Dave, it sounds like the floor is done up right, enough loops and proper insulation, so thats great. The hard parts done. I think your heater choice may be a sound one, I understand that you need 1kw per 100 SF, and with the heat loss of a garage door you may be right in there, if not a bit large.. I would still shop around and check other brands, costs etc..
Your garage is a bit larger than mine, I am only 720 SF so you may need a larger heater than I used, but my dial is set between low and middle heat output, so It could keep up but may not live as long.
I ended up setting mine up with a "Low and slow" approach, 110º out of that little heater is kinda loafing, and my pump usually runs on low, if my loops were any longer I would just run the pump on medium.
I toyed with sending in 140º into the floor at the beginning, but the heater drew a lot of juice and while it may have ran half as long it worked twice as hard and I figured it would be more likely to die running at 100% duty cycle, that and it made the lights flicker (Even with a dedicated 60 amp run).
But, remember that a BTU is a BTU, it does not matter if you use a tank or a tankless to achieve the fluid temps. Many will argue that a tank is great, but I am not storing hot water, I am storing HEAT, and it is stored only in the slab! If I go to a solar collector on the roof I will need to add a tank style heater and a second pump since my hot water from that system will be more finite, I will need to mix.
With that many loops you will need to understand what "Feet of head" is for pump selection (If you have not already purchased a pump), google that for yourself and understand it, understanding is ownership, opinions on the web are just that.
I know that many hear live or die by a heat loss calculation, I dont blame them but reality sets in once a building is built and you cannot achieve a better heat loss ratio, so insulate the hell out of your building and use good doors.
I would at least use 6" studs, if not 6" + another row of 4" on top, or bang up some good foam board on the block, then build an insulated stud wall over that. At least with the block wall your studs will not be touching the block tight (Right?) so you will not get convection of the cold through the wall.
Just remember that heat does not rise, hot air rises and you re not heating air, so double check to make sure you are not sending heat up a post or beam that can **** BTU out of the floor. I would even go so far as to set the bottom plate for the interior walls on some sorta expansion or insulation board, and prevent the wall board from sitting tight on the floor. Some guys get hung up on heat loss calcs but never consider convection, and that skews the numbers somewhat.
Make sure you have a decent basic understanding of layouts on a simple system, where the pump is in relation to the air vent does matter, and the amount of 90º elbows does not help when you are trying to bleed the air from the system. Making a fill/drain system to purge the air out is the only way to go IMHO, you can see it in my pic.
Feel free to PM me if you want more info about my little system.
