PEX up & over 2nd floor attic?

[Nathan Sweaney]

Short version:
Is it okay to run PEX up and over the 2nd floor attic to a 1st floor bathroom?

Long version:
I've got a 2-story house, 3400 sq ft, with all the copper pipes in the slab. The master bath and guest 1/2 bath is on the far side of the house from the water main & water heater. Above it is a 2nd floor full bath. For a remodel, I need to rerun the water lines to these three bathrooms, but there's no easy way to get there without going up and over the 2nd floor in the attic. Will going up two floors and then back down reduce the pressure too much? It's roughly 75 ft across the house, plus ~18 ft up & 18 ft down.

Bonus question: The current copper lines are all 1/2". Should I run 3/4" PEX instead?

 

[Jadnashua]

Pressure due to elevation change would null out between the up and down. It's a gravity thing...

1/2" PEX is (barely) enough volume to handle one bathroom, so, you'd be better off with 3/4" or maybe even 1" run, then reduce for each bathroom as needed. Copper in a slab can last a long time, but it's expensive to path and a major pain to modify.

Dynamic pressure on any piping system is dependent on the volume, the tubing size, and the effective length. Effective length is a combination of the linear distance covered along with an add-on for any fittings (primarily, changes of direction). Each fitting will add extra feet to the effective length. The fittings on PEX are smaller than the smaller ID of copper, where the wall is thinner and the fittings are essentially full diameter. For PEXx, the fitting must fit inside the pipe. PEX-A with expansion fittings will have less dynamic pressure losses because of the fittings being larger ID than for the compression ring on the other types. On PEX-A, the fitting won't go into the tubing until you've expanded it with a tool, meaning the ID can be larger at the same thickness than the other types where it must slide in without expanding the tubing.

PEX is UV sensitive, so any vents, etc., in the attic that might let some in, you'd need to cover the PEX to protect it long-term. Plus, like any water pipe, if not insulated, and it gets cold, it could freeze, so ideally, you'd have it close to the ceiling with insulation over it. Note, insulation doesn't 'make' heat, but traps it, but that also means if there's a lot of insulation between the ceiling and the heated room below, not that much heat will get to it.

So, yes, you can run water pipe in the attic, but you still need to protect it from freezing and in the case of PEX, from UV as well. PEX may not split if frozen, but that assumes you don't have any fittings up there...those being rigid, are likely to split if frozen. PEX is best used when you can minimize the number of fittings used and that will maximize the flow and minimize the dynamic pressure loss.

While you're running pipe up there, consider running a hot water recirculation return line. At 75' in the attic, even below insulation, in the winter, it will take a long time to get that line warm the first use or after any delays between them. That could easily be a 1/2" line, and even a 3/8" one might work.

 

[Sponsor]

 

[Nathan Sweaney]

Thanks for the feedback. I hadn't considered a recirculation line. That's not a bad idea.