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  1. #1
    Join Date
    Jan 2007
    Location
    Stongsville Oh
    Posts
    931

    Reheat coil capacity

    I have a hot water reheat coil that I pulled out many years ago and have been saving it thinking that I may use it someday. Some day is now here. I have a crawl space area that I am thinking about using this in with a small fan, Domestic pump, aquastat remote bulb space stat, relay and control valve all of which I own already (except the pump). The coil is 16 x 20 with 2 rows of tubes. 10 tubes/row 1/2" tubes. I am thinking of using the how water from the tank which runs about 120° to 130°F. Any thoughts on the btuh capacity I can get out of this coil at these temps. I am not sure of cfm I will get out of the small fan that I have attached. The application is not critical it is just for tempering the space. The kitchen is above and the floor is cold when we reach ambient of about 20°. Control scheme is pretty simple,Call for heat starts pump, aquastat starts fan. Satisfied stops pump, aquastat stops fan. Crawl space has a heat run already so freezing is not an issue. Main question is BTUH of coil. I have no idea how to calculate this. Just want to see if the effort is worth it.
    Thanks
    ckartson
    I didn't write the book I just read it!

  2. #2
    Join Date
    Oct 2005
    Location
    B.C. Canada
    Posts
    903
    There is a formula, i dont know it off the top of my head, but im sure somone around here will.... pretty sure u neet to know your cfm though

  3. #3
    Join Date
    Mar 2011
    Location
    North Carolina Piedmont Area
    Posts
    451
    Water temperature drop = 2.04 x MBH/gpm

    Air temperature rise = 927 x MBH/cfm

    Correction Factors for Other Entering Conditions
    Δ T 50 60 70 80 90 100 110 125 140 150
    Factor 0.40 0.48 0.56 0.64 0.72 0.80 0.88 1.00 1.12 1.20

    You can easily calculate 3 to 5 GPM but you will have to use the correction factor for temperature rise

  4. #4
    Join Date
    Dec 2003
    Posts
    2,426
    There is not really a way to calculate what you are looking for. If you know a rep that sells vav boxes you could figure out what size box has a reheat coil similar in size, give him a gpm and cfm, and he could tell you how many btus. It would prolly work.

    On another note its probably against some code to use domestic hot water for this if thats the plan. The water would just lay in the coil during the summer and maybe get funky.

  5. #5
    Join Date
    Nov 2006
    Location
    MA
    Posts
    61
    Quote Originally Posted by bigtime View Post
    On another note its probably against some code to use domestic hot water for this if thats the plan. The water would just lay in the coil during the summer and maybe get funky.
    A way around the funky water could be a timer on the bronze circulator to turnover the water in the coil every few hours.

  6. #6
    Join Date
    Oct 2005
    Location
    B.C. Canada
    Posts
    903
    Not the proper way to dovit, but if it werks at his house...fine by me, its ok to hack at home....

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