# 1/2" vs 3/4" hot water line



## Tinstaafl (Jan 6, 2008)

Leaving aside the overall volume of flow, at what point is it better in terms of hot water NOW at the faucet to use a 1/2" line to take advantage of the higher velocity of flow? Does that question make any sense? :blink:


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## jb4211 (Jul 13, 2010)

I'd think "higher velocity" does not equate to more GPM's. And I'd also think that it's the GPM's that you need to increase to get the hot water NOW, not the pressure. Meaning you have to evacuate the cold, idol water before the hot water will flow.

Does that answer or am I confused which is usually the case.


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## Tinstaafl (Jan 6, 2008)

jb4211 said:


> Does that answer or am I confused which is usually the case.


I think you're confused. :laughing:

I'm specifically ignoring GPM for this. At a given pressure, water will move faster in a smaller pipe. I think. At least that's my intuitive take on it.

Even if it doesn't, it would seem to make sense that if the pipe holds a gallon of cold water, the faucet flow rate (oops, there's your GPM) would keep the hot water from getting there faster than with a pipe containing only a quart of cold water.


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## TimelessQuality (Sep 23, 2007)

A 3/4 line will 'hold' more water that will get cold when not in use... When you want hot, you'll have to evacuate all that water before the hot will reach your fixture... 1/2 will be less...

Is that what you mean??


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## Cole82 (Nov 22, 2008)

Tin this link covers manifold design benefits over traditional systems. 

http://www.huduser.org/portal/publications/pex_design_guide.pdf

Flow Rate ft/sec 
GPM 3/8” 1/2” 5/8” 3/4” 1” 1 1/4” 1 1/2” 2” 
0.2 0.67 0.36 0.25 0.18 0.11 0.07 0.05 0.03 
0.3 1.00 0.54 0.37 0.27 0.16 0.11 0.08 0.05 
0.4 1.33 0.72 0.50 0.36 0.22 0.15 0.11 0.06 
0.5 1.67 0.91 0.62 0.45 0.27 0.18 0.13 0.08 
0.6 2.00 1.09 0.74 0.54 0.33 0.22 0.16 0.09 
0.7 2.33 1.27 0.87 0.64 0.38 0.26 0.18 0.11 
0.8 2.67 1.45 0.99 0.73 0.44 0.29 0.21 0.12 
0.9 3.00 1.63 1.12 0.82 0.49 0.33 0.24 0.14 
1.0 3.33 1.81 1.24 0.91 0.55 0.37 0.26 0.15 
1.1 3.67 1.99 1.36 1.00 0.60 0.40 0.29 0.17 
1.2 4.00 2.17 1.49 1.09 0.66 0.44 0.32 0.18 
1.3 4.34 2.35 1.61 1.18 0.71 0.48 0.34 0.20 
1.4 4.67 2.53 1.74 1.27 0.76 0.51 0.37 0.22 
1.5 5.00 2.72 1.86 1.36 0.82 0.55 0.40 0.23 
1.6 5.34 2.90 1.98 1.45 0.87 0.59 0.42 0.25 
1.7 5.67 3.08 2.11 1.54 0.93 0.62 0.45 0.26 
1.8 6.00 3.26 2.23 1.63 0.98 0.66 0.47 0.28 
1.9 6.34 3.44 2.36 1.72 1.04 0.70 0.50 0.29 
2.0 6.67 3.62 2.48 1.81 1.09 0.73 0.53 0.31 
2.5 8.34 4.53 3.10 2.27 1.36 0.92 0.66 0.38 
3.0 10.00 5.43 3.72 2.72 1.64 1.10 0.79 0.46 
3.5 11.67 6.34 4.34 3.18 1.91 1.28 0.92 0.54 
4.0 7.24 4.96 3.63 2.18 1.47 1.05 0.62 
4.5 8.15 5.58 4.08 2.46 1.65 1.19 0.69 
5.0 9.05 6.20 4.54 2.73 1.84 1.32 0.77 
6.0 10.86 7.44 5.44 3.28 2.20 1.58 0.92 
7.0 8.68 6.35 3.82 2.57 1.84 1.08 
8.0 9.92 7.26 4.37 2.94 2.11 1.23 
9.0 11.16 8.17 4.91 3.30 2.37 1.39


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## KillerToiletSpider (May 27, 2007)

Every house should be plumbed with 3/8" water lines, who cares that you can only use one fixture at a time when you save almost a second and a half on getting hot water to the fixture.


