Dual Shower Temperature Problem

[tg337]

I recently converted my regular tub shower combo to a dual head standing shower. Each shower head has its own control via pressure balancing valve, and I plumbed it (basically) as shown in my attached crude drawing. I’m a novice when it comes to plumbing, but based on some research, I built the layout and hoped it would work without compromising pressure and/or temperature.

I was pleased to find that when both showers are on, there is no apparent reduction in pressure at all. However, after about 15 minutes, both showers start to gradually output colder and colder water.
I have tried to reason out why the hot water seems to fade away. At first I thought it was the (somewhat inexpensive) Luxier fixtures and valves I used from Home Depot. I ended up trying different mixing cartridges, and eventually even replacing one valve with a much more expensive Kohler valve. Nothing seems to be helping.

I’m starting to suspect that it could be my plumbing layout… Do I need a pressure balance loop here? Or should I change the valves to thermostatic?

I thought maybe my water heater can’t keep up with the output, but if I can fill the tub upstairs without running out, I figured it can’t be out of hot water. Also, I seem to still get warm/hot water from other fixtures in the house after the showers run cold, so it wouldn’t seem like the water heater is running out.

Could it be the hot water line doesn’t have good pressure on it? Or maybe that the two pressure balance valves are fighting each other somehow?
Any advice anyone can offer would be greatly appreciated.

Thanks everyone!

 

[Breplum]

Colorful and cute drawing.
Water heaters mix cold water into the bottom of the tank with a dip tube that goes 3/4 of the way down.
The hot output from a WH is only about 65% of the rated gallons, before the mixed cold affects the hot output and begins to lower the output temperature.
You'd need a thermometer to know if you are just running out of hot water when two showers are running. There is nothing wrong with your layout.

 

[Sponsor]

 

[tg337]

Thank you so much for the reply! I really appreciate it. Do you think a tankless water heater would help this situation?

 

[Breplum]

Thank you so much for the reply! I really appreciate it. Do you think a tankless water heater would help this situation?

Tankless, when sized and installed properly provides unlimited hot water. Just expensive.

 

[Reach4]

If you are taking a shower, can you turn the temperature control higher to still get the temperature that you want? If so, a thermostatic valve might cure this for you.

You could also consider turning up the hot. Do you have a mixing valve for the water coming out of the WH to cool it to 120 or less? New construction requires that. I don't know when you need to add that for other changes.

But anyway, if you turn your WH hotter, one way or another there will be less hot needed. So that might make the water last.

Regarding tankless, do you have gas? I think it is hard to do tankless with electric.

 

[tg337]

If you are taking a shower, can you turn the temperature control higher to still get the temperature that you want? If so, a thermostatic valve might cure this for you.

You could also consider turning up the hot. Do you have a mixing valve for the water coming out of the WH to cool it to 120 or less? New construction requires that. I don't know when you need to add that for other changes.

But anyway, if you turn your WH hotter, one way or another there will be less hot needed. So that might make the water last.

Regarding tankless, do you have gas? I think it is hard to do tankless with electric.

Oh yes, this is one of the reasons I discounted the water heater theory early on. If you have the showers set to a certain temperature, they will start to cool off from there. But then, if you turn them more hot, they will get hot again (suggesting there was more hot water in the tank) and then after a while go cold again.

Of course, you can only turn the handles up till they stop, so eventually it just gets cool and there’s nothing more you can do about it.

I actually did increase the temperature output of my water heater just to see if that would help. Maybe I should try to increase it even more, but I suspect I’d have the same result.

Thanks so much guys, for bringing up these ideas. It really helps.

P.S. I DO have gas, actually! I’ve always thought if I did tankless, I’d choose a gas one.

 

[Fitter30]

Model and brand of heater, shower heads and mix valves?

 

[tg337]

Model and brand of heater, shower heads and mix valves?

Apologies for the delayed reply. I was traveling and couldn't get this information exactly by memory.
Here are the details of all the hardware.

Water Heater
Brand: A.O.Smith
Model: ProMax GCV 40 200
Capacity: 40.0
Gas Type: Natural
Input BTU/Hr: 40,000
Recovery Gal/Hr: 39.32

Shower Valve & Head #1
Brand: Luxier
Model: Rainfall Shower System (Brushed Nickel, SS-B01-TB)
Shower Head Diameter: 8 in
Shower Head Flow Rate: 1.8 Gal/Min
Valve Type: Pressure Balancing
Home Depot Link to Product

Shower Valve #2 (Head is same Luxier as #1, after switch to Kohler parts)
Brand: Kohler
Model: Rite-Temp Pressure Balancing Valve Kit with Service Stops and PEX Connections (K-8304-PS-NA)
Shower Handle: Purist Pressure Balanced Valve Trim Only with Single Lever Handle - Less Rough In (K-TS14423-4-BN)
Valve from build.com
Handle from build.com

Correct me if I'm wrong, but laying it out here, I've done the math... And it's pretty obvious that the water heater simply can't keep up.
At 39.32 Gal/Hr recovery rate (= 0.6553 Gal/min), I graphed the depletion of the water heater starting at a full 40 Gallons (see attached).
The blue line is the recovery rate of the tank.
The green line is how quickly the shower heads would drain the water, starting at 40 gal.
The gray line is the combined efforts of the tank and the shower heads.
And voila! At roughly 15 minutes, I'm completely out of hot water.

Does this seem reasonable to you guys?

 

[Fitter30]

Here's a water temp calculator just change setting
Sample 35 gallons of hot 13 minutes

Water Mixing Calculator | Spirax Sarco

This calculator will help to consider the mixed temperature of two water streams, its flowrate, percentage flow and actual flow of each stream.

www.spiraxsarco.com

 

[tg337]

Here's a water temp calculator just change setting
Sample 35 gallons of hot 13 minutesView attachment 93124

Water Mixing Calculator | Spirax Sarco

This calculator will help to consider the mixed temperature of two water streams, its flowrate, percentage flow and actual flow of each stream.

www.spiraxsarco.com

View attachment 93124

Ahhh thanks for this. I’ll rethink my math. Totally was forgetting that the valves are mixing in cold water.

My math assumed they were outputting 100% hot.