hydraulics 101

[GatorKenD]

Attached is a simplified drawing of how the water is supplied to the two ends of my house. Heavy black line is supply (PVC) from the meter. That ties into two 1/2" copper pipes (represented by the green line). One 1/2" copper supplies cold water to the east end of the house. The second 1/2" copper pipe travels over 100' around to the back of the laundry room. From there it ties into a 3/4" copper pipe (represented by the blue line). That 3/4" copper supplies cold water to the two bathrooms a the west end of the house and to the hot water heater. The hot water supply is a 3/4" copper loop (red line) (which has a recirculating pump on a timer).

The water pressure in the two west bedrooms is adequate for one shower, but less than adequate if a second fixture (shower, sink, toilet) is running on the west side.

I'm thinking that having 100+' of 1/2" copper in front of the 3/4" copper supply to the west bedrooms is robbing me of a lot of pressure. Is that correct? I'm thinking that if I abandon that long length of 1/2" copper and replace with 3/4" or larger PVC from the main supply line, I'll markedly increase the water pressure in the west bathrooms. Is that correct?

 

[hj]

1. ANY 1/2" line no matter how long is too small and increasing it to 3/4" at the end was foolish. DO NOT, and you really cannot legally, use PVC. If you use 3/4" PEX, or CPVC, you will not help yourself too much since they are marginally larger than 1/2" copper.

 

[Sponsor]

 

[Jadnashua]

The wall thickness on plastic pipe is greater than copper, so the ID is smaller than the equivalent in copper. 1/2" copper is often considered adequate for ONE device, not supplying multiple ones. Your dynamic pressure loss goes up with a smaller pipe, too, which can rob a shower from not only volume but pressure as well. According to the copper pipe handbook, on 1/2" pipe, depending on the type (K, L, or M), the pressure loss at max recommended volume per foot is: 0.189 to 0.130 pounds. ANd, that's only 5gpm, which exceeds the max recommended velocity on that size pipe. As a comparison, on 3/4" copper, at the same volume, the pressure drop is: 0.035 to 0.023 pounds per foot and even at double the volume, the pressure loss is about 50% less than 1/2" copper.

IOW, pipe diameter does play a big role in both volume and pressure...think soda straw verses fire hose...both may operate at the same inlet pressure, but the differences in delivered volume and pressure are immense.

 

[GatorKenD]

Thanks for the clarification on the correct plastic pipe. I really would have checked before I put it in! I DON'T think it I would have considered the minimum increase of the INSIDE diameter of the plastic pipe. Just got home and checked the supply coming into the house is 1". Replace the 1/2" copper w/ 1" PEX or CPVC?

 

[Jadnashua]

IF you want to maintain all of the volume available, neither! Typical 1" cpvc will have about 14% less volume than 1" copper and more dynamic pressure loss because the typical cross section is about 0.050" smaller in diameter. Throw in a bunch of fittings and elbows, and the dynamic pressure losses go up, so keeping all you have is worthwhile. But, anything bigger will be a help.

 

[GatorKenD]

so just to be clear: If my supply from the meter is 1" CPVC, going UP to 1-1/2" CPVC for that 100' run would be better than keeping the pipe diameter the same (i.e., 1").

 

[Jadnashua]

No, if the supply is 1" cpvc, I don't see any advantage to going to something bigger. THen, I'm just looking at the pipe diameter and I'd assumed you had copper coming in, not cpvc.

 

[Reach4]

so just to be clear: If my supply from the meter is 1" CPVC, going UP to 1-1/2" CPVC for that 100' run would be better than keeping the pipe diameter the same (i.e., 1").

How do you define better? Less pressure drop? If so, yes, the pressure drop would be less than if you did not replace the smaller pipe with larger pipe.

If you define it as minimizing the power to keep your recirculating hot water working, then it is not better I don't think. It will take longer for the hot water to get to the endpoint if you don't increase the pumping rate. Thus the hot water will cool more during its trip.

 

[hj]

You might have a difficult time finding ANY CPVC larger than 1" as a stock item.

 

[GatorKenD]

Let me get side tracked for a second here. The 1" supply pipe coming into the house... what are the implications if it is PVC? Would that have acceptable to use when the home was built in 1979. I've got a few feet of this pipe exposed behind some HVAC equipment where I might be able to read something on the pipe, but my quick attempt just now didn't yield anything. It's pretty "white" (i.e., it doesn't have that white-gray tint I associate with CPVC).

 

[GatorKenD]

HJ - back to my first question... you look like the only guy on this thread who does this for living. You got a favorite brand of closet flange guaranteed to last the ages?

 

[Jadnashua]

There are pvc water pipes that can be used underground as supply pipe...it cannot be extended INTO the house for use there. IN the house, you need CPVC if you want to stay with that family of pipe.