How do I connect a Honeywell RLV310 wall thermostat between main circuit box and a 240 volt baseboard heater using 12-2wg wiring?

I should like to hope I have your answer.
If yours is like most U.S. model, it is called a 220 volt unit.
That is not really 220 volts, it is really 2 separate 110 volt units.
What that means is, there are actually 2 separate 110 volt hot wires to the water heater, usually one hot wire is black, and the other hot wire is red.
One is for the upper heating unit, one is for the lower heating unit. Perhaps your lower unit has blown the circuit breaker for that line.
I mean the circuit breaker in the main fuse box/circuit breaker box.
Did you test the lower wiring to see if the hot wire is hot?
If it is hot, be sure to test the ground wire, and also the wiring between the bottom and the top of the heater inside it.
God bless your efforts.

Test across top two screws on upper thermostat for 240 Volt
Press reset button located on upper thermostat
Reset circuit breaker
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#new

http://waterheatertimer.org/images/Upper-thermostat-w-wire-a-2.jpg

Otherwise elements will test 120 Volt to ground at all times.
http://waterheatertimer.org/See-inside-main-breaker-box.html#main

http://waterheatertimer.org/How-to-reset-circuit-breaker.html

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

My limited research indicates this heater is designed for USA electrical systems. That would mean single phase, 240/208 volts, 60 Hz. The drawings I reviewed shows a 2 pole, 240 volt breaker feeding each heating circuit. That means no "neutral wire" goes to the heater and therefore no 120 volts is needed. This is good. Kenya's system is single phase, 240 volts, 50 Hz. The heater should work reasonably well on the African system. Connect the power just like you would in the US.

It sounds like upper or lower thermostat.
Thermostats turn off one leg (or one wire) in the 240Volt circuit, so each elements will test for 120V even when thermostat is off.
When thermostat turns ON, it closes circuit by adding the second leg to 240V circuit, and element turns ON.
http://waterheatertimer.org/How-to-wire-EH40-hot-water-heater-timer.html#120-240
http://waterheatertimer.org/How-water-heater-thermostat-works.html

Take 30 minutes and troubleshoot before replacing parts:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

You have electric water heater.
You replaced elements and thermostats but heater still won't work.
And tank was completely full of water, and water was running out of a faucet before turning power ON.
If elements are not fully immersed in water, they will instantly burn out.
If upper element is burned out, then tank will have NO hot water.
If lower element is burned out, and upper element is not burned out, then tank will have some hot water.
http://waterheatertimer.org/How-to-test-water-heater-element.html
http://waterheatertimer.org/How-it-works.html

If elements did not burn out, then that leaves 1 main suspect: the electricity.

1) Push in ECO red reset button on upper thermostat. If ECO is tripped out, then heater will not work.

2) Click circuit breaker all the way one direction and then back the other direction to make sure breaker is latched.

3) If tank has 2 elements, then it is 240volt.
If 1 leg of 240Volt circuit is not working, then 240V appliance will not work.
240Volt circuit needs both hot wires to complete the circuit:
http://waterheatertimer.org/How-to-wire-EH40-hot-water-heater-timer.html#120-240

Buy cheap multimeter from Home Center and set dial to 240-250-277VAC
Tape tester leads to woods sticks to keep hands away from power.
Power is ON
Test across top 2 screws on upper element should show 220-240 volts.
If you have 240 at top two screws, then skip down to step 6.
If no reading across top two screws, then move to top of tank.

4) Open connection point where wire connects to heater.
Test across both insulated wires should get reading of 220-240 volts.
If no reading, move to circuit breaker box.

5) Mark water heater breaker.
Turn off all breakers except water heater breaker and main breaker. (and breaker to light fixture where box is located)
Do not hold or touch or lean against anything metal.
Stand on dry boards and wear dry clothes.
If main breaker is OFF, the big wires that connect to main breaker are still HOT.

Remove box cover.
240Volt breaker has 2 wires connected with screws.
Test across both screws for 240Volt.
See if wires are loose.
If there is no reading, then breaker is not working on one leg.
To double check, move water heater wires to another same-size 240V breaker and then put ear against water heater to listen for bubbly-fizzing sound.

6) If you are getting 240 to water heater, then wire inside tank could be loose.
Do 30 minute troubleshoot:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on timer.

