Are these numbers close?

[2011laramie]

Hello,

I'm just trying my hand at some manual J calcs.

My shop is 44x64x16, and will be divided into a 24x44x16 bay that I will heat full time, and then a 40x44x16 bay that will be heated when i need to be out there.
This is in Alberta where it easily hits -40F. with a room target of 50F

shop details for the small bay
2x8 studs on 16" centres going to use roxul mineral wool. assuming effective R value of R22.
has roughly 56q ft of windows, going to assume R4, they are double pane low E with Argon I believe
Overhead door is 168 sq ft of R16
63sq ft of man door (1 single to outside, and a double door to other bay), Assume R10.
the roof will be blown in R60
the floor has 2" of foam board under the slab, then 1.5" down the footing on the inside and 2" down the footing on the exterior.
the interior divider wall is also 2x8 but on 24" centres.

so time for math.

1889ft2 of 2x8 wall. U=.0454
1056 of roof U= .0167
1056 of floor U=??? i don't know what to use?
168 of OH door U=.0625
56 windows U=.25
63 man door U=.1

Q=UxAxdelta T

.0454x1889x90 = 7718
.0167x1056x90 = 1587
.0625x168x90 = 945
.25x56x90 = 1260
.1x63x90 = 567

total BTU is 12077.

now for the big bay.
only going to consider a heat loss out 3 wall cause the other wall will always be at 50. technically it would actually be a heat gain into this bay.

total wall area = 2683 ft2 of 2x8 wall.
windows = 64ft2
Overhead doors = 368ft2
man door 21ft2
roof = 1760ft2
floor = 1760ft2

Q=UxAxdelta T

.0454x2683x90 = 10962
.25x64x90 = 1440
.1x21x90 = 189
.0167x1760x90 = 2645
.0625x368x90 = 2070

total for this bay 16006btu

That gives me a grand whopping total of 28083 btu/hr. No I haven't accounted for air leakage or the loss through the floor. because I don't know how to account for those values. what is the general rule?

are these numbers close or did i mess up royally. when The manual J calcs are final, how much extra does a person add so that the heater isn't running 24 hrs a day to meet the load?

I don't have a solid plan, but what my plan as of right now is to put a 50000 btu boiler in and run the slab on the small side, then run a glycol fan coil unit in the big side when i want heat over there. I picked this route because i read that gas unit heaters shouldn't be left to go below 5C for corrosion.

Any professionals got any input to this??

 

[Jackfre]

I would suggest that you look at Rinnai Energysaver wall furnaces for the space. An EX38T will fire from 13,200-38,000 btu. Easy install, 2.5” hole for the direct vent system, which comes in the box (will adjust from 4.5-9.5” wall thickness). Programmable stat is built in. That is a big space but with the modulation and your calc’d load it works. Comparing the cost of a boiler, venting, installation, air handler and space savings the Rinnai is a slam dunk.

 

[bobbyjean]

im sure there are some radiant experts here...i think you are short on input...i have used supply houses for these kind of load calcs. in the past.
the quick way is square footage multiplied by a btu number per square foot-pretty sure this works for older home construction ...meaning 2x4 with shoddy insulation...the numbers i have used are from 30 to 40 depending on condition's..location
even using 30 btu per your roughly 2000k sq. heated side is 60k-i guess i would work with your supply house to get a good calculation done.
question-are you planning on using a plate type heat exchanger? it may be a good idea ....(thermal shock) when you cycle the fan coil would be a concern

 

[yeldogt]

It all come down to insulation .. my very well insulated 1700sf studio has a 40k sealed combustion propane. It's undergoing renovations -- the 4kw electric heater is keeping the space at about 65 w/ temps in the high 30's .. and not running much.

Keeping a space at temp and bringing a space up to temp from cold -- requires a different heater. Depending on how often you cycle -- keeping a minimum tamp is not always more expensive.

 

[2011laramie]

Ya ive read all sorts of rough calculators, 10-15 for well sealed to 45 for poorly insulated leaky buildings. I definitely want a boiler to heat my slab heat. I rolled 2500ft of line between the 2 bays.

Im also working on a 12x19ft solar collector to warm glycol to run in the slab of the large bay. I figure if i recover 50% efficiency of that panel, that should be about 150btu x 228sq ft = 34000 btu per hour for hopefully 4-5hrs a day on sunny cold days. I figure that should help take the edge off the cold bay.