What is your planned floor covering? What works or is "OK" with carpet will never work with porcelain tile.

The layers of subfloor and the total thickness will also dictate the deflection rating. If code is "like 17 inches" then why are they recommending 24"?

For extra stiffness from the other forums I read they recommended 22", and the local builders said 24" is what they recommend and one opposed it. the flooring is lvt tile with radiant heat.

The install "sounds" light enough (as in about as heavy as a decent carpet). Just for giggles, please cross reference the LVT installation requirements regarding ELECTRIC in-floor radiant heat. With your description, I'm assuming this is NOT hydronic heating tubes but the less expensive and more limiting electric heating wires/mats.

The flooring manufacturer is the one who specifies use with electric in-floor heating systems. Vinyl doesn't like heat - at all. Which means it is a very special vinyl product that will allow for this style of heating. You might want to get your application approved in writing by the flooring manufacturer before continuing. You are in the design phase so this is the PERFECT time to get this in writing.

There are MANY vinyl products that require the heating "elements" (either wires or water tubes) to be "ENCASED". That means "in cement" or "in the subfloor" (as in heating tubes laid in or under the plywood subfloor. This type of assembly adds HUGE amounts of weight/stress onto a a build. When you add weight, you NARROW the gap between joists.

I HIGHLY recommend you ask a few more questions about HOW this is going to work and if it is allowed by the LVT manufacturer.

It's hydro radiant heat. The installer highly recommend the floors with garentees and warrentees. I'm just curious if 28' span open web floor truss at 24" is solid

Contact Truss MFG it should be stamped on the truss somewhere Inquire about Load/span calculations. Theres a lot more to youre question that cannot be answered with Information supplied. -best