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Old 04-09-2012, 07:41 AM   #1
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24v not getting to termostat


Lennox split unit. 240v getting to and from contactor inside the condenser but not getting the 24v on the side posts. Condenser works when the reset button is depressed.

At the circuit board: I have power getting to board via the black wire through the cover switch and a small amount of power (like 10v) getting to the board via the white wire but do not know where to follow it from there. Also, the diagnosis lights are completely off (no flashing sequences of any kind.

and, not getting any power to the thermostat. I've tried bypassing the thermo by connecting the green and yellow wires and by running off of batteries.

Any suggestions?

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Old 04-09-2012, 08:44 AM   #2
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24v not getting to termostat


Power is out at the furnace control board. You need a good volt-ohm test meter. Shut off the incoming power to the furnace. Before restoring power check to see if there is a fuse or a circuit breaker on the transformer or on the control board. If the fuse is blown or the breaker is tripped, most newer Lennox's have a rocker type breaker and there will be a white tab showing. Don't replace the fuse or reset the breaker at this time. Go to the 24 volt terminal strip where the wires from the thermostat and the wires to the outdoor unit connect. Find the wires 'Y' and 'C' and remove the C wire from the terminal, OHM across the two without putting your fingers on the bare part of the wires otherwise you will be part of the circuit and we don't want that. You should get measureable ohms which means that the contactor coil is OK. If you get a very low reading like 5 ohms or very near what you see when you cross the test leads of your ohm meter together there is a short or the contactor coil is burned out. At the outdoor unit shut off all power to the unit, either the circuit breaker or the disconnect, Check both sets of high voltage wires to make sure that they are dead. Pull one wire off of the contactor coil on the sides of the contactor and ohm the contactor coil terminals. These are the thermostat wires that connect with spade connectors. If the reading is either infinity or near zero the coil has failed and the contactor must be replaced. If the reading is not, say you get a reasonable measureable ohm reading, ohm the wires going back to the house, there may be a short and just the wire can be replaced. If the contactor ohms ok the problem likely lies with the furnace transformer. Kill all of the power to the furnace and check to make sure it is dead. Find the furnace transformer, ohm both the line voltage and the 24 volt leads of the transformer. Again, you should get measureable ohms on both sides. If you should get infinity or zero, the transformer is gone and needs to be replaced. After replacing the failed part replace the fuse or reset the breaker and power up the furnace and try the operation. Just to be clear, when I say infinity I mean that the needle or the readout either doesn't change or the reading is very high like 25k or 250k if your meter reads that high. Hope that helps. By the way, another trick that I have done is, when you are picking up parts buy an automotive type in line fuse holder and a 2 or 3 amp fuse and install it on the 'Y' terminal at the terminal strip. If it blows you have just saved yourself a whole lot of troubleshooting and you won't kill the whole system if the contactor fails.


Last edited by GarryE; 04-09-2012 at 08:54 AM.
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Old 04-10-2012, 08:58 AM   #3
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24v not getting to termostat


Thank you GarryE for your expertise. Really appreciate it. One question: What if the fuse on the circuit board is NOT blown?
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Old 04-10-2012, 10:52 AM   #4
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24v not getting to termostat


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Originally Posted by mojabe View Post
Thank you GarryE for your expertise. Really appreciate it. One question: What if the fuse on the circuit board is NOT blown?
Then it almost has to be the control board because the diagnostic lights should be operable. I should have asked what model you have but you said "lights" so I assume it is a late model unit and there are either two red or one red and one green light, they should be on when the furnace is on. Now, you are closing the safety interlock switch, correct? That black tab that is held in by the blower cover. If you have power to the board and you have done the above service checks and it still doesn't fire, it's the board. Be sitting down when they tell you the price!
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Old 04-10-2012, 11:03 AM   #5
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24v not getting to termostat


Thanks again GarryE. Yeah, the intertlock switch is engaged. I checked power before it and after it (both engaged and disengaged) and that seems to be fine; meaning power is getting to the board when the switch is engaged, etc.

I have not done the above mentioned diagnostic routines yet.

The fuse that I am speaking of is an automotive-type 3amp (maybe 5) and is on the board itself.

There are 2 green diagnostic lights (both off).
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Old 04-10-2012, 11:37 AM   #6
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24v not getting to termostat


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Originally Posted by mojabe View Post
Thanks again GarryE. Yeah, the intertlock switch is engaged. I checked power before it and after it (both engaged and disengaged) and that seems to be fine; meaning power is getting to the board when the switch is engaged, etc.

I have not done the above mentioned diagnostic routines yet.

The fuse that I am speaking of is an automotive-type 3amp (maybe 5) and is on the board itself.

There are 2 green diagnostic lights (both off).
Ya, do the tests, don't guess, eliminate one thing at a time. The fuse is usually a 3 amp and can sometimes be hard to find outside of a heating contractor most auto stuff bottoms at 5 amps. If you have the installation packet, in the back of the manual there is a diagnostic tree that you can use to help you out and refer to the schematic diagram as you work so you can familiarize yourself with the correct flow of power. Your meter is your most important tool. The best is one that has volts, ohms and an amp clamp on the end. You don't need a $350.00 Fluke but buy the best quality one that you can reasonably afford and practice with it and learn how to use it. If you don't use it all of the time take the battery out. I got to buy myself a new one on account of that!!

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