested partially shutting off some of the vents on the north half of the downstairs was thinking. However, another guy said it's not good to do that as it can potentially damage the system. Other than the whistling sounds that would probably result around the partially shut-off vents, is it advisable to do this or not?

  6. #6
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,371
    If the ductwork is undersized, I don't see how that would help, but rather make the problem worse. Many ductwork systems are undersized to begin with. Unwarranted increases in tonnage only make the problem worse. Upping the 4 ton unit to a 5 ton unit will require ductwork modification 99% of the time, especially on the return air side.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,876
    Shutting off too many. Or partially shutting off too many too far. Can cause harm.
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  8. #8
    Join Date
    May 2009
    Posts
    13
    All right, thanks. I think that helps with the ductwork/vent question. Anyone have any opinion about the three proposals?

    1. 4-ton and 2.5-ton Amana GSX13 13-SEER units w/ GMV95 100,000 BTU and 70,000 BTU 95% two-stage gas furnaces.

    2. 4-ton and 2.5-ton Trane 13-SEER units w/ 95% gas furnaces. (Bid didn't specify models or furnace sizes.)

    3. 5-ton and 3-ton Coleman 13-SEER TCGD series units w/ LX series 120,000 BTU and 80,000 BTU 95% two-stage gas furnaces.

    I will get #3 to price the 4/2.5 combo w/ proportionately smaller furnaces, so please base any opinions on all systems being the same size.

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