Hi, I have a Rheem RHSA HM4221JA air handler with

SOURCE: rheem 92 gal. no hot water. breaker not tripping.

Have you checked to make sure that the elements are good by checking the ohms. Be sure that the power is off and the wires are disconnected from elements and then also be sure to check the ohms across the tank to make sure there is not a dead short in the element reading through the tank. If both elements are good then I suggest replacing both thermos then they will both be new and should work for you . Thermos can be problematic when trying to test them sometimes they won't always give you correct readings. Make sure you have the breaker off whenever you mess with a waterheater, and make sure you install the thermos correctly with the right wires going where they should.

SOURCE: Rheem tankless shuts down on it's own.

Check to make sure wire is solid copper thermostat wire and not stranded

SOURCE: Originally was receiving error code

Code 13 is oxygen depletion check the wiring to the oxegen sensor

SOURCE: My water is not heating

Electric water heater can be repaired by homeowner
If I understand correctly, water heater is producing no hot water.
Reset button clicks in, but still no heat.

Three initial things can be causing this:
1) No power to water heater. Test across top two screws on upper thermostat for 240V. Red and black wire are attached to these screws. Screws are marked 1 and 3. Upper thermostat is same that has reset button. If power tests 120 across screws, then breaker may be off on one leg. If no power, then start testing at breaker.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

2) Upper thermostat is bad. Replace thermostat with generic upper thermostat available at hardware store.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

3) Upper element is bad. Test and replace element using Rheem 4500 watt element available at hardware store. Image shown below. Look at label on side of tank for element wattage. Rheem puts 4500 watt elements into all their 240V electric water heaters.
http://waterheatertimer.org/How-to-test-water-heater-element.html

If upper element is burned out the tank will not heat any water.
http://waterheatertimer.org/How-it-works.html

Rheem 4500Watt element


Upper thermostats are generic and interchangeable:

SOURCE: no hot water, replaced both elements, replaced

I had this same EXACT problem. I replaced both elements, and STILL no hot water. Power to everything, but no hot water. So, I got online, and got educated.

THEN what I did was to replace the Thermostats, both the top (master) and the bottom. Cost was reasonable, I think all told it was 24.00 for both elements, and about 40.00 for both thermos. Got the thermos at the same plumbing supply store where I got the elements.

This fixed my problem, I now have great hot water.

The way the water heater works is that the top thermo is the master, and when the water needs to be heated, it turns on the bottom one first. This is because that when you turn on a tap, the hot water is taken from the TOP of the tank, and the new water coming into the tank comes in to the BOTTOM. This means that during normal operation, the water in the top of the tank will pretty much stay hot, and only the water in the bottom will need to be heated. Your bad Thermostats are not reading the temperature any more, so it isn't sending power, (or rather grounding) the circuit to the elements. Since the entire tank is cold, it is probably the top one that is bad, since it turns on the the lower element (or, I should say, the lower thermostat) FIRST. The elements always have power, the switch in the thermos just works by grounding, and thereby completing, the circuit.

Either one of your thermos could be bad, or both. I thought for myself that if one went bad, then it won't be long till the other goes bad too, so I just replaced them both. On mine, they just snapped out, and snapped back in, they just kind of sit against the tank, and there is only 4 or 5 wires to hook back up.

I am assuming that you are competent to do it, since you already know how to replace the elements, and obviously have a voltage tester to see if you have power everywhere you supposed to have it. One thing, too, make sure that both of the Temperature Stettings on the thermostats are exactly the same. You do not want one to read 125, and the other to read 150, or whatever. Set them both to the same temp.

This should fix ya, and it is a heck of a lot cheaper than a whole new water heater. The only reason to completely replace an electric hot water heater is if the tank has rusted through and is leaking, or if it has filled up with sediment. Or physically damaged in some irrepairable way. Other than that, this cheap repair should do it.

Good Luck To You.