Navien water feed size

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Faderus

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Hi all,

I am starting a plumbing cleanup/pex replacement in my 1960 raised ranch which was triggered by a hole in my tile tub surround. I ordered an Uponor expansion tool, and and was trying to decide what sizes/lengths of pex and fittings I would need.

A couple of years ago I had a natural gas Navien Combi unit installed to provide hot water and baseboard heat, that acts as a supplement to my high efficiency heatpump. I live in NY.

When I was chasing pipes to see what sizes went where I noticed that the cold water feed to the Navien is mostly 3/4" pex but runs through a section of 1/2" copper and the rest is a tangle of pipes from 60 years of equipment replacements which I intend to greatly simplify and cleanup with the pex. The only functional complaint I have with this system as it works right now is that when the bath filler is opened up wide the hot water temp drops. It's an easy work around to throttle it back a bit, but could this section of 1/2" be the cause?

As an aside, for a standard 1 bathroom raised ranch with aall the pipes in a single wall would a manifold (3/4" in, 1/2" out) or 3/4" trunk and 1/2" branch be recommended?

Is there any benefit to running 3/4" to the shower/bath valve?

Based on the recommendation above, how much 1/2" should I order? would 100' each red and blue 1/2" be sufficient, or should I just buy 300' of white?

Is there any place to order short lengths of 3/4" uponor pex?

Thanks for any help you can offer.
 

Jadnashua

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A tankless system heats the water as it flow by. IF the pipe was larger, it would try to flow through the heater faster, meaning it would gain less heat. So, a larger inlet pipe would still require you to throttle the flow into the tub to keep the temperature up. Now, if the tankless hasn't been serviced regularly by periodic deliming, the heat exchanger may not be as efficient as it could be.

A tankless in the winter of NYS probably is a one bath at a time thing. Might support two showers in the summer, maybe marginally in the winter, but a tub filler might use as much as 3x as much as a shower, so you're just heat deprived.

Feeding it with 1/2" pex means that's going to be a throttle for the entire hot water system. I'd use at least 3/4" pex...individual runs after it could be 1/2" if you ran it to a manifold and ran direct. A large capacity tub might dictate keeping that 3/4" pex all the way to it. Assuming you're using a 1/2" supply valve, that would let it max out with whatever heated water you had...1/2" pex would be limiting on volume. So, in the summer with warmer incoming water, you'd be able to get maximum fill rate in the tub. You'd still havre to throttle it back in the winter to let it absorb more heat along the way. FWIW, on pex, they recommend a maximum flow rate of 8fps, which on 1/2" pex is all of 5gpm. Going to 3/4" doubles the volume.
 

Dana

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What jadnashua said- your tub filling rate is burner-limited, not plumbing limited, something you just have to live with. It's part of the package deal that comes with a tankless water heater in cold-water country.
 
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