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@GroundUp

And I'm not arguing that a piece of concrete will melt snow or ice regardless of the size of, or spacing of, tubing in it.

I agree with that and have for years promoted tight 6" spacing, 10- 15 ∆ in residential heating, specifically for comfort, quick ramp, and lowest SWT. I don't see these 6" tube spacing numbers numbers being critical for SIM design.

My point is larger tube give you more BTU capacity, longer loop length, less pump power requirement. I doubt many are barefooted in carwash bays in the winter??

All I suggest is the OP read some of the attached info I present.

I suspect the screenshots I offered throughout this post were generated in SIM software from Uponor, PPI, Rehau, others.

There is no specific way SIM HAS to be done. But there are industry guidelines based on hundreds of systems that perform to expectations.

The tube size recommendations, the 25-30 delta, tube spacing, loop length, SWT, flow rate, etc, etc all come from the manufacturers design example. These are not numbers I'm pulling from thin air.

When you have a load calc done by one of the reps or manufacturers, and sign off on the assumption doc., they guarantee the system will perform as designed.

If someone designs for a 25- 30 and only attains a 50∆, they missed something. A 50∆ in a 200' loop seems awful high?

While running tighter delta increases the slab btu output a 20∆ probably isn't juice worth the squeeze for melting snow, considering the pumping power required to get there. An example attached of 3 different ∆Ts and their output change. Same SWT in all cases. I'm seeing a specific pattern from the 3 or so examples I have shown which is 5/8 or 3/4 tube size, 25- 30 delta.

( a heat transfer demo example, not intended for a SIM design) Tighter ∆ obtained by higher flow rate= higher BTU output in the loop and section of slab it is in. You would see the same variance if you plotted a 20, 30 and 50∆ for a SIM example. Which is why I suspect the industry looks at 25- 30 ∆ in SIM designs, reasonable output, reasonable pump requirement. Which is mentioned in all the manufacturers guidelines.

Pinning down the BTU required for the OP specific job is best accomplished by the industry guidelines, in my opinion.

I did check around some carwash forms and found even wilder suggestions of 100,000 btu/hr/ bay!?

Although in the OP case the 125 btu/ hr I suggested comes out to 75,000 BTU/hr. per bay. A bit shy of what the owner/ operators on those forums suggest.

Granted these attached designs are snowmelt, the concept of melting snow or warming a slab to melt ice, and turn to liquid, or keep the floor ice free, maybe even dry after evaporation, within the same temperature ∆ would be similar in my mind. At days end we are trying to keep the slab surface around 38°F at a specific OAT.

Digging deeper at all the manufacturers sites may turn up a 3 walled carwash specific design. My money suggests they would be 100 btu/ft or higher designs.

Don't shoot the messenger, I'm just parroting readily available, industry accepted numbers and suggestions.

I suppose 1/4 copper tube loops 25' long at 2" OC could be made to work also😉