HVAC formulas



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    Susan Arnold

    I never did get an answer for these formulas before my test, but here are names for some other reference formulas to help others locate relevant resources:

    Zonal Cavity Calculation/ Average Maintained Illumination Level:

    Footcandles= (lamp lumens x lamps per fixture x number of fixtures x CU x LLF) / area in SF

    Heat Loss through Air Infiltration (also, it appears that rearranging the formula can be used to calculate CFM required for cooling in MEEB section "Psychrometrics and Refrigeration"):

    BTU/h = cfm x 1.08 x deltaT

    Heat Flow through Envelope aka Heat Loss through Transmission through walls, windows, doors and roofs:

    BTU/h = U x A x difference in T between indoors and outdoors

    Heat loss through Envelope (one formula for wall, one formula for roof, but they work the same):

    U total wall = [(U x A for window) + (U x A for opaque wall )]/ A total

    There's several acoustical formulas for sound absorption that appear to be a series that build one upon the other. The first helps you find the sound absorption of a particular material in a space (a = SAC x S). The second helps you find total sound absorption in the room by adding the absorption of the different surfaces. The third relates that to the reverberation time.


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    Rebekka O'Melia

    Hi Susan, 

    For questions like that, NCARB would probably give you a geographic location and info about heat loss thru building.  You can look up in Graphic Standards the Heating Degree Day by location, which is how you find the BTUs you need.  It's based on the difference between the outdoor temp and the indoor temp.  Every day, every degree is added up.  Colder climates need heat for more hours/year and thus need more BTUs.  

    Hope this helps!

    Rebekka O'Melia, R.A., NCARB, B. Arch, M. Ed, NOMA, Step UP ARE 5.0 Courses



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