Water Heater temperature droop

On a tank water heater, with the thermostat set to 120 F, how low can the water temperature droop before the thermostat activates the element to bring it back to the set point?

 

Welcome to the forums.

There is no easy answer to that question. A typical electric water heater has two thermostats and two heating elements. The top one heats until the water reaches the set temperature. Then it shuts off and the bottom element starts to heat. Once the temperature in the bottom reaches that set temperature..... heating stops.

Cold water enters thru a dip tube and is delivered to the bottom of the tank. That thermostat starts heating as soon as it senses the cold water. As the hot water is drawn from the top.... the cold water gradually fills the lower part of the tank and heating is switched back to the upper element.

The actual thermostat could have a 10-15 degree differential.

 

Thanks for the reply PJmax. I've seen that explanation before. I was hoping that someone could be more specific. The thermostat senses the cold water because the temperature has dropped below a given threshold. I'm wondering what that threshold is. Say there was no demand for hot water. The water temperature in the tank would slowly drop over time. At some point, the temperature at the lower thermostat would reach a threshold and the element would be activated. I figure this is pretty universal since thermostats are pretty universal. Does anyone have the specifics on thermostats?

 

Those are not high quality sophisticated thermostat.
As I mentioned there could be a 10-15 degree differential.

That means if you have it set for 120°.... it could come on as low as 110° and shutoff as high as 130°.

 

Why are you so concerned about the exact on/off points for water heater thermostats? What are you using them for?

Most water heaters have low tech analog thermostats. They are not digital with one specific temperature. If you had a box of identical thermostats and tested them all you'd probably find a range of on/off points and not one specific temperature. This range is typically beyond what humans can detect so it goes unnoticed.

 

Use experience or use trial and error.

If on some occasions you find the water temperature too cool then turn up the thermostat(s) a tad. Then the heater will kick on a tad sooner.

Note: Sometimes the thermostat turn on and turn off spread (hysteresis) can enlarge as years go by, particularly for a gas water heater. If you turn up the dial a notch and the burner kicks on and turn the dial slowly back to where it was and the heater keeps going for more than 10 minutes chances are you have a sluggish thermostat. (Hard to detect with an electric heater because you cannot hear it.)

 

Quite true. In addition, these thermostats do not have an immersion element and must sense the water temperature through the wall of the steel tank.

You can find a little more information on these bimetal disc thermostats here at pages 82-87.

https://climate.emerson.com/document...us-5312494.pdf