I install gas furnaces every day almost and as far as I am aware you dont need to elevate it. If the flame is less than 18" which I doubt, the manufacturers purposely design them that way so that the flame is above that. If you have a Lennox 90% your good because that is sealed combustion. If it concerns you heed the advice above or do what I do and put it up on bricks and cork pads. If you already have concrete on your floor it is so not worth it to retrofit to radiant floor heat, unless you do concrete for a living and have a buddy that does hydronics for a living.
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