upper & lower thermostat settings

Boon

Hey folks, Is there a right way to set upper & lower thermostats on an electric water heater? Should they both be set at the same temperature, which is what I've always done, or should one be higher than the other?

• The manual says they are both factory set at 120°.
  
• One of my tenants changed them both to 150°. I admit I'm embarrassed I didn't catch the higher water temp and I've added a DHW temperature check to my turnover checklist.
  
• A service tech, who was there to test the elements & thermostats, set the upper to 130° and the lower to 135°. This would make the lower element do almost all the work, right? The tech claimed that Rheem suggested these settings even though it ships from the factory with 120 & 120.
  

Any thoughts on the 'right' way to set those stats?

Thanks,
David

PS - the elements and thermostats were working properly; tenant wasn't used to slow recovery of an electric. I've also added a "electric water heater chat" to my tenant walk-through checklist. This was an expensive lesson!

Jamie Hall

First off, unless you have a thermostatic mixing valve, the temperature of both should be 120. You run a serious risk of having a tenant burn themselves -- at which point they would sue you -- if they are set higher.

Second, they can be set the same -- the warm water will rise and turn the upper one off, at which point the lower one will come on. Or, if the dip tube is right, the cold water coming in will turn the lower one on. And so on.

Boon

10-4 on the mixing valve.

Ironman

The upper one has to be satisfied before it will allow the lower to be energized. They cannot operate simultaneously. The reasoning for this is that if the tank is cold, the upper one will heat the top 1/3rd of the tank to give some hot water quicker than if the whole tank had to be heated. Once the top stat is satisfied, it sends power to the lower stat.

We always set them the same. 125* max unless there's a tempering valve. For a rental, 120* as Jamie said.