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How to test for 24 vac between R and C

I have a problem with my carrier weathermaker 9200 furnace which causes the LED display on my carrier thermostat to go blank. Prior to the thermostat going blank it will flash E2 which indicates that there is a brownout condition or too low voltage to my thermostat. The manual says to double check wiring and check for 24 vac between R and C. In addition, rather than continuously running, my furnace continues to cycle on and off before getting to the correct heating temperature. I replaced the thermostat but am still having problems with the furnace running until it reaches desired temperature. How do I check for 24 vac and could this be the problem with the cycle time on the furnace?

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  • strummin4jc Jan 18, 2009

    Thanks for the help skeeter. The problem appears to be with my furnace filter. Although the filter did not appear to be that dirty, I removed it from the system and the blower is running very strong until the system gets to the desired temperature.

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Yes you are right the control voltage of your furnace is 24 volts. Really without a load on the transformer usually runs 28 volts then you put a load on it and it drops down to 24-26 volts. Check the voltage between the R terminal and the W terminal with the selector switch in heat mode. Or move the thermostat switch to heat. R to C would do it also. You could also do the voltage check right at the transformer one side of the transformer is 120 volts and the other side is 24. Look for the black and white wires that is the 120 volt side the blue and yellow or off colors is the secondary side.

Now if it checks low say 24 or 22 your gas valve needs min. 24 volts to hold open the valve that allows the gas to flow into the burners. So if one of the symptoms is related to burners come on for about 3 to 5 min. then shut down and the fan keeps going it is loosing the necessary voltage to hold open the gas valve.

One more thing make sure the air filter is clean as this will cause the heat to build up in the heat exchanger and hit the high limit safety device and cut the gas off. Until the air cools enough to allow the high limit to reset then the gas valve will open and start the process all over again. Dirty A-Coil will do the same thing as the air goes through the coil to get to house. let me know how it goes ken

Posted on Jan 18, 2009

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