Pex spacing

Rule of thumb doesn’t always work. What is your finished floor (bare concrete, hardwood......), city and state, how good is your insulation, size and type of windows, ceiling height, etc.?
A responsible professional would do a heat loss analysis.
A wider spacing is cheaper to install, but may force you to run higher water temps. Higher water temps. will reduce boiler efficiency.

Make sure you insulate under the slab, even in southern climes.

While the 24" centers may meet the heat load, you're not likely to enjoy the striping that occurs on the bare concrete with your bare feet. It'll be toasty, possibly even uncomfortably hot directly above the tubing while it's less than favorable a foot away, halfway between. We have very limited information on your project, but if the contractor's excuse is simply "because it worked last time", you may want to start looking for a new contractor. That layout will give you 1600 feet of tubing, and with a standard 20 degree delta T you're only looking at about 45,000 BTU of output total- not all of that will go into the building as you'll lose some to the ground both under and around the slab, so a well insulated slab is really important here. With 20ish windows and a ceiling high enough for a 2nd story, I'd be willing to bet your heat load is close to double that on a design day in MO. That would mean either much higher supply water temps and larger delta T, or more tubing to obtain the necessary BTU. More tubing will make for a more even/comfortable heat, as well as lower water temperatures which also means higher boiler efficiency. "Rule of thumb" is typically 12" OC but a proper heat loss calculation is the only way to determine that. Most would prefer 6-9" OC for optimal comfort.

Side note, running off the wood boiler is another concern. Is this contractor also the wood boiler dealer by any chance? Might I ask what make and model of OWB you'll be using as well as what sort of underground lines will be buried between the OWB and building? These systems are kind of my bread and butter, and the vast majority are very poorly installed so I'd like to try and save you a little wood and corrosion if at all possible.

Proper design is ESSENTIAL to a radiant floor. Has a scientific heat loss calculation (like a Manual J) been done? It's the foundation for designing and sizing everything.

24" OC spacing?? We never go more than 18" on a garage! In living areas, never more than 9" OC.

You need to get rid of that hack and get a real radiant pro.

Tell me you're not planning on connecting the radiant floor directly to an unpressurized wood boiler.

Tell me that you ARE using a properly sized heat exchanger between them.

Tell me that you are using O2 barrier heat pex in the floor.

Tell me that you ARE using good quality manufactured manifolds like Caleffi, Rehau, Uponor or the like.

Tell me the the SWT to the floor will be controlled with OutDoor Reset so that the floor won't overheat for hours and hours.

And please tell me that the design was done by a radiant professional, not a carpenter that thinks all you have to is drop tubing in the floor.

I'll probably get some flack for this, but I generally find ODR unfavorable in a system like this. A simple 3 way thermostatic mixing valve does the job very well in most cases. If you're able to get your spacing down to 12", setting the mixer to an even 100 degrees has never done me wrong in our -40F MN winters and tighter spacing would allow an even lower SWT. Again, this will be dependent on a heat loss calculation.

Another helpful graph comparing tube spacing. ideally we would have as little variations possible across a residential slab, which of course comes down to tube spacing.

What happens often with tube that is spaced too wide is you end up cranking the SWT. You may require as high a 130F SWT to get mid 20 btu/ ft output. That would be a very uncomfortable slab to walk on, especially barefoot.

The common range is 80- 82°F floor surface temperature. Somewhere around the average persons skin temperature will be comfortable, if you cover the load.

Hopefully these will convince you and your contractor to rethink 24" spacing. Do some number crunching of your own to determine the path forward.

Start with a room by room load calc

https://www.slantfin.com/products/virtual-heat-loss-calculator/

You only get one chance to put tube is a concrete slab.

Help. I am trying to find a contractor for a job. Uponor hot water radiant heat. Add on room (on slab) needs to be remodeled into 2 rooms, living and sleeping. There is an adjoining space between this room and the main house (will make into powder room) that could hold the boiler. 2 family members could install but I need a plumber to do the match and install the boiler and the manifolds, etc. Is there anyone out there I could talk to about this? House is in Niles,Michigan. Thank you.

I agree in the spacing .. wow ..24" no way. I did 6" on my last one to match the wood floor loops temp.

9" would be my max and 12" for a garage. Warmboard does 12" -- but with AL.