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  1. #1
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    Question The right compressor to avoid the bypass in a zone addition?

    Hello,
    I asked my contractor to add an extra force air zone with electronic dampers and another thermostat but avoiding the bypass and instead to use my current variable speed blower to control the air pressure so the furnace is not affected in the long run. I was wondering if you guys can help me out with this question:
    Is it possible to have two zones without the bypass using a variable speed blower and a SINGLE stage compressor?

    My contractor is telling me that I need to upgrade to at least a TWO-stage compressor with the variable speed blower if I want to avoid the bypass. Meaning I will need to replace a 8 months old single stage compressor for a two stage or variable speed compressor.

    This is a new construction house that came with an unfinished attic, so the plan is to add a zone to the existing house hvac system located in the attic to also heat and cool the finished attic.

    The system comes with the following units: Variable speed furnace American Standard Gold Model: S9V2B080U3, American Standard coil model: 4TXCB004DS3HCA, American Standard 2.5 ton single stage compressor model: 4A7A3030.

    Thanks!

  2. #2
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    Have the duct for each zone large enough to carry at least 80% of the total air volume. Two and a half ton needs around 1000 CFM total so each zone should be made to carry 800-1000 CFM . You get required air flow to keep the system happy and no bypass or dump.

  3. #3
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    Quote Originally Posted by USAGIAN View Post
    Hello,
    I asked my contractor to add an extra force air zone with electronic dampers and another thermostat but avoiding the bypass and instead to use my current variable speed blower to control the air pressure so the furnace is not affected in the long run. I was wondering if you guys can help me out with this question:
    Is it possible to have two zones without the bypass using a variable speed blower and a SINGLE stage compressor?

    My contractor is telling me that I need to upgrade to at least a TWO-stage compressor with the variable speed blower if I want to avoid the bypass. Meaning I will need to replace a 8 months old single stage compressor for a two stage or variable speed compressor.

    This is a new construction house that came with an unfinished attic, so the plan is to add a zone to the existing house hvac system located in the attic to also heat and cool the finished attic.

    The system comes with the following units: Variable speed furnace American Standard Gold Model: S9V2B080U3, American Standard coil model: 4TXCB004DS3HCA, American Standard 2.5 ton single stage compressor model: 4A7A3030.

    Thanks!
    A good contractor would already know this and not have to be told!

  4. #4
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    Has a load calc been done to determine if the existing system is able carry the additional load? It probably can't, at least in cooling, and even if it could , you're gonna have large temperature swings.

    You'd be better off adding another system, possibly a ductless.

    Your contractor may be trying to gain more capacity with a larger two staged unit since they don't make them in 1/2 ton increments.

    Do the load calc and get it right!
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  5. #5
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    Thread Starter
    Thanks martyinlincoln. I will consider your suggestion.

  6. #6
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    Thread Starter
    Quote Originally Posted by bobboan View Post
    Has a load calc been done to determine if the existing system is able carry the additional load? It probably can't, at least in cooling, and even if it could , you're gonna have large temperature swings.

    You'd be better off adding another system, possibly a ductless.

    Your contractor may be trying to gain more capacity with a larger two staged unit since they don't make them in 1/2 ton increments.

    Do the load calc and get it right!
    Bob, I will definitely contact a heat and cool load calculation company to get a Manual J done. Thank you!

  7. #7
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    Quote Originally Posted by USAGIAN View Post
    Bob, I will definitely contact a heat and cool load calculation company to get a Manual J done. Thank you!
    You do not need to get a hold of a special place to get a Manual J done. A decent residential HVAC contractor should be able to do it themselves and should have had it done already in the case of your residence. You need to find a different contractor that can perform both a a Manual J and in your case a Manual D. They can set down with the numbers come up with scenarios that will make what you want work using hard facts to dictate the right direction to go. With out this NO ONE can help you, especially here with our shear speculation and assumptions flying blind on a few paragraphed posts. Best to invest in a well documented game plan to ultimately achieve what you desire...there is no room for guessing and conjecturing when it comes to zone systems.

  8. #8
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    Is the roof foamed so the attic is inside the thermal barrier? If not then you're going to want that done first.

  9. #9
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    Thread Starter
    Quote Originally Posted by martyinlincoln View Post
    Is the roof foamed so the attic is inside the thermal barrier? If not then you're going to want that done first.
    It is foamed with 3-4 inches of high density foam.

  10. #10
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    You NEED a load calculation.

    You also need some way of bringing in outside air, did this contractor explain that to you also?

  11. #11
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    Thread Starter
    Well, I just moved in few months ago and the builder that sold me the house never said anything. The couple of contractors that have come to check on the zoning upgrade project have not mentioned anything yet. However I have read about it and was thinking on addressing it. I mentioned it to the builder and he said it is not necessary. But actually he left one of the bathroom vent constantly running. I assume it is because of that.
    How do you bring constant air to the house if not opening a window?
    Thanks!

  12. #12
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    Quote Originally Posted by USAGIAN View Post
    Well, I just moved in few months ago and the builder that sold me the house never said anything. The couple of contractors that have come to check on the zoning upgrade project have not mentioned anything yet. However I have read about it and was thinking on addressing it. I mentioned it to the builder and he said it is not necessary. But actually he left one of the bathroom vent constantly running. I assume it is because of that.
    How do you bring constant air to the house if not opening a window?
    Thanks!
    If the contractors aren't mention that show them the door!
    Fan on 24/7 ask him if hes paying the electric and the cleaning bill!

    HRV's
    ERV's

    Teddy Bear promotes a Whole House Dehumidifier……...If its in your budget well worth it.

  13. #13
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    Make sure it is a high end zone panel that stages the equipment base on load. Honeywell does % of zones calling, Jackson Systems does # of zones calling. Jackson has a panel that opens non-calling dampers a bit if the pressure in the supply gets too high. Really slick system. Kinda like what the high end proprietary systems from Carrier & Trane do.

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