Electric water heater thermostat issue

Old 01-28-23, 08:49 AM
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Electric water heater thermostat issue

I am having issues with the operation of the thermostats on a 4 year old Rheem electric water heater. We began running out of hot water in the shower and upon testing both thermostats and heating elements we determined that the upper thermostat was faulty and only the lower thermostat/element were functioning. Rheem sent both new thermostats and lower element because after testing both elements the Rheem tech felt the lower element was not within specs. The power supply to the heater is 240-243 volts and the 5,500-watt upper element tested at 10.3 ohms and lower element at only 9.4 ohms.

After replacing both elements with Rheem SP10552PH resistored elements, and both thermostats with Rheem supplied thermostats, when heating is demanded the upper thermostats automatically sends power to only the lower thermostat/element unless the upper thermostat is set at almost 150 degrees. Even when the lower thermostat is set to 90 degrees and the upper thermostat is set to 125 degrees the unit sends power to the lower thermostat/element. When I was replacing the thermostats and elements I replaced the annode and inspected the dip tube. We have grandchild visit often and are concerned with scalding with water temps at 140 degrees.

Am I missing something?
Old 01-28-23, 09:22 AM
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It is normal on an electric water heater for only one element to be on at a time. Here's a decent explanation of the sequence of operation: https://www.google.com/url?sa=t&rct=...pQgtKYxx7XhjPf

A common way to increase the amount of hot water available from a given electric tank is to operate the tank at a relatively high temperature, such as 140 degrees, and then use an anti-scald valve after the tank to mix in some cold water to reduce the temperature to a safe value, typically 120 degrees. Here's an example of such a valve that mounts to the heater top in a straightforward way: https://www.lowes.com/pd/Cash-Acme-3...E&gclsrc=aw.ds

These valves increase the amount of available hot water because for every gallon of hot water used, less than one gallon comes from the tank; the rest is mixed in cold water.

Running the tank at a higher temperature also provides protection against the Legionella bacteria that can cause severe illness. It can thrive in lower temperature water.

Operating the tank at a higher temperature does increase operating cost slightly because the standby loss is a little greater; this is usually not a big factor.
Old 01-28-23, 09:29 AM
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Try increasing the lower thermostat to at least 120.

Not sure how your water heater operates but on some models if you set the lower thermostat too low then it shuts off everything too soon.

Turning the upper thermostat dial or pointer switches between upper element and lower element. But it is very hard to know when it should switch because you don't know what the water temperature is in the region of the tank where the thermostat sensor is located.

Adding to the confusion is that the thermostat dials are not precise.
Old 01-29-23, 05:41 AM
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Thank you for the replies and suggestions. The example of turning the upper thermostat to 140 degrees and lower thermostat to 90 degrees was only to view how the water heater was functioning with high hot water demand from running hot water in two shower/tubs. What was happening to cause me to diagnose the hot water shortages was determined to be a defective upper thermostat. When I replaced the upper thermostat I discovered that the lower thermostat/element was the only element heating the water unless I turned the upper thermostat to 135/140. Rheem actually sent me two new thermostats and both new thermostats behave the same.

As I understand the operation of an electric, two element water heater the upper thermostat first heats the water and when that water temperature reaches the thermostat setting the upper thermostat closes the circuit to the upper element and if the water temperature in the lower portion of the tank is below the thermostat setting, the lower thermostat then sends current to the lower element to heat water. In my original circumstance, the upper thermostat was defective and not sending current to the upper element. Because of this, only the lower element was functioning and was not able to effectively heat the entire tank.

I currently have the upper thermostat set to 135 and lower thermostat set to 125 degrees. While I do understand that the thermostats are basically low-tech devices that use bi-metal disc to determine temperature, I don't understand why I can't set the upper thermostat to 120 to 125 degrees instead of the current 135 degrees to activate the upper element. I am wondering if the light metal thermostat bracket connection either transfers heat to the thermostat, or the position of the thermostat relative to the exterior of the water tank itself transfers heat to the bi-metal plate within the thermostat.
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Old 01-29-23, 02:26 PM
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The upper thermostat control everything. It doesn't care what the lower one is set to. However if the lower one is set much higher than the upper one... the water could stay hot enough to keep the top element from coming on.

The bottom element will attempt to keep the incoming water hot but this time of year it's almost impossible so the the upper stat should take over fairly quickly. Hot water rises in the tank.

The element resistances may vary some but the elements are good or bad.
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Old 03-31-24, 05:25 AM
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Bramz, I have turn my thermostat up to 140 degrees to get hot water, replaced the thermostat and still the same. My conclusion is that the bimetal thermostats are terribly inaccurate as well as once it turns on the temperature has to increase about 15 degrees before it shuts off. Further aggravating the problem is building codes allow hot water pipes to be installed without insulation under a slab floor so water cools off rapidly in a long run.

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