General Design Question - Garage Floor

oldbugger

Hello all, I am building a garage 28'x42' and at today's prices wont be able to get everything done at once. I want to get tubes in the floor and add rest of system down the road, hopefully sooner rather than later.
I grabbed a copy of LoopCAD to get the circuit layout mapped for me but have a couple of newbie/DIY questions hoping you all could help.
Perimeter inside 4' deep 2" EPS R-8
below slab 2" R-10XPS on top of 2" of sand on top of 6" of 1' gravel
Walls 10' 2x6 with R-19 glass on top of 18" exposed foundation
Ceiling is a little uncertain as I may add some storage but 5/8 rock, minimum of R-30
Location Central Iowa
LoopCAD show 4 circuits at 270' to 280' each - 6" spacing for outer wall at least the first foot then 12" spacing in field.
Biggest question I have is 1/2" or 5/8"?
Second question is 4 circuits sufficient or should I step it up to 5?
Third question is one layout method better than another serpentine vs. serpentine counterflow?

hot_rod

Pex coils are typically 300, 500, or 1000’. 
Since you have about 1200 sq ft of slab four 300’ tools would work. No sense in wasting 20-30’ of tube

i build almost an identical size shop last year. I put the tube 6” on center, so you would need eight 300’ loops

The tight spacing allows  faster warm up, more consistence floor temperature and the lowest supply temperature, about 12 degrees lower fluid temperature for the same btu output

This would allow you to use an air to water heat pump efficiently and also do some cooling with the floor, if you want an electric option for the heat source. What do you pay for electricity?

The tube may be the least expensive component, and one that cannot be changed after the pour

Zman

Your insulation looks good, I think I would use XPS everywhere. Don't forget the edge detail, tons of heat loss there.
Personally, I would use 1/2" tubing and go with 6 loops at 250'. I would burn the extra tubing by doing 6" spacing in front of any big heat loss areas. If you keep the loops within 10% difference in length you can save a few bucks and buy simple manifolds with just isolation valves. 1/2" tubing is easier to work with and find fittings for but it is better to not go 300'.
I would do whatever, layout pattern is easiest. If you keep the lengths fairly short, you won't see much difference in temp so the pattern is less relevant.

Dave Carpentier

We have a 30x19 x11h (10ft on pony wall) attached garage, 2x6 R20 and a bit better in the ceiling.
I dont remember the pipe pattern but we did do the 2 6" perimeter loops and then 12" spacing, all in one run of 1/2".
We dont keep it at "room" temperature, more like 55-60f (I find it a good working temperature anyhow). It will hold that temp when it's below -20f.
Unless we open the door and bring an ambient frozen vehicle in. There's no quick recovery from that with this setup. I'll probably put an air coil unit heater on it's own hydronic circuit at some point, or a propane or electric unit heater of some sort.

One thing I didnt count on with the warm slab, is that the little tounge of concrete that sticks out under the garage door stays completely snow free. Its a good thing, the door seal never freezes to the slab, but Im certainly losing btu. I might get some kind of "sock" tube to lay down there ?

Solid_Fuel_Man

When I do radiant garages, I like to keep the tubing back 12" from where overhead doors will be. Still melts the snow under the door and the exposed concrete stays dry outside. But I feel it is less possibility for freezing in the case of prolonged heat failure. I've done several garages this way, my own included. 

I do 6" spacing near the doors after that 12" of no tubing and 6" for the first few runs at the walls to burn up any extra tube, as had been said. 

I've always done supply to the outside walls where the 6" spacing is and return is yhe end of the run toward the middle of the floor. For any reasonably sized radiant slab (like yours) 1/2" tube is my go to. 5/8 or 3/4 allow for longer runs for commercial large slabs. I've never had any issues with 300' runs of 1/2" pumped with Alpha1 or 0015e etc. 

Insulation and air sealing are your friends! If you apply some foam over your 2x6 after insulation and tape the seams you'll have a very tight structure. If you decide to use metal for wall finish or even sheetrock, you can surface mount electrical boxes and conduit to avoid putting holes in the building envelope.