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Extrapolating Air Handler heating response for lower supply water temps ?

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Dave Carpentier
Dave Carpentier Member Posts: 603
The manual for my air handler has some charts that show btu per supplied water at a given entering air temperature.
But the lowest they show is 130f.
I plotted them and found it to be a straight line.
Is it okay to extrapolate like this ? If I supply it with 110f water, should I get fairly close to 27Mbtu ? 27k is my air design point, and 110 is my in-floor temp.. so this might work out nice (purely by accident though.. lol).


30+ yrs in telecom outside plant.
Currently in building maintenance.

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  • Jamie Hall
    Jamie Hall Member Posts: 23,544
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    should work. If you continue that straight line back to 0 output, it hits at about 70 or so, which makes sense.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,353
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    Are you reading any of the Idronics I’ve linked for you? #25 shows how to derate most any heat emitter.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    mattmia2
  • Dave Carpentier
    Dave Carpentier Member Posts: 603
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    Thanks Hotrod, and it works out to approx 30k with the ratio... so, perfect !

    I dont think I read #25 yet, but I have indeed read a bunch.
    Its a fantastic resource, pass along my thanks for sure.

    Wish there would have been an equivalent to that in the outside telecom field when I was starting out.
    We had some manuals for reference, but they were very dry and poorly organized, and covered a lot of stuff that our company didnt use.

    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • mattmia2
    mattmia2 Member Posts: 9,840
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    You might want to slow the airflow down somehow so the air is warmer and less drafty.
    Hot_water_fan
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,899
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    Similarly, speeding up the liquid flow rate raises the average water temp and ups capacity, all else equal. 
  • pedmec
    pedmec Member Posts: 994
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    he will never get comfortable with that air

    maximum supply air temp, 110 probably not gonna get it.

    cfm= btu/(delta t x 1.08)

    cfm =30,000/(40 x 1.o8) assuming return air temp of 70f

    cfm =30,000/43.2

    cfm =694 best case scenario @110 supply air

    that is to make sure you have 110 degree air temperature. anything faster than that you are getting cooler supply air out of your system. that is less than 2 tons of airflow.

    td= btu/(cfm x 1.08)

    td= 30,000/(800x1.08)

    td= 30,000/864

    td= 34.72

    RAt= 70 +35 = 105 SAt

    what size air handler are you using.

    at 110 degree supply water temperature you are leaving out the comfort.

    are you adding humidifier to this?


  • Dave Carpentier
    Dave Carpentier Member Posts: 603
    edited May 2022
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    Its a 2001 Enerzone AHT 47-30.
    According to their 2008 product info for the same model#, it does 1000cfm at 0.5" static.
    It might be a 3 speed motor , it's wired for 2 speed (fan, or fan+circ) with red and black hot wires, but there's a blue wire shown unused at the motor.
    No humidifier being added, but hopefully an A-coil for a/c at some point (which will slow the air down some)
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • pedmec
    pedmec Member Posts: 994
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    TD= btu/ (cfm x 1.08)

    TD= 30,000/(1000 x 1.08)

    TD= 30,000/ 1080

    TD= 27

    RAt=70

    SAt= 97 (70 + 27)

    97 degree supply air @ 1000 cfms is going to feel cold. you really need to rethink this 110 supply water temperature. there is a reason design temperatures for the water supply side are normally designed for 140 degrees. air temperatures will be too cold. that is the most likely reason the manufacturer only plots down to 130 degrees.

    and you will need a 20 degree delta on your water side to get the 30k.
  • Dave Carpentier
    Dave Carpentier Member Posts: 603
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    Okay. Determined that its a 3-speed PSC motor, 1070rpm.
    Heating mode would be the highest speed (black wire), so thats the 1000cfm.
    I do have a dwyer differential gauge that I could check the static to see where I am in the table.
    But I cant find any info on what rpm or cfm that the other wires do, and no static table for the other speeds to figure it out that way.
    Everything Ive found so far online just say High/Med/Low.

    This might just be a trial and error thing next winter, if we start to get cold feeling air then bump up the ODR range and/or drop the speed.
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • mattmia2
    mattmia2 Member Posts: 9,840
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    The ratings plate on the motor might tell you. Since it is an induction motor there are only certain ways you can divide it.
  • Dave Carpentier
    Dave Carpentier Member Posts: 603
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    The manufacturer Enerzone got back to my query immediately, very cool for 20 year support.

    Its about 100-150 cfm drop per speed on this one.
    So, medium would be around 850-900cfm, and low would be 700-750.
    Low (currently used as fan) seems to move a nice amount of air, but the proof will be next winter.
    Would save a smidge of electricity too.
    30+ yrs in telecom outside plant.
    Currently in building maintenance.