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## Inner10 (Mar 12, 2009)

> Every house should be plumbed with 3/8" water lines, who cares that you can only use one fixture at a time when you save almost a second and a half on getting hot water to the fixture.


What hackery, I demand 1/4" or less!


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## Cole82 (Nov 22, 2008)

KillerToiletSpider said:


> Every house should be plumbed with 3/8" water lines, who cares that you can only use one fixture at a time when you save almost a second and a half on getting hot water to the fixture.


For some reason this post made me actually LOL :laughing:


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## griz (Nov 26, 2009)

KillerToiletSpider said:


> Every house should be plumbed with 3/8" water lines, who cares that you can only use one fixture at a time when you save almost a second and a half on getting hot water to the fixture.


Ah hell, use 1/4" line, be green use less water...:whistling

Plumb it all in at* least* 3/4". One can always slow down the flow, but you'll go nuts trying to get a 1/2 line to run two showers and someone else turns on water some where...If you're that concerned about how fast it gets hot put a circ loop in it.


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## Tinstaafl (Jan 6, 2008)

KillerToiletSpider said:


> Every house should be plumbed with 3/8" water lines, who cares that you can only use one fixture at a time when you save almost a second and a half on getting hot water to the fixture.


:laughing:

Cruel as your sarcasm can be, that's exactly what I was looking for, without going nuts looking up tables and doing math. The question was actually prompted by a buddy's run-in with an inspector.

Would it truly be only a second or two's difference in, say, a 100' run?


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## cleveman (Dec 28, 2007)

I once ran a 3/4 line to a kitchen sink. It did serve a dishwasher as well. What a mistake.

There is some logic, oddly enough, to 3/8" lines. The faucets are all restricted to 1/4".

Plumbing codes are another matter.


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## MALCO.New.York (Feb 27, 2008)

End Orifice!


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## KillerToiletSpider (May 27, 2007)

Tinstaafl said:


> :laughing:
> 
> Cruel as your sarcasm can be, that's exactly what I was looking for, without going nuts looking up tables and doing math. The question was actually prompted by a buddy's run-in with an inspector.
> 
> Would it truly be only a second or two's difference in, say, a 100' run?


It wouldn't be more than ten seconds in a 100' run.


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## thom (Nov 3, 2006)

one of the pex systems promotes a manifold system where each fixture gets an independent pair of 3/8 lines. The point is to waste less water and time while waiting for hot AND provide adequate volume when using multiple fixtures at one time.


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## KillerToiletSpider (May 27, 2007)

thom said:


> one of the pex systems promotes a manifold system where each fixture gets an independent pair of 3/8 lines. The point is to waste less water and time while waiting for hot AND provide adequate volume when using multiple fixtures at one time.


And on the plus side for the manufacturer, they sell an extra thousand feet of pipe for every manifold system that gets installed, even though the whole idea is stupid at it's very core.


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## TimelessQuality (Sep 23, 2007)

Lots and lots of Manablocs in this town.... 

There's quite a few that work at Viega, and 'take their work home':whistling


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## Cole82 (Nov 22, 2008)

KillerToiletSpider said:


> And on the plus side for the manufacturer, they sell an extra thousand feet of pipe for every manifold system that gets installed, even though the whole idea is stupid at it's very core.


Could you elaborate?

I have scene nothing but manifold systems for 10 if not 15 years. No body in this area does it the other way.

Cole


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## Inner10 (Mar 12, 2009)

> Could you elaborate?
> 
> I have scene nothing but manifold systems for 10 if not 15 years. No body in this area does it the other way.


What a strange would you live in, I've seen a few but most just run one trunk and branch off. Why bother wasting pipe with individual runs to a manifold?


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## Cole82 (Nov 22, 2008)

Inner10 said:


> What a strange would you live in, I've seen a few but most just run one trunk and branch off. Why bother wasting pipe with individual runs to a manifold?


All the shutoffs are in one location and less fittings. Just the supply line connection and the manifold connection. The topic of this thread as well I have always been told that 1/2" line gets hot water faster that a main run with branches. Pex pipe is cheaper than fitting to connect to main line. Other than that I don't know that's why I asked.:thumbup:

Cole


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## jb4211 (Jul 13, 2010)

My mom's house has 3/4" coming into the meter exiting through 1/2" then reduced to 3/8" at each fixture. I dont know why, but it was a PITA doing repairs on Sunday afternoon years and years ago: suppliers closed. She lives in a row home in NE Philly, probably built in the early 60's I think???


