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  1. #1
    Join Date
    Jul 2012
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    7

    New Construction Duct sizes, A/C runs almost constantly.

    I recently purchased a new home and feel the A/C is running way too much. Set at 74 on an 85 low humidity day the A/C runs 45-50min per hour for 16 hours a day. After some quick searches I think the main factor is related to the return/filter sizes.
    The house is 3300sq ft. The Furnace and A/C units are Carrier, the A/C is a 4 ton unit (24ABB348). I have multiple returns in the house feeding into one 24x10 return ending with a 16x24 Air filter at the furnace. I believe from reading this forum that my return/filter setup would be more in line with a 2ton unit. The calculations start getting a little confusing, but am I wrong in thinking that the ducts/filter should be almost double this size, obviously a load calc would determine much more accurately but is it possible that there is a scenario where this setup would actually work properly?

    I have the HVAC company coming today to "take a look" and would like to at least have a better idea of the things we should look at.

  2. #2
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    5,060
    Quote Originally Posted by lweisenb View Post
    I recently purchased a new home and feel the A/C is running way too much. Set at 74 on an 85 low humidity day the A/C runs 45-50min per hour for 16 hours a day. After some quick searches I think the main factor is related to the return/filter sizes.
    The house is 3300sq ft. The Furnace and A/C units are Carrier, the A/C is a 4 ton unit (24ABB348). I have multiple returns in the house feeding into one 24x10 return ending with a 16x24 Air filter at the furnace. I believe from reading this forum that my return/filter setup would be more in line with a 2ton unit. The calculations start getting a little confusing, but am I wrong in thinking that the ducts/filter should be almost double this size, obviously a load calc would determine much more accurately but is it possible that there is a scenario where this setup would actually work properly?

    I have the HVAC company coming today to "take a look" and would like to at least have a better idea of the things we should look at.
    Your return would be to small and only capable of delivering 1320 at 0.07 static and your system requires 1600 cfm

  3. #3
    Join Date
    Jul 2012
    Posts
    7
    With that information should I demand they add an additional return?

    And the filter? I used a calculator I found here and believe I came up with number around 900cfm airflow at the filter versus the recommended 400. The current ducting can not accommodate a larger filter (possibly a 4" unit versus the 1" that is in there now).

  4. #4
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Quote Originally Posted by lweisenb View Post
    With that information should I demand they add an additional return?

    And the filter? I used a calculator I found here and believe I came up with number around 900cfm airflow at the filter versus the recommended 400. The current ducting can not accommodate a larger filter (possibly a 4" unit versus the 1" that is in there now).
    Is this a new home to you as in just built or just acquired? Can you also post your location?

  5. #5
    Join Date
    Jul 2012
    Posts
    7
    New construction built 2011, we just purchased from the builder (did not have it built to our specs). Metro Detroit.

  6. #6
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by lweisenb View Post
    New construction built 2011, we just purchased from the builder (did not have it built to our specs). Metro Detroit.
    Tell the builder to have the HVAC company come out and "benchmark" your equipment.

    This will involve measurement showing the airflow, refrigerant charge, electrical consumption and actual BTU out put of the equipment. Have them document all measurements and then we can tell you how the equipment is performing.

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    +1 on benchmarking.

    In Detroit, on a 85F day, if it's running 75% of the time, it's not too far off on sizing and capacity. At 90F and above you should be running continously for a good 4-8 hours in the evening to maintain 74-75F. You design temperature in Detroit is only 87F. Winter is 5F. So in those conditions you equipment should be running at full capacity. Above that you should slowly lose ground. With a larger home, the system will have longer run times because the air volume and mass of the floor and wall materials is propeortionally larger than a smaller home compared to the rate of heat gain, which is determined by the exterior surfae area. Make sense?

    In your climate, a well constructed spray foam insulation home might only need 3 tons or less.

    What are your supply temps?

    The return is a little undersized, but not grossly undersized. You still likely getting 350CFM/ton that Carrier often uses on their equipment and which isn't uncommon for a humid climate. You're only moderately humid, but a full 400CFM/ton shouldn't make or break you.

