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PEX plumbing rough in help / to use or not to use home run / trunk-branch / remote manifold

stitzcrew's picture

I have a few quesitons on a new build using Pex. 

Is there a reason to run an HR system for cold lines if there is no concern on waiting for hot water?  My thoughts are that a trunk and branch system seems the most practical. 

I planned on using an HR system for hot supply using 1/2" for most areas.  One area that I am concerned with is the master bath that will require roughly 50' of piping runs to most fixtures.  From reading it seems that a 1/2" pex line will hold around .46 gallons at 50'.  This seems that it would still take a decent amount of time to clear the cold water.  Although this is not as wastefull as a 3/4" trunk line it is more of a concern on wait time.

Here is what I thought as a solution to this problem of waiting on hot water.  I was looking at having a 2nd 10-20 gallon electric water heater in my closet (not wifes) adjacent to the shower.  I would have it drained under the bench seat in shower (out of sight).  If I run a 3/4" hot supply into the cold end of WH the room temp water in the line is mixed into the 2nd WH which is set slightly hotter than main to adjust for room temp water.  The hot end of WH would then run to a manifold where 1/2" pex lines are run to each fixture in master bath. This would cut the runs to ~6'-15' compared to 50'.  The 2nd WH would not use as much electricity b/c it is always fed by a hot line (room temp at worse).  I feel this is a better solution than a re-circ system b/c the master bath is the only place where hot water use is most important.  

The rest of the homes hot water supply would be run on 1/2" HR where lengths are considerbly less than master bath.  I was thinking of a 66 gal main WH with a 20 gal 2nd giving plenty of hot water for us and guests. 

So it seems that my thoughts are using Trunk and Branch for cold supply, HR for most hot supply and remote manifold (off 2nd WH for master bath).  

Hopefully some of you can make sense of this and provide some input in a kind manner.  I attached picture if that helps of master bath.  Thanks 



The system you describe has (post #207122, reply #1 of 12)

The system you describe has been discussed numerous times here.  There's no real problem with it, and it's probably (slightly) fallen out of favor only because the Taco undersink recirc pumps are simpler to deal with.  (Plus of course the heater takes up space.)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Recirc pump (post #207122, reply #2 of 12)

So if I ran a remote manifold for both cold and hot supplies to the master bath, would I then put the recirc pump between hot and cold at that point to be able to service all fixtures in bath?  It seems that you can get pumps that run by a temp setting or possibly by timed intervals. 

How do you avoid having water that is above the recirc pump turn on temp (not sure what temp so pump doesnt run all the time) and below the temp needed to create ideal shower temp ~105.

Although 90 deg water is better than 70, its still not 105.  There may be a simple answer and a post out there asking the same thing.  Feel free to send any links that are helpful. 

My thought on the smaller WH in closet was to insure water hot enough for almost instant shower temp and add an extra amount of hot water that guests do not have access to. 

I do like the idea of a recirc pump for space and reliability (one less WH to worry about breaking). 

Thanks for your input

None of the schemes is (post #207122, reply #3 of 12)

None of the schemes is perfect.  With the recirc pump scheme you can have a pushbutton that you press on entering the room, to start the pump.  It stops automatically when the thermostat says so.  You don't quite get instant hot water, but pretty quick, without running water down the drain, and with relatively little fuss.  You can add a timer to trigger it for your usual morning shower time, etc.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Regardless of which re-heat (post #207122, reply #4 of 12)

Regardless of which re-heat method you choose, it will be a good idea to use 3/8" pex for the hot supplies to the vanity sinks. The wasted water from 3/8" is nearly half of what 1/2" loses, and a 3/8" line will deliver far more water than what the water-conserving faucet can use.

Even showers can be fed with 3/8" because of the low-flow heads that have become standard. Tub fillers need 1/2", however.

