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# Thermostat voltage and resistor value

#1
11-21-19, 09:02 AM
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Join Date: Dec 2014
Posts: 230
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Thermostat voltage and resistor value

Hello all,

I have a few questions regarding the testing the thermostat.

When the thermostat cover is removed, the voltage between Y and R should be ~24V AC , the voltage between Y and C should be 0, and there should be continuity between Y and R.

Q1> Since there is a continuity between Y and R, but the Y has no voltage, so it is true that there is a large resistor between the Y and R?

Q2> When the thermostat cover is installed back and the unit calls for cooling, the Voltage between R and Y should be 0 and Y and C should be ~24V AC. Is that correct?
Is it true that somehow the thermostat shorts the Y and C when it calls for cooling?

Q3>Unrelated question.
When I measure the wall outlet, the voltage between hot and neutral is 120V. The voltage between the hot and the ground is 120V while hot has no continuity to ground.

When I measure the voltage between R and myself. Why there is no voltage?
Why not 24V?

Thank you

#2
11-21-19, 09:45 AM
Member
Join Date: Jun 2012
Location: USA
Posts: 1,471
with cover removed there should not be continuity between Y and R. Normally Y goes to the contactor in the outside unit which also has a connection to C to activate its coil.

Thermostat connects Y (and usually G) to R with a cooling call.

Measuring between R and you (or to most anything else) is meaningless. Sometimes will get voltage if C is connected to ground at the HVAC unit but that is not always the case. Same issue when birds sit on a high voltage line and don't get fried..

#3
11-22-19, 07:57 AM
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 62,067
while hot has no continuity to ground.
Be SURE..... you never try that with power on. Your meter could explode.

Continuity (ohms) should never be measured in a powered circuit.