Dueling water heaters

[foredeck1]

I am doing a complete bathroom remodel on a home that we purchased. The home has a 40 gallon gas water heater. In the course of the bathroom remodel, we are installing a custom shower setup that will have a total of 7 kohler watertiles in addition to a showerhead. I need more hot water capacity to run the shower system unless I want to have 2 minutes worth of hot water when we turn the tiles on. I have purchased a 50 gallon electric water heater to augment the system (electric so that I don't have to deal with venting and gas piping). It has been suggested that I run the electric water heater in sequence after the original hot water heater, and that I then run a loop from the output side of the electric heater back to the input side of the gas water heater, and install a circulating pump in this loop. This is being suggested as a low cost alternative to having the electric water heater actually heating any water, instead it would act as a holding tank.

My questions are as follows:
1) does this make sense, as the gas water heater is then going to be doing twice the work, and I still have to pay to operate (and in the future maintian) the circulating pump.
2) The branch that is going to feed the shower system is proposed to come off of a T out of the output side of the electric water heater, the other end of the T being the loopback. Would the operation of the circulation pump decrease the amount of water pressure that would be delivered to the shower?

Thanks in advance for advice.

[homebild]

I don't like the system that has been recommended to you.

You would do better to use the 50 gallon supplemental heater plumber directly to this shower only or to have a tankless electric heater installed that would serve the shower only.

There is no sense involving the rest of the house's hot water supply when only 1 fixture is affected.

[homebild]

I thought more about this and really question whether you will need an additional water heater for this application.

The flow to the shower heads is restricted by the riser that supplies it.

If you have a 1/2" riser pipe that serves 1 shower head or 7 shower heads you are going to use the same amount of water. All heads (USA) today have a restricted flow rate of about 2.5 gpm and that rate is limited by the size of the pipe that serves the shower heads.

So if you have one 1/2" riser pipe that will serve all 7 shower heads, you will use no more water and the shower will last just as long as if you had only 1 shower head.

The only way that you will use a larger amount of water with the Shower Tile setup is if you have individually supplied riser pipes/branches to each shower head that come directly from the main supply line or if you increase the diameter of the pipe that serves the heads.

Otherwise flow is not regulated by the number of outlet fixtures, flow is regulated by the size of the pipe that serves those fixtures.

So if you use a single riser and the same diameter pipe, you will not need to have a second water heater at all....

[suemarkp]

A 1/2" pipe flows much more than 2.5 GPM. Every low flow shower head I've seen has a restriction plate in it with about an 1/8" diameter hole. I've drilled these out to increase the flow.

At some point you're right, adding more shower heads isn't going to do much for total flow (and the pressure loss will just be getting so high that water will dribble out all of the shower heads instead of shooting out). I'm guessing that at around 3 shower heads you'll reach the point of diminishing returns because the 1/2" pipe can't flow any more.

[foredeck1]

I am running 3/4" dedicated lines off of the water heater and the water main to the shower system to make sure that i have all of the pressure I can get. The pressure that is delivered to the showerheads is actually limited by the shower valve(s) that are being used. I am told that if I have all 7 watertiles running at once, I will suffer degradation in waterpressure, for what it is worth. I read in the Kohler user guide to the shower valve that water at shower temperature is about 75% hot, and 25% cold. With 3 showerheads going for example, I will be burning 7.5 gallons of water, which equates to roughly 5.5 gallons of hot water, which on a 40 gallon water heater will give me a 7+ minute shower (give or take). We are adding the water heater to give us longer shower time.

[homebild]

Running 3/4" lines to your bath is a good idea to increase the volume of water available for flow, but increasing the pipe size reduces the pressure in those lines.

Seems to me you'd do best to use a tankless heater as a point of use water heater dedicated to this shower only so you have no restrictions on shower length at all and then decide to use 1/2" pipe to keep the pressure as high as possible or 3/4" pipe to get the most flow.

Whatever you do, here, it appears you're going to have a tradeoff between pressure and flow especially if you are on a well.....

[suemarkp]

Running 3/4" lines to your bath is a good idea to increase the volume of water available for flow, but increasing the pipe size reduces the pressure in those lines.

I can't agree with this statement. The pressure in the pipes is related to the system pressure from either the well pump or the serving water utility. With no water flowing in the pipes, your pressure will be lets say 60 PSI. When water flows, there is friction which causes a pressure drop. The pressure in your pipes will not be higher than the system pressure regardless of how small of a pipe you go to. Smaller pipe has more friction at a given flow rate which further eats away at your available working pressure.

The only way I can agree with your statement is if you mean velocity instead of pressure. But the only reason you care about the velocity inside a pipe is to keep it low enough to minimize friction losses and erosion effects. Your shower head is the ultimate velocity increaser -- small holes give small streams of water at high velocity.

[homebild]

Mark,
You are correct. I meant velocity, not pressure, and the smaller the pipe diameter the lower the pressure because of flow restriction due to friction.

In my haste to head out the door this morning I stated 'pressure' when in fact I should have stated velocity.

Thanks for the heads up.

Here's a helpful link regarding pressure, velocity and pipe diameter:

http://www.bobvila.com/ArticleLibrary/Subject/Plumbing/Water_Heaters/WaterPressure.html