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  1. #1
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    AC Sizing and CFM requirements impact to tonnage

    I posted prior to the migration around my duct work in my new home being in conditioned space not the attic (Also attic being encapsulated). I got confirmation in my research that being in conditioned space the HVAC company was incorrectly factoring gain/loss inflating the system size requirements to be 4 ton 2 stage (duct was adding .5 tons to heat gain) (3042 sqft home).

    I have discussed this with the contractor and requested they correctly factor the first 2 floors having duct in the conditioned space along with fixing the directional orientation to reflect zero windows/doors face West (they have it wrong).

    The new counter to my requests is that it wont change it down from a 4 ton 2 stage because the CFM requirements of the home demand 1540 CFM so a 4 ton is required (Also long duct runs from 3rd floor to first floor so pressure issues, making assumptions that all 3 zones will call for cooling together). Further the ACCA sizing allows for a 2 stage system to be up to 130% in size and the Heating to be up to 300% in size so it wont change.

    I am looking for advice on these counter arguments, it is looking more and more like I will just have to accept the unit as designed under the incorrect characteristics and deal with supplemental dehumidification (Houston, TX). I know ACs aren’t primarily responsible for humidity control but it would factor in how much supplemental dehumidification I would require.

  2. #2
    Join Date
    Jun 2003
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    Madison, WI/Cape Coral, FL
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    The a/c removes moisture based on the amount of air flow through the cooling coil. Less air flow increases moisture removal and less sensible cooling. The air flow adjustment is critical more so slightly over-size.
    assume that you are including a whole house dehumidifier?
    Keep us posted.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  3. #3
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    Apr 2016
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    Thread Starter
    I am more resigned to the fact that I will require a whole house dehumidifier just as I had to install in my current 2014 built 1.5 story home to deal with excessive humidity, just a matter of how much capacity which to me hinges on them sizing the AC right. They don't have one in their design and stated to me on a discussion call that "The most you will encounter will be around 60% in the shoulder months" as though that is acceptable design. I don't think I have any supporting code requirements to pressure them to install one in their design if the humidity is problematic as I expect it to be so I will likely install one after moving in (as I did with my current home). The unit they have installed is a 4 ton 2 stage paired to a 5 ton coil (my understanding is this increases sensible efficiency but robs latent) with a 90000 BTU variable speed furnace (122% cooling capacity and 253% heating capacity based on design conditions, it will go higher once they fix what I requested they fix in their calculations).

    If they recalculate the load based on my corrections to the design (no west facing windows doors, ducts in conditioned space) the likelyhood is high the actual cooling load would necessitate a 3 ton 2 stage system. That said, the argument now gets to the CFM requirements of the home being 1540 CFM which is too much for a 3 ton unit so they have to oversize; I guess it doesn't make full sense to me being 3 zones and a 2 stage unit already. I know going from 3rd to 1st floor has some long trunk runs so there is airflow degradation to account for and going too high in CFM on a smaller unit to overcome distance will reduce delta-t.

    I am hopeful that this HVAC company will correctly configure the variable speed blower for the home needs and system. Considering they did Manual J on incorrect design data (no actual review of architectural plans based on wrong orientation and larger sqft than designed) I have my doubts on how well this install will be. I had to adjust the taps on my existing homes unit after we moved in as they left it at default max 2200 CFM instead of lowering to 1600 per design conditions.

  4. #4
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    Mar 2016
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    Dayton Oh
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    if your writing the check you dont have to accept anything you dont want or want changed.

    I dont see how if you really only need 2 tons of cooling how you would need 1600 cfm. Long runs can be calculated for. if they are sizing by ASP, the find the TEL of the duct, and do the math and use a duct calculator to find the size duct needed for these runs.

    Maybe you should push to have this contractor removed and find an ACCA accredited contractor.

    I dont have my calculator in front of me to spout off a random number example of how the length of the runs would affect the sizing requirements. Ill try and see where I hid it tonight and try and make a response with an example in the morning.

