Info on new build

You should start with a load calculation and radiant design. It will give you the heat load for the building and radiant loops and lengths.

For best comfort and consistent floor temperature I would not exceed 12" on center. The tighter the spacing the more even the floor temperature. 18" will have some very wide, noticeable temperature gradients across the floor. It may be acceptable for a shop, not so nice for the residence.

Also the more tube, the lower you can run your supply temperature. once you have the load calculation in BTU/ sq. ft. you can look at different tube spacings and loop lengths.

1/2 loops 300' in length are common for many systems, I'd consider 9" on center also, it's worth the extra tube cost.

It cannot be changed once the concrete is poured.

Don't scrimp on underslab and edge insulation, that drives the operating cost for the life of the building. 2" minimum under and around the edges.

I'm seeing that 300' loops of 1/2" PEX (0.475" actual) with 12" spacing works pretty well, but shorter loops and closer spacing would be better if it works out for your layout and budget.

This morning I'm bringing a patio/three-season room up to temp. Floor is 1.5" concrete pavers with the PEX buried at the bottom of 1" of stone dust. Floor is 600 ft^2, with two 300 ft loops of 1/2" PEX with 12" spacing. Flow is calculated to be running at 1.75 feet-per-second/1.0 gpm. IR gun measures 125 degF supply and 104 degF return, so approximate rate of heat delivery is 20,000 btu per hour, or 33 btu per ft^2.

At the start of the loops I'm seeing 72 degF max / 63 degF min floor temperatures, and towards the end it's 67 degF max / 54 degF min. Shorter loops and closer spacing would bring all four numbers closer together.

This would be kind of a worst case situation for a concrete type of floor. I'm using a high supply temperature to bring the room up to temperature more quickly so the floor temperature hasn't had a chance to even out more. Plus the PEX is not a couple inches deep in concrete, it's just a 1/2" below the 1.5" pavers.

The differences in floor temperature are noticeable with stocking feet, but otherwise comfort is close to ideal.

And as Bob points out, need to pay close attention to insulation detail at the outside boundaries. It was recommended to me to use 4" thick horizontal for two feet in all along the outside perimeter. Search and study on http://www.greenbuildingadvisor.com/ for ideas on how to get this right.

Don't use that boiler. Find a different one.

I spent 17 years with one.

The wood side of that boiler is VERY inefficient, poor heat return from the amount of wood it burns. I don't think you could find a more inefficient design.

Plus word on the street in the town they are made in is they are closing operations - can't confirm, but it's what I heard.