Has anyone ever played w/ running pex a tad narrower (and narrower) on return lines? It just seems a logical conclusion as the water is cooler on return lines. Is this possibly significant, or just splitting frog hairs?

I've never actually split a frog hair but..... As the surface around the upstream pipe becomes warmer the temp in your return lines does too and applying path of least reistance thinking, return pipe offers more btu's (to surrounding surfaces) at the end of the cycle than supply.
J

lol do frogs even have hair?

Joe: Something does not compute here, and maybe I am misreading. The only way for the return temp to be higher than the supply temp is for the concrete to give it some energy, no? If the concrete is warmer than the supply temp, then you surely are not heating concrete w/ that water. Did I miss something? Jimmy: OUR frogs have hair. How do you think they get through winter up here? I've seen frogs give a big ol' bellow at 55 F below and their puffed out "crop" gets flash frozen, solid. Poor buggers have to deal w/ that till spring, but they never do it a second time, smart little devils that they are.

LOL on the frogs.

I might not be doing this the right way but what I did was run two 250 foot runs per zone, then I ran hot in one end and where that one returned I ran hot in on the opposite pipe. I figured somewhere in the middle they would be close to the same temp.

Having said that I don’t think it really matters since my return temps are about 2F lower then the outgoing temps anyway.

Brock: Surprisingly, we really do have frogs here. They are about 1.5" long; fairly rare, too. On your pipe temps, if none of the pros chime in here, you may want to investigate what is "wrong". Somewhere I think I read that you should have about a 10 degree drop in water temp. If you don't, you are not heating efficiently; perhaps pushing the water too fast, loops too short, something like that. I dunno, but that seems to be what's in my memory banks.

Didn't suggest return became warmer than supply,simply put (path of least resistance), you first discharge most heat upstream, then as that surface becomes warmer more heat is discharged downstream. The "water" is moving at a constant rate, but heat moves more quickly to areas with less heat so cooler areas (downstream) eventually get the lion's share.
j

Yes, in our situation I am circulating 90F water and I am most likely moving it to fast, I sized the pump for 10 250-foot runs but usually only have 5 on.