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  1. #1

    Heat Loss/Gain Calculation

    Considering this question is regarding a machine shop located in a pole barn, it is possible that this question should be asked in Commercial Forum. If not--

    I have installed walls, ceiling and insulation (r-17 spray foam 2.5", plus r-13 cavity in walls and r-30 in ceilings) in my pole barn where I plan on operating a machine shop. The barn was erected in two separate adjacent sections. The interior of the south section is 38 x 30 with 9' ceilings and the north section is 31' x 31' x 12'. The south section has an insulated metal entry door and an insulated 10 x 8 overhead door on the south wall, one 2 x 4 double pane window on the west wall and an insulated metal door in the north wall(common between the two sections). The north section has an insulated 12 x 10 overhead door in the north wall. Exterior temperatures were figured for 46725 zip code. I used 50% relative humidity for my calculations not knowing if this is appropriate for the intended usage.

    It is possible that the north section will not require constant HVAC and I am wondering if there is a way to size the furnace for the entire building, but only heat one half consistently with occasional heating of the other section to economize on HVAC operating cost, without causing the furnace to short cycle? Perhaps this isn't a good idea to begin with!

    I have attempted to calculate proper furnace sizing using HVAC calculation software available online, but the software is designed for residential application, therefore I have come to realize that there are some inputs not available that would be included in commercial applications that might alter my overall results such as lighting, activity and air quality. The difference in BTUs when figuring the common wall as an exterior wall is 5000.

    Any advice before I proceed will be greatly appreciated!
    Last edited by stierdas; 11-26-2012 at 05:19 PM. Reason: Additional information

  2. #2
    Join Date
    Oct 2010
    Anderson, South Carolina, United States
    HVAC calc load calc software has a commercial program. Commercial buildings require bringing in outdoor air and loads for equipment inside and other things. It would probably be a good idea to put in two independent systems or you could just put a swinging heater or radiant heater (spaceray) in the one side to provide heating and a central system in the other side.

  3. #3
    I did see that they offered a commercial calculator after purchasing the residential 2 month version. Unfortunately it wouldn't make sense to spend the additional amount for a one time use.

    In the mean time I have spoken with a local HVAC professional who has suggested that I size the furnace for the half that will be consistently heated (figuring all four walls as exterior walls) and on the infrequent occasion that heat is desired in the other half, open the zone shut off for that area allowing for a longer period of time to bring it to the desired temperature. This seems to make the most sense. Do you agree?

  4. #4
    Join Date
    Oct 2010
    Anderson, South Carolina, United States
    Yea that will probably work to knock the chill off

  5. #5
    Join Date
    Jun 2007
    Minneapolis, MN
    You have made a good observation on commercial versus residential loads. However, most steel buildings or pole barns have loads even lower than the typical residence e.g. less glass, architecture, traffic.

    First, lets get rid of the "furnace" and get a condensing boiler. Since you have a perfectly good radiator in the slab and someone will be standing on the cold concrete next to a big cold machine it pays to look down.

    All condensing boilers modulate output so heating one side will not be a problem. You can use a fan coil for the occasional side and be perfectly comfortable and spend much less doing it in the heated shop.

    Exhaust and makeup air should be handled separately.

    A good designer will have heat loads and air changes for you.

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