One contractor says trunk lines too small, the other says they are fine?!
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  1. #1
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    One contractor says trunk lines too small, the other says they are fine?!

    So I am getting quotes to replace my furnace. Looking at variable speed blowers and 2 stage heating. Anyways, I have a 4 ton system, which is the correct size for my house. However, I have 2 trunk lines feeding branches to all the registers. One is 22x8, the other is 12x8. One contractor says that I need to up the size of my trunks about 1/2 the total length of their run to get the correct air flow to all the rooms. The other one says that I have the correct size and dont need to change a thing. What cfm's are those trunks able to handle? I found charts online but the total CFM's come out between 1240 and 1500. My return is also 28x12 if that matters at all.

  2. #2
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    The physical dimensions of the trunks are only one small part of the equation. A full load analysis and Manual 'D' duct design should be performed. I'd recommend Manual 'T' also be used extensively for the analysis. The reason for the attention to the duct design is that each change in direction, no matter how small, affects the resistance to air flowing. That resistance is recorded as static pressure when the constructed duct is tested. If you've got a 4-ton blower currently connected to the system, I'd have a static pressure test run. It takes only a few minutes and any good contractor should be able to preform the test. In fact, it should be performed at each tune-up as a matter of maintenance and good industry practice. A total static of 4. to .6 would be acceptable and indicate that the entire duct system is able to handle a variable speed blower for a 4.0-ton system. A lower than .4 static might be questionable (depending on how far below .4 it actually is) as far as airflow spread, across a room for example. If it's higher than .6 (with a dry coil, m7 with a wet coil) then I'd suspect there are issues that should be addressed before installing a variable speed blower.

    Whether the main trunk, branch supplies, supply or return grilles are at fault for high static pressure readings takes some detective work, unless you've got a very restrictive air filter, in which case not much detective work is needed.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  3. #3
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    I have yet to find ANYONE in my area (Fairfax, VA) that will actually take the time to do these calculations. If anyone knows the name of someone who will, then please email me at ! Everyone (large companies too) just come in, and say they will price out a new furnace w/o these calculations. I have had 7 different companies come and in and still nothing.
    Last edited by BaldLoonie; 02-25-2012 at 04:05 PM. Reason: please put e-mail address in your profile

  4. #4
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    I hear your frustration. Do your public utility companies offer any rebates? If so, they may be a source for partner companies or those who do load analysis to comply with rebate requirements. The size of the company has nothing at all to do with cutting corners. Heaven knows, some of the biggest companies cut the biggest corners, so there's no peace of mind in dealing with large companies or big box retailers. And yes, you may have to interview 100 companies to find one or two who do load analysis. Sit down with your phone book and make phone calls, one after another. Just ask them how they size replacement units. If they don't volunteer to do a load analysis, thank them and move on. You'll also find out who answers their phones during the business week and who uses a pager or beeper or 2nd phone number. I'd suggest you're looking for a legitimate company (they have someone answering their phones in a real office M-F) and a 24/7 answering service for emergency needs and yes, they also do load analysis. Another source if YOU want to cut corners might be to consult with HERS raters or BPI building and envelope analysts who could recommend a company or two who they know to be doing load analysis.

    It's not always easy to find qualified companies that don't cut corners. But doing your homework will pay dividends in comfort.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  5. #5
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    This company may serve your area, or know who you can call in your area. They're out of Virginia Beach.

    LowesMech

    He is a member here.

    You can also click on the Contractor Locator link in my sig, to find other members close to you.
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    How many times must one fix something before it is fixed?

  6. #6
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    Quote Originally Posted by Border411 View Post
    I have yet to find ANYONE in my area (Fairfax, VA) that will actually take the time to do these calculations. If anyone knows the name of someone who will, then please email me at ! Everyone (large companies too) just come in, and say they will price out a new furnace w/o these calculations. I have had 7 different companies come and in and still nothing.
    Skippedover is correct, but, in reality, no one will take the time to do those time consuming calculations when the competition won't.

    Someone that knows ductwork sizing and asks the right questions should be able to get it pretty close. Normally we can't change the ductwork unless it's accessable in the basement.

    To answer your question, the 22x8 is good for 1,000 cfm and the 12x8 is good for 500 cfm.

    A four (4) ton system needs 1,600 cfm. Do you have any take offs (supplies)coming off the plenum?

    The return side is normally the problem child quite frankly.