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## Inner10 (Mar 12, 2009)

Cole82 said:


> All the shutoffs are in one location and less fittings. Just the supply line connection and the manifold connection. The topic of this thread as well I have always been told that 1/2" line gets hot water faster that a main run with branches. Pex pipe is cheaper than fitting to connect to main line. Other than that I don't know that's why I asked.:thumbup:
> 
> Cole


Yes and no...

I'd be hard pressed to think the savings of not buying 3/4" T's would be more than buying all that extra 1/2 pipe and shut-off valves.

Think about this, you take a dump, then go to wash your hands with hot water, now the trunk is hot, you get hot water right away at the shower instead of draining long 1/2 inch lines for each fixture.


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## Tinstaafl (Jan 6, 2008)

Inner10 said:


> *Think about this*, you take a dump, then go to wash your hands with hot water, now the trunk is hot, you get hot water right away at the shower instead of draining long 1/2 inch lines for each fixture.


:blink:

I did for a moment. Sounded like you had the commode hooked to the hot water line. :laughing:


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## Inner10 (Mar 12, 2009)

> I did for a moment. Sounded like you had the commode hooked to the hot water line.


I'm not that rich!:laughing:

...But I do wash my hands with warm water. :thumbsup:


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## Cole82 (Nov 22, 2008)

Inner10 said:


> Yes and no...
> 
> I'd be hard pressed to think the savings of not buying 3/4" T's would be more than buying all that extra 1/2 pipe and shut-off valves.
> 
> Think about this, you take a dump, then go to wash your hands with hot water, now the trunk is hot, you get hot water right away at the shower instead of draining long 1/2 inch lines for each fixture.


There are no extra shut off valves the valves are on the manifold built into it. 

You are assuming the trunk/main line is warm but after sitting idle for 10 hours a normal work day. It isn't very warm anymore. Of coarse if you have a stay at home wife she could be doing laundry or dishes through out the day keeping it warm. We will say it's cold now though after the 10 hours. You go to wash your hands after that monster poop you speak of. Now you have to drain that large main line before you get warm water. With the manifold setup it's only a 1/2" line instead of 3/4" or even 1".

Not trying to argue just wondering as the branch setup is unheard of here. I was actually trying to remember the last one I saw on a new house and I think it was 98. I remember back in the late 80's early 90's they were using manifolds in the new development behind our house.

Cole


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## Inner10 (Mar 12, 2009)

> Not trying to argue just wondering as the branch setup is unheard of here. I was actually trying to remember the last one I saw on a new house and I think it was 98. I remember back in the late 80's early 90's they were using manifolds in the new development behind our house.


Must be a regional thing, only time I've ever seen manifolds being used for potable water was when it was a DIY job and the homeowner though it was better for some reason. Never seen a real plumber do it that way.


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## Cole82 (Nov 22, 2008)

Must be a regional thing.

On a side note. If I ran hot water to my toilet would it keep the seat warm?

Cole


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## KillerToiletSpider (May 27, 2007)

Cole82 said:


> Must be a regional thing.
> 
> On a side note. If I ran hot water to my toilet would it keep the seat warm?
> 
> Cole


No, but it will keep the tank from sweating in the summer time.


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## TimNJ (Sep 7, 2005)

Cole82 said:


> Must be a regional thing.
> 
> On a side note. If I ran hot water to my toilet would it keep the seat warm?
> 
> Cole


When I worked on a military base there was always some plumber who would end up hooking up a toilet to the hot water line. 
Gave new meaning to "take a steaming sh**".
The janitors thought it was great. They could sanitize the toilet just by flushing it once or twice.


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## TimNJ (Sep 7, 2005)

Cole82 said:


> Tin this link covers manifold design benefits over traditional systems.
> 
> http://www.huduser.org/portal/publications/pex_design_guide.pdf
> 
> ...



Great link there.:thumbsup:


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## TimNJ (Sep 7, 2005)

Tinstaafl said:


> Leaving aside the overall volume of flow, at what point is it better in terms of hot water NOW at the faucet to use a 1/2" line to take advantage of the higher velocity of flow? Does that question make any sense? :blink:



How is your layout set up. 
When I built my place I had my h/w in my mechanical room. After living in the house for a while I ended up relocating it so it was closest to the point of where I actually used most of my h/w. Made a big difference.


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## Tinstaafl (Jan 6, 2008)

TimNJ said:


> When I built my place I had my h/w in my mechanical room. After living in the house for a while I ended up relocating it so it was closest to the point of where I actually used most of my h/w. Made a big difference.


