Wiring two electric water heaters in series to one electrical circuit

[Sparky54]

This is my first post on this forum. I am preparing to add a second, identical 30 gallon electric water heater with two 4500 watt heating elements to my hot water system. The cold water supply will be piped into Tank #1's cold water inlet. Tank #1's hot water outlet will be connected to Tank #2's cold water inlet and then out Tank#2's hot water outlet to our home's hot water taps.

I believe the most efficient wiring is to: 1. Replace Tank #2's lower heating element thermostat SPST switch with a upper heating element thermostat SPDT switch. 2. Wire an additional 10 gauge wire to the now-available normally open (NO) contact of Tank #2's new lower heating element thermostat. 3. Route this additional wire up through the tank's wiring conduit; through the attached external conduit; and into Tank #2's electrical connection device box. 4. Attach this additional wire to one of the hot leads (line 1) leading toward Tank #1. 5. Attach Tank #1's other hot lead (line 2) to the line 2 hot wire and the hot wire (line 2) that goes to Tank #2. 6. Attach an equipment ground wire for Tank #1 to the branch circuit's equipment ground wire and the equipment ground wire that goes to Tank #2. 7. Route Tank #1's 3 supply wires through an external conduit into Tank #1's electrical connection box and connect the three wires to the appropriate connection terminals, including a DPST turnoff switch. 9. Continue routing Tank #1's 3 supply wires through an external conduit into Tank #1 and make the appropriate electrical connections.

Wired in this matter, only one, if any of the four heating elements in the two tanks will be energized at any time. Whenever Tank #2's upper heating element calls for power it will get it. If Tank #2's thermostat is satisfied, then whenever Tank #2's now DPDT lower heating element thermostat calls for power it will get it. When both of Tank #2's thermostats are satisfied, then whenever Tank #1's upper heating element thermostat calls for power it will get it. And finally, if all three of the other heating element thermostats are satisfied, then whenever Tank #1's heating element thermostat calls for power it will get it.

I will initially set Tank #2's upper thermostat at 120 degrees F.; Tank #2's lower thermostat at 115 degrees F.; Tank #1's upper thermostat at 110 degrees F.; and Tank #1's lower thermostat to 105 degrees F. I am thinking that this will help even out the time that each of the four heating elements are powered up without decreasing much of the total hot water availability.

Please comment.

 

[Reach4]

I am thinking that this will help even out the time that each of the four heating elements are powered up without decreasing much of the total hot water availability.

I think it should work as you describe.... but why?....

Hard to run a new circuit?

Trying to reduce the peak power that your smart meter sees?

No room for a 50 gallon WH? Natural gas not available?

 

[Sponsor]

 

[Sparky54]

I think it should work as you describe.... but why?....

Hard to run a new circuit?

Trying to reduce the peak power that your smart meter sees?

No room for a 50 gallon WH? Natural gas not available?

This will be easier than routing a new circuit from the main electrical panel. I am trying to keep peek demand down. I use to have a 50 gallon tank. When I had to replace that 50 gallon tank I thought of this 2 tank scheme. My wife and I are empty-nesters and currently, with just the first 30 gallon tank, we have sufficient hot water availability. However, when we have overnight guests, availability will become an issue. So, I will be adding the second tank soon. The tanks each can have two ball valves to isolate each tank and a bypass pipe with its own ball valve to bypass that tank. This is attractive to me because if one tank needs repair or replacement, I still will have hot water during the time it takes me to repair or replace that tank. I replaced our 35 year old copper pipes in our utility room recently and it was relatively easy to plumb in the additional piping and 3 valves for a second tank at that time. Finally, we live in the country on a acreage. When we started to build back in '82 and natural gas was not available. I decided to go all electric so I did not have to purchase a propane tank; periodically order propane; and worry about blowing up the house due to a leak. Plus, our water source heat pump has been less expensive to operate than a propane furnace.

Thanks for the good questions!

 

[Reach4]

Plus, you could turn off the upstream tank when you are not having visitors.

 

[Stuff]

Sounds like you are following the instructions at this site: http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html

If you keep the 1st tank at 100º does that make it a breeding ground for Legionella?

 

[Sparky54]

Stuff-

You make a good point. Maybe I should just set all four thermostats at 120 degrees F.

-Sparky 54

 

[Jadnashua]

A simpler way to extend the existing WH is to install a tempering valve (if you don't already have one) and when needed, ramp the thermostat settings way up.

 

[hj]

A 30 gallon tank, gas or electric, has to be one of the most useless items you can purchase. With all thermostats at the same temperature, which is how they should be set, the lower element on #1 will operate most of the time and the upper thermostat on #2 will only function when you are about to run out of hot water, the same as if you had 4 staged elements in a 60 gallon heater.