Breaker could be bad.
Let's do a quick check and save you money.
Open top cover on water heater and push in red reset button that might be tripped.
If reset is tripped, water heater should turn ON.
If reset will not reset, then replace upper thermostat:
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
If reset is not tripped out, then it is OK, so test two top wires on upper thermostat.
Use cheap multimeter from home center, and set dial to 240-250-277 VAC.
Test across top two screws on upper thermostat to see if you have 240Volts.
To do full troubleshoot on ordinary tank electric water heater takes 30 minutes and multimeter
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

If water heater is not receiving 240V, then the breaker is suspect, or a wire between breaker and water heater is loose.
See basic water heater circuit
http://waterheatertimer.org/240-v-water-heater-circuit.html

Now let's look at circuit breaker:
Mark suspect 240V breaker.
Identify all your breakers, and also mark another 240 Volt breaker that is same amperage as water heater 240V breaker.
Turn off main breaker.
Take front panel off breaker box.
The wires and lugs on main breaker are HOT, so stay away from that area.
http://waterheatertimer.org/See-inside-main-breaker-box.html
Suspect breaker has 2 wires coming from breaker.
See if one or both wires are loose, and then tighten.

If wires are tight, then move these wires to the other 240V breaker that is rated for same amps as water heater.
Now turn on main breaker.
Put ear against water heater tank to hear bubbly fizzing sound.
This will identify if circuit breaker is bad, or if water heater needs a full troubleshoot.
It is not a good idea to run two 240V appliances on same 240V circuit breaker, so you may have to decide one or the other appliance until you can replace breaker.
Instructions to replace circuit breaker:
http://waterheatertimer.org/How-to-replace-circuit-breaker.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on water heater circuit.

You have water heater with two elements.
Two-element water heaters are 240Volts.

1) Lets talk about what makes a 240Volt circuit
See basic water heater circuit:
http://waterheatertimer.org/B220C.html

Open link, and illustration at top of page is your water heater circuit

Notice that two insulated wires come from the circuit breaker and go to the water heater.
These two wires are 120volt each. When these two 120Volt wires combine, they create 240Volts
These are called hot wires because they come from the hot busbars.
There is also a bare copper wire that comes from neutral busbar.
The bare copper wire is a safeguard, and is not necessary for the 240V circuit to work.

Inside each circuit breaker box is 1 neutral busbar, and 2 hot busbars.
Open image below, and it shows typical residential breaker box
http://waterheatertimer.org/images/Inside-Main-Breaker-Box-420.jpg

In a residential home, each circuit takes two wires to complete the circuit.
For 120Volt circuit, you need 1 neutral wire and 1 hot wire to complete the circuit. These are usually a black and white wire. The bare copper wire is a safeguard.
For 240Volt circuit, you need 2 hot wires to complete the circuit, and each hot wire comes off a different busbar. These can be a black and white wire, and sometimes a black and red wire. The bare copper wire is a safeguard.

With a 240Volt water heater the two hot wires connect to the black and red wires located on top of water heater.

2) Let's talk about wiring inside a water heater.
http://waterheatertimer.org/images/WH-w-combined-530.jpg

Open image on link above, and it shows your 240volt water heater wiring.
The 2 hot wires from breaker box connect to the red and black wires on top of heater.
Inside each water heater, the wires are color-coded and will appear the same, or nearly the same, as shown on link above.
As long as your wiring appears like the image, and the hot wires from breaker box connect correctly, and the circuit breaker is working fine, then your water heater will work.

Add a comment any time.

3) More electric water heater links about water heaters, thermostats and tank wiring:
http://waterheatertimer.org/How-electric-water-heater-works.html
http://waterheatertimer.org/How-water-heater-thermostat-works.html
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html

Your testing may be off here. Or you might have special order 'simultaneous' wired water heater.

Water heaters with two elements are 240V.
Elements test for 120V at all times, but with ordinary non-simultaneous water heater, only one element at a time has 240. This is called non-simultaneous operation.
Open links below for understanding of water heater operation.
http://waterheatertimer.org/How-it-works.html
http://waterheatertimer.org/How-water-heater-thermostats-work.html
http://waterheatertimer.org/B220C.html

If both elements were drawing 240 at one time, then upper thermostat is bad, or else your water heater is wired for simultaneous operation, and one of the elements is burned out.
http://waterheatertimer.org/How-to-test-water-heater-element.html
Your tank might be wired for simultaneous operation, if water heater was special order from Rheem, or if somebody changed wiring and set lower thermostat-and-element on a separate circuit breaker.