  7. #7
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    Quote Originally Posted by George2 View Post

    To answer your question, the 22x8 is good for 1,000 cfm and the 12x8 is good for 500 cfm.

    Thats if the air handler/furnace has the ASP to lose .08" in the supply.

    A four (4) ton system needs 1,600 cfm. Do you have any take offs (supplies)coming off the plenum?

    The return side is normally the problem child quite frankly.
    The return side is probably no better then the supply.
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  8. #8
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    Quote Originally Posted by beenthere View Post
    The return side is probably no better then the supply.
    Yes....probably the worst part is what I find.

    Because, with the way they build now, they design them with the interior walls removed for that "open" feeling. There are less places to put the returns.

  9. #9
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    the CFM ranges in those ducts. theres no way of saying "yea 12x8 is good for 500 cfm" we do not know the static in the ductwork. it could be good for 200cfm or 1000cfm. this is why it is important to measure the static pressure and then you will know for sure how many cfms it will handle.

  10. #10
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    Quote Originally Posted by Border411 View Post
    I have yet to find ANYONE in my area (Fairfax, VA) that will actually take the time to do these calculations. If anyone knows the name of someone who will, then please email me at ! Everyone (large companies too) just come in, and say they will price out a new furnace w/o these calculations. I have had 7 different companies come and in and still nothing.
    Send me an Email it is in my profile.

  11. #11
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    Quote Originally Posted by George2 View Post

    A four (4) ton system needs 1,600 cfm. Do you have any take offs (supplies)coming off the plenum?

    The return side is normally the problem child quite frankly.
    No take offs. Second opinion you have an email inbound.

    I was also looking to adding a return in the rec room downstairs as there are supplies but no returns (can feel air blowing through the the door opening at the top of the stairs.)

    The company that said they wanted to add bigger duct work about 1/2 way on each trunk said that so the bigger duct work can serve 6 out of the 11 supplies on the 22x8 side and 3 of the 6 on the 12x8.

  12. #12
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    Quote Originally Posted by George2 View Post
    Skippedover is correct, but, in reality, no one will take the time to do those time consuming calculations when the competition won't.

    Someone that knows ductwork sizing and asks the right questions should be able to get it pretty close. Normally we can't change the ductwork unless it's accessable in the basement.

    To answer your question, the 22x8 is good for 1,000 cfm and the 12x8 is good for 500 cfm. I agree here... although I believe the volume of CFM to be a little less in those size ducts.

    A four (4) ton system needs 1,600 cfm.


    Quote Originally Posted by Border411 View Post
    No take offs. Second opinion you have an email inbound.

    I was also looking to adding a return in the rec room downstairs as there are supplies but no returns (can feel air blowing through the the door opening at the top of the stairs.)

    The company that said they wanted to add bigger duct work about 1/2 way on each trunk said that so the bigger duct work can serve 6 out of the 11 supplies on the 22x8 side and 3 of the 6 on the 12x8.
    Without doing the proper load calcs and all... that "sounds" like a much better design than you have now.

  13. #13
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    Quote Originally Posted by George2 View Post
    Skippedover is correct, but, in reality, no one will take the time to do those time consuming calculations when the competition won't.

    Someone that knows ductwork sizing and asks the right questions should be able to get it pretty close. Normally we can't change the ductwork unless it's accessable in the basement.
    Well George, you're not competing with me, that's for sure. I do those time consuming load calcs all the time. And we NEVER have a complaint about noise of discomfort, not even with our zoned systems. And we have other companies that will walk away from repairs and problems and refer them to us to resolve. Does it cost the HO to get those 'pretty good guess' jobs fixed. You bet. And generally the repairs are more expensive than if they'd done it properly from the start. Sure, it's tough to sell against a company that's going to do it for $X,XXX.xx less but is guessing. And sure, there seems to be no end to the customer's who are just chasing the price of the single event (that job) and ignore all the other signs of guessing and substandard work. For them I have no sympathy. In fact, once we quote on a job, we will not provide service for at least 5-years if they go with another company, unless the client pays us the difference in the 2-quotes first. Never had anyone pay the difference but have had several call and want us to 'fix' the problems. Our advice is to call the installing contractor back and have him/her fix it. It's a tough work out there but that's no excuse for cutting corners. Cutting corners just puts you in the running for quoting on price alone, the race to the bottom. Good luck with that.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

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