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  1. #1
    Join Date
    Apr 2011
    Location
    Barrie, Ontario
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    3

    cfm vs temp rise

    Okay here goes...

    Customer has a small house 1000sf. main duct sizing is 8x12, according to my charts max cfm is 400. The lowest setting on the furnace is 757 cfm. I don't want any temp rise issues, (not enough maybe) or excess noise. Can I cheat, so to speak, by using a denser air filter and rechecking?
    I am not going to re size his duct-work for him...

    Ramhammer

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,125
    400 CFM on a 40,000BTU input 80% efficient furnace will give you a temp rise of 74F.
    Whats the allowable temp rise on the furnace?
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  3. #3
    Join Date
    Oct 2004
    Location
    So Cal
    Posts
    3,527
    main duct sizing is 8x12. I am not going to re size his duct-work for him...
    Then walk away from the job.

  4. #4
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Why do you think 400 CFM is maximum on a 8x12 duct? Is that the return or the supply? By charts, do you mean the blower fan curve charts based on static pressure?

    A thicker filter might make a bigger difference than you might think.

    Before doing that , you could check the static with no filter as a reference. If yu can't get minimum CFM wiht no filter on that small main duct, then it won't matter how big of a filter you put on there.

  5. #5
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,302
    Only one 8x12 for the whole house?

  6. #6
    Join Date
    Apr 2011
    Location
    Barrie, Ontario
    Posts
    3
    Thanks all for you help and suggestions and sorry for not making my self clear or providing all the necessary info/data..

    Precision HVAC ?
    Just walk away, so the only solution you could come up with was to redo his whole supply duct system ? Or maybe walk away so someone else could come in and overcharge the crap out of him.Your stats/profile say you are more resourceful than that.This guy wants a good job at a good price, I have to compromise here. Will it be a perfect system,no, is the system housing ICBM's or microchips,no, making the place comfortable that's all.

    Anyhow...

    http://www.scribd.com/doc/42157897/F...t-Sizing-Chart is the cfm info I use.

    Yeah only one 8x12, on the supply side. He has six 5" ducts running to other rooms.

    46,000 btu furnace @ 95% pushing 757 on LOW. Temp rise is between 35-65 the sweet spot being 50.

    I sure as hell don't know all that is involved with the proper set up and balancing of a system, but I'm sure as hell going to try...

    I went over this today with an HVAC professional 35yrs experience.
    He suggested that I make a "pressure" box after the plenum to be able to supply a new run of 6" (which is were I was having trouble with the cfm's) couldn't steal it from the 8x12.
    He said between the plenum 16x20x2ft and the pressure box 8x22x3ft (mounting the 22 flush against the end of the plenum) I should have no
    problem with temp rise?

    This monster is in a crawl space so appearance is not important. NOW don't everyone jump on me because I said that...Professionalism and a nice neat clean job are always paramount. Duct lines are straight as well as all gas lines,hydro and venting and yes I will even put my tag on it,Not a cowboy here..

    Holy crap I type a lot....latter and thanks all Ramhammer

  7. #7
    Join Date
    May 2004
    Posts
    538
    sounds like you have a lot going on.

    some basic math. 8x12=96 sq inches.
    take that and divide it by 6 which = 16. or 16,000 btu's of cooling.
    So at 400 cfm per ton that would give you just under 1.5 tons of cooling.
    building code here is 6sq inches per 1000 btus.
    But now all change out's have to be tested by a 3rd parity too. This is to make sure the duct work is the correct size and low leakage.


    Maybe install a mini split heat pump or wall furnace. The new mini splits can be connected to duct work too.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,125
    To hit your sweet spot of 50 delta, will require 800 CFM.
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  9. #9
    Join Date
    Mar 2010
    Location
    Spud Country
    Posts
    326
    Add a few 8" runs if possible.
    I am an experienced, caring, trusted, service provider who solves my customers issues beyond there expectations by educating them and guiding them with value-building options to make excellent buying decisions while building security and wealth for my company, myself and my family.