  5. #5
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    Aug 2002
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    Trying to move air 3 stories is insane except in commercial applications.

    I also don't know where your getting 130% and 300% #'s from.

    Each floor requires a separate system for true comfort.

  6. #6
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    Quote Originally Posted by pecmsg View Post
    Trying to move air 3 stories is insane except in commercial applications.

    I also don't know where your getting 130% and 300% #'s from.

    Each floor requires a separate system for true comfort.
    eventually I was going to get to that point. Or hopefully a knowledgeable contractor on site would bring that up.

  7. #7
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    Quote Originally Posted by jbhenergy View Post
    eventually I was going to get to that point. Or hopefully a knowledgeable contractor on site would bring that up.
    That's because were not worried about the bottom line here, a Knowledgeable contractor on site is!

  8. #8
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    Sep 2007
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    Forget your 3 zone system and put in a separate system for each area you want zoned.

    That would be a much better solution.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  9. #9
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    I didn't come up with the 130% and 300%, I told them my research showed 115% and 140% were the limits based on ACCA (found a Verifying ACCA Manual S® Procedures document on the ACCA site which discusses this). They said because it was a 2 stage system and variable furnace they could go that high which makes zero sense to me.

    They aren't ACCA Approved from what I can find on the ACCA site (cannot even find them listed on it). I argued with the guy about the ducting situation and conditioned space to where he said the software (RightSuite) doesn't let him change the calculations as it does it for him. I found the user manual, screenshot the section where it states duct in conditioned space and zero gain/loss for the software and sent it to him. Then his counter turned to the whole CFM requirements and duct distance, the real situation is they already installed the furnace, blower and condenser coil inside and likely don't want to redo things.

    I will probably have to engage the builder about the situation as the responses I get from the HVAC contractor are as I noted here but I don't believe them. I keep getting "I have been doing this 15 years" which just irritates me because I know the calculations and design is wrong.

  10. #10
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    Quote Originally Posted by VegaTX View Post
    I didn't come up with the 130% and 300%, I told them my research showed 115% and 140% were the limits based on ACCA (found a Verifying ACCA Manual S® Procedures document on the ACCA site which discusses this). They said because it was a 2 stage system and variable furnace they could go that high which makes zero sense to me.

    They aren't ACCA Approved from what I can find on the ACCA site (cannot even find them listed on it). I argued with the guy about the ducting situation and conditioned space to where he said the software (RightSuite) doesn't let him change the calculations as it does it for him. I found the user manual, screenshot the section where it states duct in conditioned space and zero gain/loss for the software and sent it to him. Then his counter turned to the whole CFM requirements and duct distance, the real situation is they already installed the furnace, blower and condenser coil inside and likely don't want to redo things.

    I will probably have to engage the builder about the situation as the responses I get from the HVAC contractor are as I noted here but I don't believe them. I keep getting "I have been doing this 15 years" which just irritates me because I know the calculations and design is wrong.
    Congratulations you found a contractor that's done it wrong for 15, 20, 30 + years.

    Believe us Zoned systems are great if done properly. Rarely does that happen so Pay a little extra, get 1 system for each floor "SIZED" properly. and be happy in the end!

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  12. #11
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    Quote Originally Posted by pecmsg View Post
    Congratulations you found a contractor that's done it wrong for 15, 20, 30 + years.

    Believe us Zoned systems are great if done properly. Rarely does that happen so Pay a little extra, get 1 system for each floor "SIZED" properly. and be happy in the end!
    My thoughts exactly.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  13. #12
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    Most companies can’t do single zone system let alone multiple zones


    Sent from my iPhone using Tapatalk

  14. #13
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    Quote Originally Posted by jbhenergy View Post
    Most companies can’t do single zone system let alone multiple zones


    Sent from my iPhone using Tapatalk
    Can't stress this idea enough. Do as pecmsg & I suggest. Get it done correctly and be fully comfortable and happy.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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