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  1. #1
    Join Date
    May 2012
    Post Likes

    Air conditioner and return air sizing

    Hi all,

    Looking for suggestions as to what to do. Live in Calgary, Alberta, Canada and looking to install central air for the 2-4 months we have that can be somewhat hot.

    2300 sqft walkout basement 2 story house, 2 years old south facing and no fancy energy efficient windows.

    Had 2 contractors recommend a 4 ton (max size based on furnace)... which sounds crazy and then note that 3.5 would work. One contractor go with 3.5 is and then another 2.5-3.

    Was going to go for the 3.5 most of them seem to recommend till I realized I am limited by my 16x25x1 furnace filter size.

    Option 1: Move to 3 ton.
    Option 2: Move to 3 ton with 3.5 ton sized evap. coil.
    Option 3: Stay with 3.5 ton and increase size of filter opening to 20x25 for $200.

    I am leaning to Option 1 but contractor scaring me saying if it doesn't get cold enough because of it don't be calling him.... his prices and reviews seem good from what I can find online and why I chose him. No one here would be willing to do a correct load calculation as it takes too long....

    Thanks for your help!

  2. #2
    Join Date
    Aug 2010
    West Monroe, LA
    Post Likes
    You need a load cal. Performed. I hate to hear that no one in your area will do one. They are just guessing at what size you need or using a rule of thumb to size your system . You can do your own load cal. If you look on the home page with all the posts. At the very top you will see a sub heading listed that says HVAC cal. You can purchase a 1 time use for your home for $49.00. Money well spent.

    As far as the return air sizing 16x25 is to small for a 3 ton. 20x25 would be better as the noise level would be less and the unit would not stave for air. Where is system located? Did the contractor tell you not to call him if the a/c does not cool properly? If so then I don't he is the proper person for the job. If you do the load cal. Then that would answer the question what size is needed. I would simply ask how he arrived at the size for the home. The sad thing is they are telling you that a load cal. Takes to long! That is just lazy to me. Keep us posted.

  3. #3
    Join Date
    May 2012
    Post Likes
    Thread Starter
    System is located in the basement. It is a Lennox GU42F 48C-110. I checked the manual and it recommends 16x25x1 and there is one filter box is attached to the side. If I get someone to upgrade it to 20x25 will it actually increase airflow as the hole on the side of the unit is already smaller than the 16x25 filter box probably like 14x22, or should they cut that out bigger too?

    Most of them use a 600-700sqft/ton rule of thumb I think.... seems crazy as Calgary doesn't get as hot as other places which I hear use anywhere between 7-800sqft/ton.

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