  10. #10
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190

    Just because your ductulator says 400 @ .08

    Doesn't mean that is all the airflow you'll get!
    12x8 is 500 at 0.1 per 100 FEET. and what about the other short runs?
    Fact is you don't know without looking at the sketch of the ENTIRE DUCT SYSTEM Not just the value of one size of duct.

    You could also add a branch to the plenum or any number of modifications if it proves to give you that high temp rise you just CALCULATED ( guess based on science and some ASSUMPTIONS)
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  11. #11
    Join Date
    May 2007
    Location
    Dry as a bone Tucson
    Posts
    4,344
    Quote Originally Posted by ramhammer View Post
    Okay here goes...

    Customer has a small house 1000sf. main duct sizing is 8x12, according to my charts max cfm is 400. The lowest setting on the furnace is 757 cfm. I don't want any temp rise issues, (not enough maybe) or excess noise. Can I cheat, so to speak, by using a denser air filter and rechecking?
    I am not going to re size his duct-work for him...

    Ramhammer
    Where is the furnace located on the duct? In the middle or on one of the ends?
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    "keeping condensing pressures low and evaporator pressures high"
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  12. #12
    Join Date
    Jun 2011
    Posts
    21
    760 CFM in a 12x8 duct puts you at about 0.2 inches of pressure loss per 100 feet of duct. Ideally you'd be at 0.1 inches, but if your duct runs are short enough (as they likely are in a 1000 sq ft house) that doesn't matter because you'll still have plenty of pressure at every register to distribute the air effectively.

    If you want to back this up you could get the fan curve for the fan in the furnace and see what the static pressure is leaving the unit, then do a pressure loss calc using 0.2 inches per 100 foot of straight run and using the tables for losses at each fitting given in the ASHRAE Fundamentals book and make sure you still have some left over by the time you get to the register that is furthest away from the fan. A 1000 sq ft house is pretty small though, so I'm guessing that the calcs would tell you that you're fine.

    It will, however, be louder than it would be if the ductwork were sized properly, because you're blowing about 1200 FPM through the main.

    If you're worried about blowing too much air in the heating season when you don't want to feel like there's a fan in the room, I would definitely NOT try to choke the air with extra filters! Test the system out, and if you think the airflow is too high for the winter explain to your customer that you need a way to slow the fan down during the winter. If you're in a climate where cooling is a big deal, you can tell them that they need the fan power in the summer but need to pull it back in the winter. Either way the best way to fix the problem would be to put a VFD on the fan and slow it down to where it needs to be. If you can't install a VFD on it there is probably a way to set it at a lower speed, but you said it was already at its lowest speed so I guess you know that already. If that's the case well, you over sized the system. You suck.

  13. #13
    Join Date
    Jun 2011
    Posts
    21
    Quote Originally Posted by r-290 View Post
    sounds like you have a lot going on.

    some basic math. 8x12=96 sq inches.
    take that and divide it by 6 which = 16. or 16,000 btu's of cooling.
    So at 400 cfm per ton that would give you just under 1.5 tons of cooling.
    building code here is 6sq inches per 1000 btus.
    But now all change out's have to be tested by a 3rd parity too. This is to make sure the duct work is the correct size and low leakage.


    Maybe install a mini split heat pump or wall furnace. The new mini splits can be connected to duct work too.
    Some more accurate math...
    96 sq inches of duct relates the volume of air pushed through it (which is whatever the fan pushes and is not a function of duct size) to the speed of the air (given in the US in Feet Per Minute or FPM) and the loss of pressure as you get further from the fan.

    Actually if you want to get technical about it isn't just square inches of the cross section. Rectangular duct behaves differently than round duct of the same cross sectional area because of turbulence in the corners, but only the geeks at ASHRAE (and I mean that in a good way) care about things like that.

    Regardless of duct size or shape, CFM has no direct relation to BTUs. It depends on what temperature the air in the duct is. If you blow 65 degree air in to a 65 degree room, you're not doing any cooling or heating, you're just blowing air. Changing the temperature of a space is a function of both air temperature and volume. You can have the same effect with a lot of slightly cooler air as you would with a little bit of much cooler air.

    Don't listen to this guy. All change-outs have to be tested by a third "parity" for a reason.

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