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  1. #1

    Oversizing air handler to improve air flow

    Hi. I'm looking to replace my current 15-year old 3.5 ton furnace/air conditioner. The current system cools well, except for an upstairs loft. The loft only has one small air duct, and has windows facing the afternoon sun, so it can get toasty up there in the summers. I live in Indiana, so 90+ degree days are common in July - August.

    I've had a well-respected local contractor out to quote a new system. He's suggested that I go with a larger air handler/furnace to increase air flow to the loft. Specifically, he's recommending a 5 ton furnace mated with a 3.5 ton air conditioner.

    I've never had any heating problems with the current 3.5 ton system, so if I went with the larger unit, it would only be to increase air flow to the loft in the summers.

    The higher temps in the loft are a nuisance, but not a big one. I'm OK spending a bit more on the system if it improves the air flow, but it's not a big enough deal to justify installing new duct work.

    So...the questions are...do you think the 5 ton system will help improve the air flow and other than the installed cost, what is the downside to having a slightly oversized furnance/air handler?

    Thanks much.

    Bonzai

  2. #2
    Join Date
    Dec 2009
    Location
    northeast pa
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    118
    Quote Originally Posted by Bonzai View Post
    Hi. I'm looking to replace my current 15-year old 3.5 ton furnace/air conditioner. The current system cools well, except for an upstairs loft. The loft only has one small air duct, and has windows facing the afternoon sun, so it can get toasty up there in the summers. I live in Indiana, so 90+ degree days are common in July - August.

    I've had a well-respected local contractor out to quote a new system. He's suggested that I go with a larger air handler/furnace to increase air flow to the loft. Specifically, he's recommending a 5 ton furnace mated with a 3.5 ton air conditioner.

    I've never had any heating problems with the current 3.5 ton system, so if I went with the larger unit, it would only be to increase air flow to the loft in the summers.

    The higher temps in the loft are a nuisance, but not a big one. I'm OK spending a bit more on the system if it improves the air flow, but it's not a big enough deal to justify installing new duct work.

    So...the questions are...do you think the 5 ton system will help improve the air flow and other than the installed cost, what is the downside to having a slightly oversized furnance/air handler?

    Thanks much.

    Bonzai
    If you increase the size of furnance then 31/2 ton duct is not big enough to handle cfm i would stay with tonage and try to balance air to push more air to hotter areas.

  3. #3
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    2,482
    That is the most ridiculous idea. However, it's not the first time (unfortunately) that I've heard it.

    The only way you'll improve the A/C to the loft is to greatly enlarge the ductwork or go with a little mini-split A/C.

    The mini-split is the only way to go IMHO.

    Then you can probably save (downsize) the old system and save money on that end.

  4. #4
    Join Date
    Aug 2011
    Posts
    568

    Hmm

    Quote Originally Posted by tech93 View Post
    If you increase the size of furnance then 31/2 ton duct is not big enough to handle cfm i would stay with tonage and try to balance air to push more air to hotter areas.
    maybe the duct is oversized in the first place? fact is we don't have the facts! lod calc,duct sizes,etc!

  5. #5
    Join Date
    Jun 2011
    Posts
    21
    If I were a betting man I'd tell you that the contractor wants to sell you a bigger system because that's how he makes his money!

    Attics are going to be hot because heat rises. How much that effects the livability of your uppermost floor has to do with a lot of different aspects of the construction of your house. Simply up sizing the air conditioner would be the "brute force" way to handle the problem but a) it will cost you more money to buy the extra electricity and b) it may over-cool the other rooms in your house.

    The problem is that heat accumulates near the top of your house. Installing an attic fan to blow the hot air trapped above the ceiling outside might work. If the top floor of your house is right at the roof, so you don't have an attic, then it's going to be problematic by design. In that case I would consider a separate system for that room, maybe a mini-split, through-the wall, or just a window AC unit, that you can turn on only when you need to.

  6. #6
    Join Date
    Jun 2011
    Posts
    21
    oh, so I forgot to mention the most important part... to answer your original question, over sizing (as in going from a 3.5 ton to a 5 ton unit) will have NO effect on the airflow in your loft. 1 ton = 12000 BTUs, it is a measure of cooling capacity, not airflow. The capacity of your AC unit is a measure of how much heat it can remove from your house. It may do that by blowing more air or blowing less, colder air. Either method would provide the same capacity.

    If you are not getting enough AIRFLOW to your upstairs room (as in you put your hand on the register and don't feel a lot of air coming out), then the problem has nothing to do with capacity. You might need to install some balancing dampers to direct flow up the riser to the loft, or you might need a bigger fan (more CFM), but there is no reason to think you need more capacity.

  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
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    Smile

    Quote Originally Posted by Bonzai View Post
    I've had a well-respected local contractor out to quote a new system. He's suggested that I go with a larger air handler/furnace to increase air flow to the loft. Specifically, he's recommending a 5 ton furnace mated with a 3.5 ton air conditioner.

    I've never had any heating problems with the current 3.5 ton system, so if I went with the larger unit, it would only be to increase air flow to the loft in the summers.


    . I'm OK spending a bit more on the system if it improves the air flow, but it's not a big enough deal to justify installing new duct work.

    Bonzai
    A variable speed Motor would be an improvement.
    You may have a 1/2 HP motor now, and a 3/4 or1 HP might be appropriate.

    1600 CFM would be a maximum desired air flow.
    Anything higher than 1400 CFM will tend to decrease the humidity removal capability.

    The air flow can be adjusted
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    66,807
    If you use a larger blower to increase air flow to the loft. You will also increase air flow to the rest of the house. And could also end up with a very noisy system.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  9. #9
    Join Date
    Mar 2007
    Location
    South Texas
    Posts
    203
    Look into a damper system, increasing the size of the unit or duct will not solve the problem as the unit will follow the commands of the thermostat which I assume is located downstairs. Remember hot air rises

  10. #10
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    33,399
    Anything higher than 1400 CFM will tend to decrease the humidity removal capability.
    That sums up the craziness of his idea right there. Good for the desert, nuts for Indiana. Been outside today?

  11. #11
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
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    3,958
    Install the minisplit or fix the ductwork.

  12. #12
    Join Date
    Aug 2011
    Posts
    568
    sometimes a contractor see a duct system in an existing house, hears a customer complaint ti hot on the upper floor of a 2-3 story house that has one zone one system. the home owner is on a budget and is looking for a potential fix! this contractor offer one.. so you end up with a comprmise system. it may not be ideal but do to budgetary constraints this is the only alternative, question for the h/o what is your budget for fixing this issue the right way! it could get expensive, some times on this board it's nice to come up with all these expensive ideas, except the h/o may not or does not want to spend that kind of money.

  13. #13
    Join Date
    Nov 2003
    Location
    Skiatook, OK
    Posts
    322
    Quote Originally Posted by 54regcab View Post
    Install the minisplit or fix the ductwork.


    you can not push 1800/2000 cfm throught ductwork desizing for 1200/1400cfm
    Don't annoy the combat veteran!!!!
    All gave some - Some gave ALL

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