Rough in DWV/Pressure Design questions

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JoeJee

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Hello,

I hope someone is able to assist and give advice. I am trying to get a plumber out to quote or even sell me design specs but it is like pulling teeth. I didn't want to run the plumbing but it is looking more and more like I might have to.

I am trying to move forward on my second floor/roof. The ides is to run all plumbing north (top) out of the house and then into a 3" along building to meet up with the 3" that is on the east side. My existing roof is a concrete slab with 3-4" xps foam. I will attempt to keep the foam and add dirt/rock as needed. We will pour a slab on top of this so I need to know if possible:
1) how much space do I need between the two slabs for the DWV traps and fall?
2)Should I attempt to combine under the new slab or just run each individual line out?
3)How far away from each fixture would I need a vent?
4)Can I combine vents in attic?
5)How big should floor drains be for the Washer, Dishwasher, utility room (Water softener, Filters, HVAC drains). These will probably go to daylight unless there is a large objection.

--Water in and Hot water heater will be behind 1/2 Bath and Kitchen. For each area (#6,#7,#8) I will have an accessible manifold in the wall to junction off to each fixture.
6)Water will be PEX-A. 1" cold for 1/2 bath and Kitchen.
7)1"cold and Hot for Laundry, main bath.
8)1" cold and Hot for Master Bath.
9) 3/4 hot return from each area for quicker hot water.

Any help/advice/ is MUCH appreciated.
 

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wwhitney

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Just to back up a moment, you have an existing single story house with a concrete slab roof? And you plan to build a second story on top of it by pouring a new concrete slab elevated above the slab currently functioning as the roof? And you are going to elevate the new slab by adding soil or crushed gravel fill on top of the foam on top of the existing slab?

That sounds very unusual. Do you have an engineer advising you on the details of how to do that?

Some brief answers:

1) Enough for the height of the trap along with 1/4" per foot fall on the horizontal drains.
3) Each trap needs a vent connection (wet or dry) before the fixture drain has fallen one pipe diameter.
4) Sure

Cheers, Wayne
 

JoeJee

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Just to back up a moment, you have an existing single story house with a concrete slab roof? And you plan to build a second story on top of it by pouring a new concrete slab elevated above the slab currently functioning as the roof? And you are going to elevate the new slab by adding soil or crushed gravel fill on top of the foam on top of the existing slab?

That sounds very unusual. Do you have an engineer advising you on the details of how to do that?

Some brief answers:

1) Enough for the height of the trap along with 1/4" per foot fall on the horizontal drains.
3) Each trap needs a vent connection (wet or dry) before the fixture drain has fallen one pipe diameter.
4) Sure

Cheers, Wayne
Hello,

Thank you for the reply. You are correct in your assessment of my building and and plan. It is VERY unusual I must agree. I do have an engineer advising. They will be doing a post tension slab to help evenly distribute the weight. Otherwise I don't have more details yet on how they are doing it. We thought of using wood to bring the floor up but very expensive and really no positives other than easier to access the plumbing in the future.

If I am unable to get a plumber out to bid would someone be willing to work with me on a layout if I put in the fixtures on the floorplan? I don't know the proper fittings or the proper distance required from things.

Never in my life heard of this crazy sandwich on an existing slab.
I can take some pictures as it is happening and post. I have cored through OLD industrial buildings that had slab ontop of slab ontop of slab but this isn't really the same thing. If you think about it though, if you use SB2 or dirty base, then foam, then concrete you are doing similar. Big difference in my setup is I have concrete as my base instead of fine gravel.
 

JoeJee

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Have a plumber scheduled for Thursday so hopefully he is able to work with me but not sure if he has time.

Question about a hot water loop: (I will have laundry moved to garage so Kitchen, Laundry, and 1/2 bath will not be on hot water loop)

- Will a 1” PEX-a supply be gig enough for a regular full bath and then to 1 master bath? It calculated up to 18 fixture units. Should I worry about future add in’s for custom showers?

