Tubing choice for running discharge across dropped ceiling

[ajs123]

I'm installing a 1 cf Fleck 5800 system and the only place to run the discharge is 70 feet or so away (existing standpipe from a washer hookup that's no longer there). About 20 ft. of the run will be behind a dropped ceiling and 12 ft fished through a joist space above a portion of the basement that was finished with drywall. I'm running 3/4 PEX along most of the same path to get un-softened cold to the kitchen as well as an outdoor bib.

Questions:
(1) What kind of tubing, and what size, should I use for the discharge? Advice from a reputable plumbing supply show was that 1/2" vinyl is fine but the manual from the vendor says to use solid-wall tubing that can't get pinched, and the full fleck manual says 3/4" if going over 20' or over 7 gpm. But this unit includes an orifice to limit drain flow rate to 2.0 gpm. Pressure drop in a 20' 1/2" pipe at 7 gpm (OK per the manual) is around 10 psi. The drop at 80' and 2 gpm is less than 5 psi. As i understand it, these units use rotary valving so flow doesn't abruptly start or stop, so length per se shouldn't matter. Do I have it right?

(2) I'll support the PEX at every other joist where it's accessible. But in the 12' run behind drywall do I need to get in there and install hangers?

Thanks!

 

[Jeff H Young]

I like cpvc for this I think it looks cleaner than pex I thought 1/2 inch but wouldnt hurt to go 3/4 sounds like you are maxed out for 1./2,

 

[Sponsor]

 

[ajs123]

I like cpvc for this I think it looks cleaner than pex I thought 1/2 inch but wouldnt hurt to go 3/4 sounds like you are maxed out for 1./2,

I ended up running 1/2 inch CPVC (before seeing your answer). I'm comfortable with the 1/2 inch. If needed, I can measure the backwash flow and adjust the time. I ran it down several inches into the standpipe, through a rubber airgap cap, with a union just above so it can be removed. It does look nice and clean (in parallel with the unsoftened cold line that I had to run!).