I am currently remodeling my bathroom downstairs (same floor as the water supply entry to the house).

I want to install a Moen U thermostatic valve with (two) 1/2" inputs and (two) 1/2" outputs.

I plan to use these to install a rain shower in the ceiling and a standard showerhead on the wall. with the plan of using both simultaneously.

Question for you experts... my main line into the house is 3/4" that necks up to 1" after the water meter (all copper right now) and for some reason goes down to 1/2" when it hits the ceiling of the utility room.

that half inch feeds everything in the house. 2 bathrooms, kitchen, wet bar, urinal off of my mancave (Don't ask lol), and 2 hose connections. spread across 2 floors.

I want to get enough pressure going to the downstairs shower to supply those 2 shower heads with enough pressure to take a good shower.

If I increase my mainline to 1" (or 3/4"???) pex from where the 1 inch copper currently ends, and run 1/2" cold line to the bathroom (about 15-18 feet, give or take), and 1/2" line to the hot water tank (and 1/2" line from the hot water tank). will that give me the pressure I want?

or am I better off running the 1" (or 3/4") cold supply pipe all the way to the valve, reducing to 1/2" there, and connecting into the moen U valve? and similarly run 1" or 3/4" from cold supply to hot water tank, then 1" or 3/4" from the hot water tank to the hot side of the shower valve?

bathroom is about 75% gutted (everything but the floor and ceiling is out) so now is the time to upsize the supply to the bathroom if that is what it will take.

Thanks for everyone's input!

  • This kind of comes down to an opinion as 1/2 will be adequate for 2 shower heads unless you remove the flow restrictions. Some say use 1/2 so it won’t take as long for the water to heat up, others say use 3/4 so flushing the toilet won’t cause a temp spike of cold or hot. The most effective thing I have found is to have pressure regulators set 15-20 psi below the normal system pressure. pressure regulators make it possible to maintain a constant flow even if 1/2 plumbing is used. I start adding these in the 90’s I believe it is the way to go, (opinion as are 1/2” and 3/4” lines) even with 2. – Ed Beal Jan 7 at 17:03
  • @EdBeal Thank you! So I would ideally place pressure regulators on both the hot and cold lines dedicated to the shower? or on the lines that supply the entire bathroom? – Jay Jan 7 at 19:39
  • The shower is the critical point in my opinion I don’t want a toilet flush to ruin my rain shower experience. – Ed Beal Jan 7 at 19:46

Upsizing from 1/2 to larger pipe will help, but only if the upstream supply supports it. In this case, with a 3/4" main, it is probably worth doing. Your plan of running 1/2" to and from the water heater is no good, go with 3/4" until you reduce to each fixture leg with 1/2".

You shouldn't have to worry about big hot or cold swings in shower temperature, whatever you end up doing, because your fancy new shower valve will most assuredly be equipped with a pressure/temp balancing system "scald guard" feature. That won't prevent overall pressure drop though, so upsizing to 3/4" on all branch mains, then 1/2" to each fixture leg will be a good thing.

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Hi Jay,

Going to 3/4 pipe will help yes.

What you need to be carefull is :

  • Avoid to install un-necessary 90 elboy or splitter or connector.

Always try to run your Pex if you will use Plastic Pex, with curve, No Bend (Never).

  • If you can run your line until your mixer (shower selector) without any fittings, it's the best situation to maximize the preasure.
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