Boiler choices

DaveA

I'm in the planning stage for a woodworking shop to be built in Jerome ID. The plan is for a 30 x 40 x 10 steel building on a 4" concrete slab with 2" XPS insulation under the slab, vapor barrier, rebar and 2" XPS around the perimeter. I want four 300' loops of 1/2" PEX-A spaced at 12". Jerome winter design dry bulb temp is 9.4 deg F. I'm shooting for 65 deg. indoor temp. Looked at several heat loss calc apps. I liked two, one by Borst Engineering and the other by BuildItSolar.com. I get total loss figures of 30,127 and 31970, and applying the shop floor area to the calculation gives me 25.1 btu/sf/hr for the first and 26.6 for the second. Then I input the floor output figure into RadPad and it gave me floor temps of 78-79 deg. but avg. water temps of only 78 deg. Flow rates of .6 or .7 gpm/tube and pressure drop of approx. 5' of head. My question centers around the heat source. Looked at a lot of propane fired condensing boilers (no NG available on site) and the specs all seem to put them in overkill with resulting low efficiency. Then I found that the electric costs in Jerome are only 8.5 cents/kwh, so I then checked out Electro Industries boilers. They have a model EMB-S-9 rated at 31000 btu. The other spec of note was the water temp range, which is 90 to 160. Given the RadPad results regarding avg. water temp, my question is would this boiler be a good choice? I realize the 79 deg temp is the average so should I assume the supply temp would be about 10 deg hotter, and will that work since the boiler minimum is 90 deg.? I'm a newbie virgin at this but tried to do as much research as I could.
Thanks in advance for any feedback.

GroundUp

The EMB-S-9 is an awesome little unit, as is anything else from Electro that I've seen. I've got lots of them out there with zero hiccups aside from a test jumper being accidentally left on at the factory once. They will hold a 90* SWT, so that would be dandy for you. Operating cost however, may be more in favor of LP depending on your fuel costs. At 8.5 cents, it takes $2.16 propane to break even with a 95% efficient unit like an HTP UFT-80W (and others, but that is my go-to). The UFT has a 10:1 turndown ratio which would allow you an 8k output on low load calls/warm days with outdoor reset (which comes with the boiler). LP seldom gets over $1.50 in my area and electricity is 14 cents so it's a no brainer in most cases. Need to factor in some maintenance costs for gas units as well so that pulls from some of the savings

HVACNUT

How hot is it going to get in the shop in August?
You've certainly done your research, but I dont get why you wouldn't just go with a ductless system. Heat and A/C!
Even with radiant, you won't have warm tootsies with your OSHA approved boots, and aren't work shops supposed to be a little spartan?

GroundUp

Ductless at 0*F with an ice cold floor? No thanks.

Big Ed_4

There is nothing better then radiant slab in a work place during the Winter...Yes Ductless for the Summer ...

Zman

I think RadPad is giving you some bad temps. I think you will need more like 115 on design day.

DaveA

Thanks for the comments all. Groundup, I should have mentioned the other reason I am leaning to electric is propane in that part of Idaho is currently at $2.50/gal. The BuildItSolar yearly cost had it about $245 higher than electric.
HVACnut, I can't rule out ductless I guess. As far as the shop being spartan I'm all about comfort when I'm woodworking, since this is my retirement activity, plus it'll be just me and I'm not doing this as a business. I had experience with a radiant slab in my shop in NY. I Loved the 80 deg+ floor in the middle of western NY winters.
Zman, I was also a little suspicious of the RadPad results. The 2 inputs you enter to calculate water temp were floor output (26) and slab R-value. Right on the same page of the calculator is a chart with various R-values based on slab thickness. I used the .5 indicated for a 4" slab with tube spacing at 12" (and no floor coverings). The range is 0 to 5, so I wondered about that being so close to the minimum.

DaveA

Zman,
In the screen print of your simulator what is the variable that results in the 115 deg water temp? I'm guessing that simulator is used by pros only and I'm not even familiar with some of the terms. If I go back to RadPad and use the 115 deg figure as a given it displays a value of 2.0 for the slab R-value, which is 4x the .5 it gives as an example. What problem am I facing if I just set the supply temp at 115? Burned soles of my boots, maybe? If increasing the supply temp does result in floor temp being too high is there something else that can be done to the system to compensate?

Zman

The program is Hydronic design studio.I think you can demo for free.

With a 4" slab, your floors will barely feel warm at 115. I have 1.5" slabs and tile floors. The floors don't really even feel noticeably warm until the water reaches about 120.

I would suggest outdoor reset controls for your boiler. With an electric boiler, you could overide the boiler aquastat with a tekmar or other controller. If you post the boiler wiring diagram, I am sure someone would be willing to work that out.