System Sizing

Hello, this fall I'm doing a large project with replacing the concrete and radiant floor heat system in three of the bays at a car wash. This is for ice and snow melt.

This system will be 1900 square feet. I'm going to use 3/4 inch rehau pex. There will be two manifolds. The first will have four loops. The second will have six loops. Each loop will be about 270 feet or less.

I would like to install two navien NHB-150H boilers in series so that if one goes down I will have one still working to provide some heat.

This is the picture of a set up a have for another system. What do I need to change to install two boilers in series this time?

I would also like input on how to size the pumps for the system.

The two boilers will be installed in the upper floor of the equipment room. With the manifold installed on the lower level with the pex coming out of the ground to the manifolds. Should I have a separate pump for each manifold?

I will plan on just having a sensor in the slab to determine the temperature and when to cycle on the floor heat. Really the goal is so that any snow in the bay gets melted, and any water sprayed won't freeze on the floor. Basically it will keep a constant temperature above 32 degrees.

I plan on doing a 30-35% glycol mixture.

Thank you for all help, I greatly appreciate it. This is a huge project that i'm trying to do as much of the install myself.