    That being said, I'd make them com back and enlarge the duct, and I'd then pay then extra to install a 20Xx25" 4" or 5" thick media filter like a "Air Bear or similar". A 12 or 16"x25" return plenum would be much better... then attached to a 20'X25" filter housing mounted against the furnace.

  8. #8
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,975
    Quote Originally Posted by lweisenb View Post
    ...
    The house is 3300sq ft. The Furnace and A/C units are Carrier, the A/C is a 4 ton unit (24ABB348). I have multiple returns in the house feeding into one 24x10 return ending with a 16x24 Air filter at the furnace. I believe from reading this forum that my return/filter setup would be more in line with a 2ton unit. The calculations start getting a little confusing, but am I wrong in thinking that the ducts/filter should be almost double this size, obviously a load calc would determine much more accurately but is it possible that there is a scenario where this setup would actually work properly?

    I have the HVAC company coming today to "take a look" and would like to at least have a better idea of the things we should look at.
    Using 350-cfm per ton * 4-Ton, is 1400-cfm; you're going to have 766-fpm velocity going through that filter which; cannot be allowed; ACCA says for a media cheap-filter 300-fpm is tops for initial velocity which will rise rather rapidly as it loads reducing the free-air-area of the filter. The Filtering ability of that throwaway filter is surpassed at 500-fpm & above.

    A 16x24 is 384-sq.ins., / 144 is 2.6666-sf * .685% (Ak) free-air-area is 1.82666; 1400-cfm / 1.826621-sf is 766.4-fpm velocity; Hart & Cooley says a 16x24 will only flow 552-cfm @ 300-fpm velocity through the filter.

    You should NOT use that single furnace located filter; it needs a new adequate filtering setup...
    Last edited by udarrell; 07-23-2012 at 02:10 PM. Reason: Math error corrected...

  9. #9
    Join Date
    Jul 2012
    Posts
    7
    Quote Originally Posted by udarrell View Post
    Using 350-cfm per ton * 4-Ton, is 1400-cfm; you're going to have 766-fpm velocity going through that filter which; cannot be allowed; ACCA says for a media cheap-filter 300-fpm is tops for initial velocity which will rise rather rapidly as it loads reducing the free-air-area of the filter. The Filtering ability of that throwaway filter is surpassed at 500-fpm & above.

    A 16x24 is 384-sq.ins., / 144 is 2.6666-sf * .685% (Ak) free-air-area is 1.82666; 1400-cfm / 1.826621-sf is 766.4-fpm velocity; Hart & Cooley says a 16x24 will only flow 552-cfm @ 300-fpm velocity through the filter.

    You should NOT use that single furnace located filter; it needs a new adequate filtering setup...
    I just double checked the measurements, the main return trunk that spans the house is 24x8 that goes into a 90 angle that becomes a 24x10 (so only the last 8' are the larger size.

  10. #10
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Yup, that's a restriction and so is the air filter. Both need ot be enlarged. Honestly, it might not cost that mcuh to enlarge, you might just have it done right and pay a reputable HVAC installer to fix it and check out the rest of the system while there.

  11. #11
    Join Date
    Jul 2012
    Posts
    7
    My issue with expanding is the basement is finished, is it feasible to add a second return to the first floor portion that is exposed and y the two together?

    Also in turn is the 24x8 sufficient for delivery as well or is that too undersized?

  12. #12
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    5,060
    Quote Originally Posted by lweisenb View Post
    My issue with expanding is the basement is finished, is it feasible to add a second return to the first floor portion that is exposed and y the two together?

    Also in turn is the 24x8 sufficient for delivery as well or is that too undersized?
    930 cfm at 0.07

  13. #13
    Join Date
    Jul 2012
    Posts
    7
    The HVAC guy that came out only brought tools to test the charge. Did not have an airflow meter but agreed the return duct was too small and that they used the wrong boot. Claimed that the house was originally spec'ed for Heat only and that it would be sufficient for heat... They are putting a larger boot 20"x25" with an addition return and an air bear 20"x25"x5" filter. They are also going to tape/seal the duct work they can get to.

    Now my question is, is this going to be enough?!?

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