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  1. #1
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    Confused Whether I need a new furnace for the air conditioner

    Apologize if such question has been discussed before. I tried to search but didn't find useful answer.

    I have a 1580sf house which seems need a 3 ton air conditioner.

    The house has a 310JAV024070AAJA Bryant furnace which according to the spec, it has 66000 BTU input and 53000 BTU output. However online spec also says Tonnage = 2 ton. (What is this furnace tonnage?)

    Thus my contractor suggested me to change to a 3 ton furnace.

    However it's said 1 ton is 12000 BTU, 3 ton requests 36000 BTU which my furnace seems to be enough?

    Do I need to change the furnace in this case?

    Appreciate.

  2. #2
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    Quote Originally Posted by enight723 View Post
    Apologize if such question has been discussed before. I tried to search but didn't find useful answer.

    I have a 1580sf house which seems need a 3 ton air conditioner.

    The house has a 310JAV024070AAJA Bryant furnace which according to the spec, it has 66000 BTU input and 53000 BTU output. However online spec also says Tonnage = 2 ton. (What is this furnace tonnage?)

    Thus my contractor suggested me to change to a 3 ton furnace.

    However it's said 1 ton is 12000 BTU, 3 ton requests 36000 BTU which my furnace seems to be enough?

    Do I need to change the furnace in this case?

    Appreciate.
    Furnace tonnage = how much air it can move. If your AC is three tons, you should have a furnace capable of moving three tons of air.

    The BTU rating concerns heat output.

    The larger concern is if the AC is sized correctly and whether the current duct work can handle three tons. It would take a decent load calc to be sure; however 1500 sf is pretty damn small for three tons of AC in most cases. If the AC is too large, it won't run long enough to remove humidity if you are in that type of climate. And all that stopping and starting is hard on motors.

    Another factor concerning the furnace is it's age. If it's over 15 years old you may want to replace it while you are doing the AC and save some labor.
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  3. #3
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    What size is the duct connected to the furnace, and what material is it made from?
    “I haven’t failed. I’ve just found 10,000 ways that won’t work.” - Thomas Edison

    “It’s not whether you get knocked down, it’s whether you get up.” - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  4. #4
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    The pipe seems to be 12inch metal.

    I find another more detailed spec.
    It does say CFM is Heating 720, Cooling 900.

    Online says 450 CFM per ton which seems the furnace truly not enough.

    Thanks both.

  5. #5
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    12” square or round metal? Is that for return and supply?
    Airflow is usually 350-400 CFM per ton.
    “I haven’t failed. I’ve just found 10,000 ways that won’t work.” - Thomas Edison

    “It’s not whether you get knocked down, it’s whether you get up.” - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  6. #6
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    Why go bigger? Bigger is not better. Bigger runs less, dehumidifies less.

    Odds are your duct system can barely handle 2 tons so if you go 50% bigger, you'll need to enlarge the supply duct system, return duct system, more or larger supply registers and return grilles AND buy a new furnace.

  7. #7
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    Thread Starter
    Quote Originally Posted by rider77 View Post
    12” square or round metal? Is that for return and supply?
    Airflow is usually 350-400 CFM per ton.
    12inch round. Not sure, that's the wider one.

  8. #8
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    Quote Originally Posted by BaldLoonie View Post
    Why go bigger? Bigger is not better. Bigger runs less, dehumidifies less.

    Odds are your duct system can barely handle 2 tons so if you go 50% bigger, you'll need to enlarge the supply duct system, return duct system, more or larger supply registers and return grilles AND buy a new furnace.
    I reference that heatmap online. I live in north California (zone 2) which 1590sf needs 3 ton (or at least 2.5 ton)?

  9. #9
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    Quote Originally Posted by enight723 View Post
    12inch round. Not sure, that's the wider one.
    That’s good for ~525 CFM, or just over 1 ton of airflow. You will not get 3 tons of air through this duct, nor will a 3 ton unit work right when connected to this ducting.
    “I haven’t failed. I’ve just found 10,000 ways that won’t work.” - Thomas Edison

    “It’s not whether you get knocked down, it’s whether you get up.” - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  10. #10
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    Quote Originally Posted by enight723 View Post
    I reference that heatmap online. I live in north California (zone 2) which 1590sf needs 3 ton (or at least 2.5 ton)?
    There is no such chart or map.
    “I haven’t failed. I’ve just found 10,000 ways that won’t work.” - Thomas Edison

    “It’s not whether you get knocked down, it’s whether you get up.” - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  11. #11
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    The HVAC Contractor is not doing the job right for you; you're headed for a real mess that should have been avoided...!

    First a good load-calc & Sizing should be performed along with a duct system manual D.
    The existing 2-Ton system did not perform well for a number of reasons, one being an inadequate duct system.

    Get things done right & 2-Ton ought to handle the latent & sensible heat-gain. Things can also be done to make your home more energy efficient to make 2-Ton work with your existing furnace.

  12. #12
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    Quote Originally Posted by enight723 View Post
    I reference that heatmap online. I live in north California (zone 2) which 1590sf needs 3 ton (or at least 2.5 ton)?
    Unless your windows have no glass in them... here in NC 3 tons would be a mold factory...
    The TRUE highest cost system is the system not installed properly...

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  14. #13
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    Thread Starter
    Thanks all. We decided to use 2.5 ton AC and still change the furnace and all ducts.

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