"Jim Rainfordson" wrote in message
The unit is housed with 3/8" FPT (plastic) holding a 3/8MPT to 1/4
compression male (plastic). Do I have to step it down to 1/4 or can I
use the 3/8" FPT? I'm wondering if putting too much pressure on the
unit (by not bottlenecking it to 1/4"C) will cause it to function
improperly. I don't care about the pressure in this regard. I'm
getting the increase by using two units instead of the usual one.
However, I do want to use less fittings.

Larger is better. l It won't increase pressure, but it won't restrict flow.

What about exiting the units? If 1/4 C is the intake then that's the
bottleneck, so at that point it shouldn't matter if I use 1/4"C or
3/8" FPT, right?

Right



The faucet I have in mind is a single handle with standard 1/2 MPT.

Anyone know if the units themselves automatically act as a back flow
preventer? If not I'd be worried unequal pressure might blow water
back and forth from filtered to unfiltered water, not that I can see
any terrible consequences.

No, they do not, but I don't see that it would blow water back at any point
either. Where is the unequal pressure coming from? The hot water when
running a blend? Not a problem.