That certainly would, no doubt about it. :thumbsup:

But it's often not practical to do that for various reasons, and that's when this question comes into play. In the situation that prompted the OP, my buddy ran 3/4" cold water lines, but used 1/2" for hot--based on the theory that the HO wouldn't have to wait as long for hot water at the faucet. The inspector shot him down and made him change it to 3/4".


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## Cole82 (Nov 22, 2008)

Tinstaafl said:


> That certainly would, no doubt about it. :thumbsup:
> 
> But it's often not practical to do that for various reasons, and that's when this question comes into play. In the situation that prompted the OP, my buddy ran 3/4" cold water lines, but used 1/2" for hot--based on the theory that the HO wouldn't have to wait as long for hot water at the faucet. The inspector shot him down and made him change it to 3/4".


I just tried finding anything that requires a 3/4" line and the only thing I could find are flushing rim sinks.

That is looking at IPC table 604.5

Unless it's a local amendment I would have asked him to show you the code.

Cole


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## Tinstaafl (Jan 6, 2008)

Cole82 said:


> Unless it's a local amendment I would have asked him to show you the code.


Yeah, I know. But it's the same inspector for framing, plumbing, electric etc etc. Easier to just say yassa, Massa.


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## PowerWash (Mar 25, 2011)

Tinstaafl said:


> Yeah, I know. But it's the same inspector for framing, plumbing, electric etc etc. Easier to just say yassa, Massa.


And that is the only thing about building new houses I do not miss! Inspectors in our area are assigned a house and they inspect it from start to finish. Gotta play the game.


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## Ben81 (Feb 3, 2012)

Recirc line 1/2" dia with the pump being 1/2" dia as well. I've seen too many installations with the 3/4" pump and it eventually eating the copper in the house. That type of pump is made for the commercial end of things. This pump is only 1/250th of a hp and has the option to run continuously or on a timer. Timing wise depends where you would hook your recirc line up to. Some instances you can't always get your recirc line teed off right in the cabinet by the affected fixture but put it as close as you can and insulate the recirc line


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## Jackfre (Mar 1, 2012)

*Well...*

There are good reasons to get down to 1/2", from the 3/4" main, as soon as possible. You have to take pressure drop into consideration but I've gone thru this a lot. I represented a major tankless manuf. Tankless will not make hot water until the tap is opened. Proximity to the point of use is a consideration in the installation. Given that the cross sectional area of 1/2" is .19 sq in and 3/4" is .44, there is over twice the vol to possibly waste prior to getting hot water. A properly sized 1/2 line makes sense. Also, you have to consider the flow rate of the particular fixtures you are selling. 

On our home in MA. that we sold last year, I had a tankless water heater for 12 yrs. It was right under the kitchen sink. I remodelled the bathrooms and re-installed the tankless on the other side of the basement using the same piping with 3/4 feeding across to the front of the house where the bathrooms were located. It took forever to get hot water at the kitchen sink as I had to dump all that 3/4". I explained to my wife the changes. She then explained to me, not exactly what the changes were going to be, but what the end result would be. After 38 yrs I understand her when she speaks in this way. Especially so when she is correct. I ran a 1/2" line direct to the kitchen sink, reducing the delay and dump vol. All good!


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## bob_cntrctr (Jan 30, 2008)

Ben81 said:


> Recirc line 1/2" dia with the pump being 1/2" dia as well. I've seen too many installations with the 3/4" pump and it eventually eating the copper in the house. That type of pump is made for the commercial end of things.


Could someone please explain this? A "3/4" pump" (never seen a pump with a 1/2" fitting - but I'm thinking of well pumps) somehow will cause the copper to be corroded?


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## txgencon (Jan 4, 2011)

Inner10 said:


> Think about this, you take a dump, then go to wash your hands with hot water, now the trunk is hot, you get hot water right away at the shower instead of draining long 1/2 inch lines for each fixture.


Who washes their hands just before getting in the shower?

On another note: 

100 feet of 3/8" water line will have 0.57 gallons of water

100 feet of 1/2" water line will have 1.02 gallons of water 

100 feet of 3/4" water line will have 2.29 gallons of water


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## Inner10 (Mar 12, 2009)

I always wash my hand's after wiping my ass...but that's just me.

I'm still surprised to see that running dedicated pipes is common practice in some areas. A popular way lately has been to have a trunk and branch setup with a tankless water heater and a re-circ pump on a timer.


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## txgencon (Jan 4, 2011)

Inner10 said:


> I always wash my hand's after wiping my ass...but that's just me.


You should try using toilet paper and/or wet wipes.


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