If you have simultaneous operation, then you would have 2 circuit breakers and 2 wires going to water heater or one 50 amp breaker with 8 gauge wire to heater. If this is the case, then both elements could test for 240V and one of the elements is bad, and thermostats are ok.

If you have ordinary Rheem water heater, then it is wired for non-simultaneous operation where only one element is ON at a time. This is called non-simultaneous operation and you can check because water heater has one circuit breaker rated for 20 or 30 amps.

Rheem elements are 4500Watts.
If both elements were ON at same time, that would be 9000Watts, which would trip typical water heater 20-30 amp circuit breaker.
9000Watts divided by 240 volts = 37.5 amps
So if upper thermostat is bad, and it is turning on both elements so they both test for 240, then thermostat is bad, but also one of the elements has to be bad.
Or test is inaccurate.
Interesting puzzle.

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Your water heater has 2 elements so its 240Volts.

1) If breaker is delivering power on one leg of the 240Volt circuit, then water heater can appear to have electricity, but tank is receiving 120V. Both 120Volt legs are needed to complete the 240V circuit. Double check for 240Volts.

2) Review electric Load.
Water heater label shows wattage of tank.
If tank is 4500Watts for example, you installed 3500 or 4500 watt elements.
With 5500 Watt elements you need 30 amp breaker and orange-colored 10 gauge wire
With 4500 Watt elements you can get by on 20 amp breaker and 12 gauge wire up to 75 feet away.

3) Check ECO red reset button on upper thermostat. If reset is tripped, then ECO is reading high temperatures. Suspects at this point are defective upper thermostat, shorted wire or bad lower element.

4) Also note that insulation and cover must be re-installed over thermostat. Thermostat reads temperature through wall of tank. If outside air circulates around thermostat, that can cause a mis-read and cause tank to overheat and ECO trips.

If you don't want to test the heater, replace both thermostats for about $25. Return the upper thermostat for a refund. Then see of problem persists.

Here's what I would do.
1. Check breaker for 240
2. Check water heater for 240 across 1 and 3 screws on upper thermostat
3. Note wiring and breaker size.
4. Note label on side of tank.

5. Look at end of new element to see wattage printed on each element. Look for sings of high heat and burning. Tighten screws on elements.

6. Check ECO reset button. Look for high-heat and burning. Tighten screws on thermostat.

7. Test upper thermostat as follows:
http://waterheatertimer.org/images/Electric-diagram-water-800.jpg
Test left-side #1 screw to right-side middle #4 screw. Test should read 240V at all times. If not, then ECO is tripped, thermostat is bad, or electricity is OFF

8) Water heater is cold so upper element should be ON. Test upper thermostat.
Test right-side #3 screw to left-side bottom #2 screw and it should read 240V
Test right-side #3 screw to right-side bottom #4 screw and it should read 0 volts

8. If upper thermostat checks out. Test upper element for 240 across both screws. If upper element does not read 240 when water is cold, then element is bad or there is a shorted wire between element and upper thermostat.
How to test element:
http://waterheatertimer.org/How-to-test-water-heater-element.html

9. Do a full troubleshoot on electric water heater
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

The controls of electric water heaters are designed so that at no time are both the top and bottom heating elements energized. Nearly all electric water heaters of this capacity in the US & Canada (other places, too) operate on 240 Volts.

When the water in the tank is below the set point of the thermostat (in your case - 120 degrees), the top heating element is expected to be on - (unless there is an issue with the top thermostat or limit switch). The top most control is the "high temperature limit". It is identified by the reset button on it. Make sure this isn't tripped by depressing the button. If it clicks - it was tripped and should start to make hot water at this point. If not tripped, you should check for the presence of 240 Volts between the heating element terminal screws. Do not measure from ground to a terminal screw and believe 120 Volts is "good". To make heat, you need 240 Volts - not 120 Volts measured across the terminals - not to ground. The amount of heat created running at 120 Volts is only 1/4 of what it will do at the correct voltage.

If you don't measure 240 Volts on the top element, check the bottom element in the same manner described for the top element.

If unable to measure 240 volts on any element, either there is a problem with the power source (blown fuse or circuit breaker), high temp limit switch, or thermostat(s).

If 240 Volts is present on either heating element, and water is not warm / hot in 30 minutes or so, a defective heating element is suspect. You can change controls without draining a tank, but replacing elements will require draining the tank first. Do not power the water heater without first filling it.

You can read a very detailed "how to" article about checking water heaters here.

I hope this helps - and good luck!