My slab was poured with 1/2" pex loops in 2004 I think it was? Anyways, just getting around to sizing a pump for the floor heat now. Take a guess at my zone lengths. Yup, 1000 ft per zone. Actually a little longer even. Now luckily the longest loop is my shop so I can live with a not so warm floor or use my jackhammer to split it into a pair of 500+ ft loops. The living quarters are different. Now, the first part starts in the bathroom with 6" tube spacing so at least the floor in there will be warm! I don't have good pics of were the tubing layout goes from there other than it is 9" spacing. I can't even guess a hidden spot to jackhammer out to split the zone. I may try raising the supply temp or going euro with a 4 way on a timer to reverse the flow every 30 minutes. How did this happen? Easy, I sized my own loops trusting a table type reference I had printed out without calculating the head loss myself since we were in a real big rush due to winter coming. Turns out the table was very, very wrong. I recommend double and triple checking everything before the pour. If in doubt, shorten your loops. You can always plumb them in serial flow later.