By getting hot water from (post #207122, reply #5 of 12)

By getting hot water from that small wh near the shower you will often get a slug of not-so-hot water when the cold water that's been sitting in the line finally reaches the shower head. Small water heaters cannot heat quickly enough to compensate. Sure, it will be mixed a bit with hot water but still colder than you will want to feel. 

In any case, I'd also recommend only one feed line from the manifold to the vanity sinks. You can branch at the first sink to go onto the second and save some pipe and hot water. 

hot water into hot water? (post #207122, reply #6 of 12)

Why would his system not provide near instant hot waer and some 90 gallons of it at that?  The 20 gallons in the close heater is hot and only six feet away.  Six feet of water is out of the line before you can turn the water on and step in the shower.  And the 20 gallon heat will have no problem keeping up as you suggest because ALL the water going into the 20 gallon tank is from teh 66 gallon HOT Water tank.  The 60 feet of line (presumed cold) between the two tanks would be MASSIVELY diluted once it is added to the twenty gallon tank.

I agree with other posters that it is a shame teh water in the 20 gallon tank is never available to other guests and perhaps worse the master uses water from teh 66 tank taking it away from guests.  It would be nice to know if you could take a shower from teh 20 gallons alone if it were just plain plumb with cold water.  Probably not for most people.  But a dedicated tanik for the master does start to make sense.  What about a 30 gallon tank?


Personally I really dislike (post #207122, reply #7 of 12)

Personally I really dislike having any spike in the water temp as I'm showering so i'd go with a large number of home runs even for the cold.  It's overkill perhaps, but pex is dirt cheap....and I'd go with 1/2" over 3/8" unless it's a toilet - 1/2" just works fine.

Recirce pump is the way to go and just plumb in a return line and keep the pump by the hot water heater so you don't have to hear it under the sink.  No pump is perfectly quiet.  Single gang timers cost less than $20 or you can pay $50 for one to attach directly to your pump. 

While you're at it I'd run pex in the floor to warm the bathroom floor a bit - if you have a circ pump and return line you're halfway to what's needed for a simple floor warmer - you'll just need a few more gizmos and valves to control things.

If I could edit my location it would say I'm now in Reno :-)

Thanks for the replies.  I (post #207122, reply #8 of 12)

Thanks for the replies.  I have some time before the build and just trying to narrow down plumbing options before its too late.  We do not have NG available so its most likely electric.  Was only going to run a smaller propane tank for cooktop.  I was also thinking that the WH in master would have to be drained differently due to possiblity of scalding feet in the shower (not likely but im sure there is some code against it).  I get the change in temp which im not a huge fan of.  I was thinking if the master WH was set at a higher temp then the main that the room temp water entering would dilute it down to a temp closer to the main.  There may be a point that the mixing valve might need to be bumped up once that diluted mix hits the shower head (would think this is minimal).  I was also looking in at having a Thermstatic Mixing Valve to adjust for these temp changes and avoid scalding. 

I know there no solution that has all positives but I am looking for something better than what I have now.  Our bath now is only 20 feet away and it takes forever to get hot water.  I don't mind waiting for the shower although a waste of water.  The thing that is most annoying is waiting on the faucet to wash hands.  Hopefully soap is good enough for most germs b/c most of the time the water is cold. 

My new thought on this was to run a remote manifold to the vanity wall and have a pump and hot return line for recirculation.  Im not sure the best way to run the pump whether timer, thermistat, or manual switch.  I am guessing that a 40-50 ft run of 3/4" pex would take some time to clear out so a manual switch might be limited for a quick wash of hands.  I do think either way that 3/8" for vanity makes sense b/c I won't need a large quantity of hot water. 

I was thinking of running one line to the shower wall and into a therm mixing valve and then branching from there to shower head, body sprays and rain head.  each of these would have a shutoff valve just past the mixing valve so there is only need for 1 mixing valve.  I am guessing I would need a 3/4" line into the TMV to ensure enough water is available to run more than one shower fixture at decent pressure. 

So the main reason to home run the cold lines is to keep equal pressure?  If I was running a remote manifold system for hot in the master should I run the cold the same way or just HR it?  

