Radiant System Heat Load Seems Questionably Low

[mopar440_6]

So, since I'm not having any luck finding an HVAC contractor available/willing to look at finishing the radiant system for my shop, I decided to attempt the design myself.

I performed a ACCA Manual J calculation using the calculator on builditsolar.com and I know the free stuff online should only be used as a rough guide but the calculated heat load seemed extremely low for a 3000 sqft building. The calculator came out with 51,450 btu/hr design loss. So, to the heating experts on here, does this seem too low or am I on the right track?

Below are the specs/assumptions:
-0*F design temperature
-5500 degree days (used Harrisburg, PA as closest reference)
-50'x60'x14' pole building, steel panel walls inside and out, no housewrap
-R38 blown cellulose in 3000 sqft of ceiling
-Approximately 7" of blown cellulose in all walls assumed R21
-12'x16' R13 overhead door and standard 3' (R3 assumed) man door on North wall
-8'x8' R13 overhead door on East wall
-Standard 3' (R3 assumed) man door on South wall
-8'x8' R13 overhead door on West wall
-220' perimeter assumed to have effective R-value of 2 (has 2" R10 foamboard under entire slab and around perimeter except overhead door openings)
-42000 cuft total building volume assumed to have 0.5 air change/hr infiltration (design loss increased to 77,900 btu/hr with 1 air change/hr infiltration value)

 

[tomroblee]

Your calculation eyeballs about right.

For comparison, I have a 3500+ sq. ft. home in Indianapolis, In. It's built on a slab with 10' ceilings throughout (except for a raised ceiling in the living room.) My HVAC is a four ton geothermal system that is capable of putting out about 50,000 btu early in the season when the loop temperature is fairly high. In subzero weather, the loop temperature drops into the 30's, and the heatpump output is more likely to be in 35,000 btu range. My auxiliary heat doesn't kick in until the outside temperature is near zero.

My house doesn't have a multiple garage doors (my attached garage isn't heated by the heat pump, and it isn't included in the 3500+ sq. ft.) To offset this, I do have a ton of windows and the entire HVAC system (including ductwork) is located in the attic. I'm guessing that I have similar insulation (the house has 6" walls), but my attic insulation is probably compromised a fair amount because I have to ton of can lights in the ceilings and seven exhaust fans (one each in 4 1/2 baths and two in a "sun room" where the previous owner had intended to install a hot tub.)

I would think that the biggest variables are your overhead doors. If they aren't tight fitting and are frequently being opened, it could have a major impact.

 

[Slowgsr]

I think it looks pretty close, going by my dads house which has surprisingly low heat loss

 

[BadgerBoilerMN]

You did a good job. I would hire you.

 

[Crazy68Dart]

I came to the same conclusion on my building, it seemed low @ around 16k BTU/hr for a building a little shy of 1000 sqft. My building has lower ceilings than yours, and less overhead doors, etc. If you multiply mine x 3 it is in the same ballpark as yours. I am doing radiant as well.

All said, I am not expert on this either, but your calcs seem similar to mine from LoopCad.