-If 1” PEX-a is large enough, would a 3/4 PEX-a return be adequate?
 

John Gayewski

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Have a plumber scheduled for Thursday so hopefully he is able to work with me but not sure if he has time.

Question about a hot water loop: (I will have laundry moved to garage so Kitchen, Laundry, and 1/2 bath will not be on hot water loop)

- Will a 1” PEX-a supply be gig enough for a regular full bath and then to 1 master bath? It calculated up to 18 fixture units. Should I worry about future add in’s for custom showers?

-If 1” PEX-a is large enough, would a 3/4 PEX-a return be adequate?
Yes and yes
 

JoeJee

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We won’t be able to move the laundry near the garage. That will put hot water at 20 fixture units not including the kitchen. Not one plumber I have talked to are able to use 1-1/4 Pex-a. Can’t get a rental for one either. Should I just leave the laundry off of the hot water loop? Also, can the kitchen not be on a hot water loop or can it be on it’s own loop? Hot water heater is directly behind it.
 

JoeJee

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Looking at ordering Pex pipe next few days. Anyone able to help me on sizing? Are these hot water trunks and hot return sized correctly:

— 1” hot to master (will have up to 3 shower heads, 1 jet tub, and 2 sinks) then 3/4 to clothes washer, then 3/4 hot return
— 1” to kitchen sink/dishwasher/island, then 1” to hall bath (1 tub/shower, 2 sinks) then 3/4 to 1/2 Bath, then 3/4 return?

For cold line, is it better for home run even on the longer runs for master bath or should I do the same setup as the hot (trunk and branch)?
 

Jeff H Young

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2 1 inch lines from water heater one going to kichen one going to master and 2 3/4 returns to water heater way over kill and unusual its just a 2 bath house ? Then your talking about 1 1/4 " most of these cheaper areas no one runs bigger than 3/4 but thats small . but back east and in the south they hardly use 1 inch i guess its because the homes are so competetively priced they short cut everything
 

JoeJee

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2 1 inch lines from water heater one going to kichen one going to master and 2 3/4 returns to water heater way over kill and unusual its just a 2 bath house ? Then your talking about 1 1/4 " most of these cheaper areas no one runs bigger than 3/4 but thats small . but back east and in the south they hardly use 1 inch i guess its because the homes are so competetively priced they short cut everything

Might be overkill but I am going by the fixture count and it adds up to 1” copper/1-1/4 Pex with only 1 hot trunk unless I break things off. I could be adding wrong as well. I have been in a lot of houses with 3/4 copper (equivalent to 1” Pex) and water pressure drops when several people are using stuff. Gets worse if the distribution system isn’t great. I might be over thinking the pressure system and that is why I am here looking for advice; you guys have a ton of knowledge and I only get one chance (post tension slab).
 

wwhitney

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Other than cost of the pipes and bigger holes in the framing, there's no downside to oversizing your cold water pipes.

With a recirculating system, the only additional downside to oversizing the hot water pipes is the additional heat loss (greater surface area), which you could make up for with thicker insulation (all hot water pipes need to be insulated on a recirculating system).

Without a recirculating system, you want to right size the hot water pipes to minimize wait time for hot water. For a branch serving a bathroom with just a lavatory and a single showerhead shower, that would be a 1/2" PEX line. Not sure about other cases.

Cheers, Wayne
 

Jeff H Young

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Im thinking a 1 inch hot is adequate if you have 2 branches going out and 2 coming back to circulate youll need to ballane them some how usualy you want a single loop not 2 loops. Running copper I only bring 1/2 inch back to heater usually dont know if running 3/4 has advantage or the pro and con of that
 

John Gayewski

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Pipe segments are sized by how many fixture units are supplied by each segment. Your first segment should be the largest. After some fixtures are served the fixture count goes down then the pipe size goes down. Draw a schematic. Label how many fixture units are served at each tee and size it accordingly. It's easiest to start at the farthest fixture with the longest distance.
 
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