24 volts therm wire

I have a ? I went with a friend on a service call for air condition, anyway
he explain to me when he hooked up volt meter from air handler red to yellow
which is power to cool anyway he touched his meter to red to yellow and it read 24volts then says to me it is open at 24volts and when switch is closed
it is asking for cool and it reads zero because it is the same wire and there is
no difference . WHAT!! what is difference ,well i dont get it cause if the swith is on then shouldn’t it say 24volts he says i have it backwards . when you hook up to a outlet it reads 120 and that means you have power .i am confused can someone out there explain this better to me to understand? to me what i am getting is that when a swithc is on it should read voltage and read zero when it is off thanks. you HS are great people


Lets see If I can explain your red wire is the hot side 24 volts between red and the neutral side of the step down transformer normally will be black as neutral at the transformer low voltage side. Red, White, green, blue and orange are all the same leg or the hot side of the of the transformer at the thermostat. Neutral from the transformer does not normally travel to the thermostat unless a set back type of thermostat. The best way to think of a thermostat as being a switch for the incoming hot red wire and being simply switched by the stat to what ever mode you desire yellow for cool white for heat green for fan. Blue and orange is for auxiliaries such as heat pumps.

Controls for A/C units and furnaces basically fall into to categories they are basically a load, a switch or a combination of both such as a time delay relay.

A load requires both a hot and a neutral in order to energize just like a 110 volt wall outlet is a load. A light bulb is a load but it has a switch in one leg to control off and on.

A thermostat is a switch the gas valve is a load a Relay has a load and a switch. All safeties on heat and air units are a simple switch.

Did I confuse you:shock:


Your friend has a weird way of troubleshooting but it probably works for him.

When he put his meter from red to yellow he is getting the 24V from red and a low resistance to the 24V transformer neutral through a circuit or relay coil from the yellow wire so the meter reads 24V.

This is the way some people check fuses with power on, they just place the meter across a “powered” fuse, zero volts = good fuse and full power reading = bad fuse. This method requires the right type of load circuit or it can fool you.

Most people would put their black meter lead on the low voltage neutral black wire and the red lead on the yellow wire to see if the yellow(cooling) is energized via the thermostat with 24V. The first thing is to check the red wire to neutral and make sure 24V power is even present.

The reason he does this is that there is not always a neutral wire to all t-stats. Heat pumps have lights. Programmables need power to run. Others don’t have a neutral so you must use the “known” circuit to get it.

Actually this way of checking gives you a lot of information “at the t-stat”. You can test the relays, sequencer heaters, RV Coils and contactors on heat, cool, fan and reversing valve control circuits. Not to mention if the transformer is producing.

Rafael, Do not confuse your self trying to compare testing line circuits like control circuits. You must fully understand the circuit your testing or you may be wrong.

Thanks guys i really appreciate it . Bottom line is
thermostat wire red to yellow when hooked to vmeter and calling for cool
the red and yellow wires are one the same and closed which to me closed should
mean power in circut and therefor should read 24v not zero. why does it read zero?
it has something to do with the difference in voltage
Thanks to you all

Example “B” in my photo;

The meter has resistance through it, being the device it is.

If the switch (which has less resistance than your meter) is closed, current passes through the path of least resistance (the switch).

No power through the meter, nothing to measure.

Not to confuse things, but if the switch is bad (but still is working), You may get a low volt reading on the meeter from the current that can’t get through the bad switch with a higher resistance than your meter.

It has nothing to do with the amount of voltage, the voltage could be any range that you are measuring. A volt meter has to read **between **two points either hot to neutral or an open point on the hot leg. Let take two wires could be 110 volts or 24 volt and place a fuse box in the circuit going to a load say a 110 volt motor or a 24 volt relay with a coil as the load. With the fuse in place and the circuit energized reading across both sides of the fuse will read zero because you are not reading back through the load.

Remove the fuse and read the voltage on the L1 (inlet) side of the fuse box across the open fuse to the outlet side which allows you to pick up a neutral through the windings of the coil or motor thus you will read voltage up to the inlet side of the fuse.

Sorry David you posted while I was typing did not see you on line.

I get it now thanks you all i really appreciate it…

You better get it with us doubble tagging you! :slight_smile: lol

Yah i dont get the voltage reading 24v when switch open either
and zero when switch is closed . dont feel bad i still am at a lost
with difference in voltage when you hook volt to 120 it reads 120
and theres power. but i am not in this field just a plumber

If you have no 24V power, look for a small fuse in the airhandler, or sometimes in the exterior unit.

edited out errors…

Are you 100% sure you want to do that you have me lost on that one:shock:

sorry, mixed up my systems, comes from having worked on equipment from 4 different countries, it is 24 VAC on hvac low voltage

Not a problem we all get mixed up some times:D