I attached a picture of my thoughts on the shower       

plumbing_pic.jpg60.05 KB

Suggestion:  Install the (post #207122, reply #9 of 12)


Install the remote manifold as in your original post.

Instead of a small WH, install a Taco-type recirc pump under the vanity.

The Taco (or Grundfos, etc.) can be on a timer to match your schedule and habits, and it will give you instant hot at the vanity while also keeping the manifold hot, thereby making hot water quickly available to the short branch lines that feed the other fixtures.

You won't need a third line this way, because the Taco doesn't use one.

Insulate the hot line from the vanity to the manifold and all the way back to the WH.

Another option I was thinking (post #207122, reply #10 of 12)

Another option I was thinking about.  I like the idea of a taco pump or any recirc pump.  I was thinking of scrapping the WH in closet and going with a smaller undersink WH 4-6 gallons.  This could be fed by 1 seperate cold line.  I wouldn't think I would use enough hot water washing hands to clear the hot water in tank.  With that I would run larger hot line(3/4") to a remote manifold in the shower wall with a recirc pump on a switch that can be manually operated when needed.  I figured that hot water needed for a bath or a shower would be more routine than washing hands.  If the switched was turned on the pump could pobably clear the cold water line in 10 secs or less.  So by the time you get your clothes off you would have hot water and there is no need for the pump to run throughout the day.  With this plan there is no need to run a line from a closet or the vanity to feed the shower/tub area.  I was thinking of having the best of both worlds with a recirc pump and point of use WH at vanity.  Also the vanity need for hw would not cause cold water to enter into the main WH. 

My only thought on a seperate return line was to keep hot/warmer water out of the cold line to minimize temp changes for the mixing valve in shower.  Either way any of these options will be a vast improvement. 

Is this really a problem? (post #207122, reply #11 of 12)

Is cold water in a shower really a problem?  Don't anyone get defensive.  I really don't know.  For me I turn the water on in the shower finish disrobing, check teh temperature, adjust it and step it.  A 1/4 gal or 1/2 gal of cold water in teh hot line is gone before you disrob.  Now, when washing your hands, you turn on the water and wish to start wasing immediately so you have to wait.

So if you wish to have an instant hot water or 4 gallon unit for the sink, sounds good, but is the shower a problem?  To each there own.  Perhaps my showers ahve never had a 50 foot run.

As for home runs, why?  It seems to me a manifold at the bth makes sense.  With this set up, if you run the shower, you have primed the line for the sink with hot water adn you are good to go for 30 minutes or so and or visa vera.  You get the benefit of hot water in teh line.  With a home run each appliance has the problme wiht cold water.  What am I missing?

And if you run home runs, how do circulating pumps work?  Would you not need a pump for each line?  With a manifold you would need one pump for the line to the manifold.

Thanks all for the education.


I think I am with you (post #207122, reply #12 of 12)

I think I am with you doright.  For me hot water is really only an annoyance for washing my hands at vanity and kitchen sink.  The shower clears water much faster and is not needed as much so wasting water should be lower than waiting to wash hands with hot water.  If its a 3/4" line to manifold it will hold just under a gallon and would probably take 20-30 seconds to clear so not life altering. 

Now I need to figure out what size line to run to my shower if I with have body sprays, shower head, and rain head.  My thought was to run 3/4" to the mixing valve and split on the other side with 1/2" to each. If I only ran 1/2" its my understanding that pressure would be reduced if more than 1 was running at a time.  I don't plan on running everything to be wasteful but a combination of two here and there would be nice. 

From my readings recirc pumps and homerun systems don't really work unless you want to constantly circulate cooled water back into the WH.  I guess there are advantages/disadvantages to each of the three ways to run piping but I am leading toward remote manifold for long runs.  If i get close to instant hw at any sink I am a happy person b/c hopefully we wash our hands more times than we shower ;)


Thanks for the great posts in a friendly manner