Electrical Wiring in the Home/Thermostat Replacement


Hi Greg, I'm replacing a Mears M21 DP with a Honeywell Heat Only Non-Programmable CT410B thermostat. I'm fairly certain I have the correct replacement. I expected it to be easy, just rewire it the same way that the old one is. My issue is that the it's hard to tell which wires replace which. All 4 wires on the Mears come out of one end with the two black ones in the center. On the new stat the wires come out of the sides, both red on one side, both black on the other. The diagram references which is in and which is out but I can't tell which is which on the installed old one. I hope I have explained things clearly. Thank you,


There are a couple of ways to check which two wires are the line voltage and which two are the load wires.

One involves checking A/C voltage and the other involves checking Ohms.

Checking the line voltage is dangerous and should only be attempted if you feel qualified/knowledgeable enough to do so. I will give you both options and you can choose which one to do, or you can call an electrician.

1. LINE VOLTAGE: With the circuit breaker off separate all four wires in the box/wall and do not allow them to touch anything. Get your A/C voltage meter ready and set to the highest A/C voltage setting. Turn on the circuit breaker and touch your probes to the different wires until you get a reading of approximately 220 volts. The two wires that give you a reading of about 220 volts are the line voltage wires. Remember which ones they are and then turn the circuit breaker off. Of the two wires marked as line voltage, they will connect to the two line voltage on the t-stat. It does not matter which one because it is 220 volt A/C. The other two wires will connect to the load wires on the t-stat.
Put the t-stat back in place and turn on the circuit breaker. It should work now.

2. OHMS: Turn off the circuit breaker. Check for line voltage with the meter in the A/C voltage position. You want zero voltage present when using your ohm meter. Take the two probes on the ohm meter (just put your multimeter in the ohm position) and start touching wires until you get a reading between two wires. The actual reading will depend on the wattage of the heater. But you are looking for somewhere around the 20 ohms give or take.
You should only have two wires that give you an ohm reading and these two wires are the heating element wires. The other two wires are the Line voltage. Wire the t-stat accordingly and turn on the circuit breaker.

Hope that helps.

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Electrical Wiring in the Home

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Greg Hughes


Troubleshooting electrical including appliances and HVAC.


34 years of experience.

Both college and tech schools in this field.

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