ProPex tubing and thermostatic valve sizing

[Temp945]

Hi all,

I'm planning a bathroom remodel and am a bit confused on how to size the Uponor ProPex tubing and valves.

I am planning on having a showerhead and handheld shower rated at 2.5gpm each (5.0gpm total), and each with their own volume control/shutoff. I'd like to be able to get the full rated 5.0gpm when both are in use.

I am also re-piping the home in advance of the bathroom reno. I am using Uponor ProPex. I have good water pressure at the meter and my pressure reducing valve is set to 60 PSI.

My main question: the thermostatic valve I'd like to use is rated at 10.9gpm (Kohler K-2972-KS-NA) and has 1/2" inlets. Can I get at least 5.0gpm of hot water out of this valve with 1/2" ProPex supply lines? Based on my reading it will not be possible to get more than 2.5 - 3.0gpm through the hot supply which will obviously not be enough, even if the water heater is set quite high.

I could run 3/4" supply lines to the thermostatic valve, but it has 1/2" inlets. Will using 3/4" supply lines provide the additional volume I need even though the valve is reduced to 1/2" at the inlet?

Thanks a lot for reading!

 

[wwhitney]

As to your question about 3/4" pex to a shower valve with 1/2" inlets, that's fine in and of itself. The pressure due to the pex and the pressure drop due to the shower valve are independent, and the reducing fitting itself won't be a big effect. A brief primer on pressure drop:

Each piece of the system from the PRV to the shower head has a curve of pressure loss versus flow. The pressure you get at the outlet is the 60 psi PRV setting minus the sum of all the pressure drops of all the components at the target flow. The outlet will have an input pressure needed to get the target flow; if you have less pressure, you'll get less flow, if you have more pressure, you'll get more flow. [But the outlet may be designed to have close to a certain maximum flow over a wide range of pressures., i.e. the curve may flat-line at a certain point.]

Worst case, your hot water is 120F, your cold water is 50F (does incoming water get that low in Florida?), and you want 110F water out. So for 5 gpm mixed, you need 6/7 of that as hot, or 4.3 gpm hot. Propex's chart says that 4.5 gpm of 120F water through 100 feet of 1/2" Propex has a pressure loss of 62.5 feet of head, or 26.9 psi.

How much 1/2" pex are you using, and what will be the other sources of pressure drop between the PRV set to 60 psi and the shower head? The shower mixing valve should have a published pressure drop chart. And what pressure does the shower head need?

If your 1/2" pex is only 10 feet, it only accounts for 2.69 psi, and I can't imagine any upside to upsizing it. If it's 100 feet, that's a lot, and you need to do the calculations above to see if it is sufficient. It is worth keeping the line as small as possible, because every time you shower, you need to empty that pipe of cooled water, which adds up over time.

Cheers, Wayne

 

[Sponsor]

 

[Fitter30]

Use any shutoff valves make sure there full port less pressure drop than a standard valve.

 

[Temp945]

Wayne: thanks a million for your detailed reply. It was really helpful and pointed me in the right direction. I was able to look up the Uponor chart that you mentioned and I did some calculations.

Link to chart: https://www.uponorengineering.com/~...ressure loss charts.ashx?version=062520180400

155 ft total supply length (from PRV to water heater to shower valve)
4.0GPM hot water at 120F + 1.0GPM cold water at about 60F = 5.0GPM for two shower heads
1/2": 22PSI (50.23 head) per 100ft = 0.22 PSI/ft
3/4": 4.0PSI (9.35 head) per 100ft = 0.04 PSI/ft

1/2" from the water heater to the shower valve:
35 ft of 1/2" x 0.22PSI/ft = 7.7 PSI loss
120 ft of 3/4" x .04PSI/ft = 4.8 PSI loss
7.7 + 4.8 = 12.5 PSI loss combined
60 PSI at PRV - 12.5 PSI loss combined = 47.5 PSI at shower valve

3/4" for the full run:
155 ft x 0.04 PSI/ft = 6.2 PSI loss
60 PSI at PRV - 6.2 PSI loss = 53.8 PSI at shower valve

Does that look right to you? I have not accounted for fittings, but since I'm using Propex, they are very limited.

The spec sheet for the valve states 10.9GPM at 45PSI:
https://www.us.kohler.com/webassets/kpna/catalog/pdf/en/K-2972-KS_spec.pdf
Based on my calculations, I should be able to get close to the full 10.9GPM if I use 3/4" supply lines, but since I am only using 2 fixtures rated at a combined 5.0GPM, this is a moot point.

Also, I looked up the spec sheet for the shower head:
https://www.us.kohler.com/webassets/kpna/catalog/pdf/en/K-965-AK_spec.pdf
It does not list the PSI needed for 2.5GPM - it only states that the max PSI is 80.
I'm also confused by the note at the very end of the document: "For use with automatic compensating valves rated at 2.4 gal/min (9.1 l/min) or less." What does that mean exactly?

In short, it appears that I can use either 1/2" or 3/4" supply lines for the run from the water heater to the shower valve. Do you agree? Is there any advantage to using 1/2" vs 3/4" in this case?

 

[wwhitney]

On the last two questions:

1) Yes, your method looks correct, so if your math is correct, and you've accounted for all the sources of pressure loss, I agree.
1/2" from the water heater to the shower valve will give you the flow you want, and the extra 6 PSI available from using 3/4" doesn't buy you anything.

2) Yes, if you use 1/2" instead of 3/4", you'll be waiting about 1/2 as long to get hot water at the shower every time you use it.

The ID of 1/2" PEX is 7/9 * 5/8, and the ID of 3/4" PEX is 7/9 * 7/8. So their ratio is 7/5, and the internal area ratio is 49/25 = about 2. That means the 3/4" PEX holds twice as much water, and you have to wait twice as long to drain it.

Cheers, Wayne