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  1. #1
    Join Date
    Apr 2009
    Posts
    86

    Can you benefit from oversizing a modulating compressor system?

    Since carrier is due to come out with a modulating compressor in June, and there are other modulating systems on the market. I have been thinking about how you can use this to customize the system for different scenarios.

    We will use my house as the scenario. Duct is sized properly for 4-ton.

    If we assume that a modulating system can operate a 3 ton range. i.e. 2-4 ton or 3-5 ton.

    Would I run into problems if I oversize to 5-ton for the extra heating capacity in winter since it would most likely only run in 5-ton operation when it is extremely hot or cold. In the non extreme weather it would probably run in 3-4 ton range

    What harm is being done running 5-ton when the ductwork is designed for 4 ton?

    Obviously I would loose out on a little efficiency in the summer while dehumidifying since a properly sized 4-ton system would be able to go to 2-ton operation during humidification.

    Sound logic, or newb failboat

  2. #2
    Join Date
    Dec 2005
    Posts
    1,905
    Quote Originally Posted by djp0623 View Post
    Since carrier is due to come out with a modulating compressor in June, and there are other modulating systems on the market. I have been thinking about how you can use this to customize the system for different scenarios.

    We will use my house as the scenario. Duct is sized properly for 4-ton.

    If we assume that a modulating system can operate a 3 ton range. i.e. 2-4 ton or 3-5 ton.

    Would I run into problems if I oversize to 5-ton for the extra heating capacity in winter since it would most likely only run in 5-ton operation when it is extremely hot or cold. In the non extreme weather it would probably run in 3-4 ton range

    What harm is being done running 5-ton when the ductwork is designed for 4 ton?

    Obviously I would loose out on a little efficiency in the summer while dehumidifying since a properly sized 4-ton system would be able to go to 2-ton operation during humidification.

    Sound logic, or newb failboat
    First, we (technician) still try to master the logics behind the 2 stage compressor. so please wait and see. Trust me I am holding my breath too.

    Second, I don't think we can have a system that is "one size fit all"

    Third, I am not trying to be sarcastic. Just humor.

    Last, a little oversize (10 - 15%) would not hurt. I hate everything is exact. But way oversize is not good.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,704
    First, you would need to measure the actual static in your duct system to find out how many CFM your current 4 ton blower is moving. Just using a tape measure and saying its sized for 4 ton/1600CFM doesn't tell you what CFM its actually moving.

    Lets say it is sized for 1600 CFM at a .5 TESP with a clean new air filter. When the mod would go to 5 ton and try and move 2000 CFM through that same duct system, the TESP would increase to .78". Doesn't sound too bad. Except that is the air filter is a little dirty, and at 1600 it operating at .6" to move 1600 CFM, then to move 2000 CFM, the TESP would rise to .94". Not good.

    Of course if your 4 ton duct work is operating at .7" to move 1600 CFM, then to move 2000 CFM, the TESP would rise to 1.09".

    Check your actual static today, and see how well sized your duct work really is. And you may find that one pit fall is what looks like proper sized duct, really isn't as proper as you thought. 10X22/8X32 duct for 4 tons, isn't as large as it sounds.
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