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BillyHvac, Journeyman HVAC Tech
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Hello. Our air conditioner wont turn on, and I think the problem

This answer was rated:

Hello. Our air conditioner won't turn on, and I think the problem is the Tempstar furnace, because even when I put the fan switch (on the thermostat) from auto to "on" the furnace fan doesn't turn on.

The breaker in the house breaker box is on, and I'm sitting on the floor in the basement with the two panels off the front of the unit. My first guess is to check the power connections coming into the bottom left of the unit and which run into the box that has the push-in switch that needs to be in for the fan to run (that is held in by the lower panel). However, I can't get the ends of my electric meter to contact where these things plug in, and before I start pulling wires out, I thought I'd ask for help, since I'm sure I'm going to need it one way or the other before this is over.

(And shouldn't I be able to read like 25 volts from the WYRGC wires on the left side of the aforementioned box? Those are the wires FROM the thermostat, correct?)

Thanks for any help.


Remove the 2 doors and tape the door safety switch down securely.

Then follow the wire from the door safety switch to the circuit board..we want one meter lead here. The other meter lead to cabinet ground.

You are correct about R,G,Y,C etc....R to C should be 24v

Customer: replied 4 years ago.

That was fast. :)


The door safety switch has a black wire (hot) going into it (bottom of switch), a brown wire that looks like it comes out and goes up into the furnace area and connects to a small black box that says "blower control" on it, and then the top connection has TWO black wires, one that goes to COM on what appears to be the back side of a black box that the WYRGC thing is attached to, and the other to a clear plastic connection thing on the right side of the door safety box.


Earlier I turned off power at the breaker box, pulled the bottom black wire, and turned the power back on and the meter read 120V (well 116.4) for that wire and where the white wire plugs into the back of that box, so I assume the power is okay going to the furnace. I have since reattached that hot line. With the button pressed, I get NOTHING from R to C (or anything else on WYRGC to anything else).


I apologize for being confused. I don't see anything that goes to a circuit board?




the only way you will not have a circuit board is if the furnace is older.

is it similar to this

Customer: replied 4 years ago.

How old is old? I can't find a manufactured on date. We've lived here for 15 years or so, so it is at least older that that. I do not see anything that looks like what you posted (although the "view full image" doesn't work for me).


I did follow the black wire over to where it plugs into the back of the WYRGC thingy, and it read 120 volts with the other lead grounded on the box. However, the wire coming OUT of that thing does NOT read any voltage on it. Sorry that I'm being difficult. Should I take a photo?



A photo would confirm...but it sounds like you have a bad 24v transformer.

Post a pic and I can confirm..OR..just give me model number.


Customer: replied 4 years ago.

I think this is the model number: tempstar nugk100dh08


These photos are horrible:


24 volt transformer is what WYRGC is plugged into?

Ok, if you follow the C terminal wire it leads to the transformer. We need to measure the transformer voltage as close to the transformer as possible.

Here are schematics in this manual for you

Customer: replied 4 years ago.

Okay, if I understand you... I removed the two screws holding the brown WYRGC board so that I can get to the back of it. Behind the C are three wire plug-ins (I also have the wiring diagram on the back of the upper panel and have been looking at that.).


C has three wires attached to the back of it. A white wire that goes up into the furnace area and attaches to the gas shutoff thing (?). Another white wire that disappears in the spaghetti of wire inside the safety shut-off box. And a yellow wire that goes into the COM port on what I think is the transformer in question.


Read lead on the yellow wire/COM and black lead to the metal box (ground)?? I get 0.0 on 20 V DC and 120 V AC settings on my little digital reader thing. Am I close?

Is this close to what you are seeing...?

The R wire and C wire eventually end up on the transformer as that is where 24 is created.

Once we identify transformer it will go fast.

Customer: replied 4 years ago.

Yes. Similar. Half sticks out of the box with the cutoff switch and half is inside the box. The outside has COM (yellow wire to W), then says 40VA, and then has 25V with a black wire that runs up into the furnace and connects to the "blower control." The "inside" of it has COM, PRI, 120V. The 120V reads 118 volts, the COM on the outside reads nothing.

Great...almost there...the black wire...that is what we need tested.

24v = transformer good.
0v= bad transformer.


Customer: replied 4 years ago.

The big black wire, on the "outside" of the transformer, that reads "25 V" has no power. I set my meter on both 200 ACV and 20 DCV and 200 DCV and it read all zeroes.


Is my furnace ancient? Will I still be able to order one of these things?





The good news is that transformers are generic.

any transformer that inputs 120v and outputs 24v will work. They are very affordable also.

You can even get one from a local heating/cooling company.


Customer: replied 4 years ago.

While I can't say that I'm thrilled that the A/C won't be operational tonight, I'm fricking awesomely amazed that you helped me through this. I'll take this one out tonight and see if I can't find one in town somewhere tomorrow.


You rock, Billy. I wish I could send you a beer through the Internet.


My family thanks you as well!


Thanks again. Phew!



You are very welcome and I really wish you well on the repair. Just stop back at this same thread if you need any follow up at all


BillyHvac and other HVAC Specialists are ready to help you
Customer: replied 4 years ago.

Uh oh. Sorry to bug you again, Billy.


So I went to Menard's today (because I was in the neighborhood) and got a 120 volt --> 24 volt transformer. By the looks of the documentation, it seems like it is for doorbells.


Instead of two pluggable terminals on each side, the 24 volt side had two screws and the 120 side had two black wires and a green wire. I grounded the green wire, attached one black wire to the white wire that went into the old transformer, the other black wire to the two black wires that together went into the other "port" of the transformer, and then attached the Yellow and Black from the other side to the two screws on the 24 volt side. Voila! The fan turned on!


Then I turned the fan off at the thermostat and put the thermostat on "auto" (fan) and "cool" and turned down the temperature. Nothing. I figured I'd go to work for the rest of the day and mess with it tonight.


So we get home just now and now the a/c doesn't turn on NOR does the fan turn on. The transformer is HOT to the touch. So I turned off the power and unplugged the two black wires from the 120 side.


Will that transformer not work? Did I screw something else up?


Again, sorry to hassle you with this again....




this means we know what the part affected is...but there is an existing short.

Odds are it is inthe wire that runs outside to unit, or the contactor in the unit. Can you follow the thermostat wire from Y and C out to the unit and look for any damage...weedwacked, chewed, pulled etc...
Customer: replied 4 years ago.

I do not see any damage. A month ago the neutral and one of the hots going to the a/c unit broke/melted because (I think) when the last guy that worked on it installed a new (transformer, or something) they didn't tighten down the wires very well.


So I put a new box outside, ran the old power into the box, new breaker, new 10/3 from the box to the power connections in the unit (with no splices in the way, like it was before). In doing this, I no longer had the 24 volt pieces going straight there, so I put in two new wires from the new box through the flex conduit and into the a/c unit. All of those connections still look good.


Is this something where I can put all the stuff back together again downstairs and make it hot and turn the thermostat to cool and see if I'm getting 24 volts at the outside unit? This is such a pain in the neck... :)

Try this first...unhook Y and C at the indoor unit..turn power back on..turn fan switch to on.

Tell me if fan runs

Customer: replied 4 years ago.

No. Can I possibly have burned out this new transformer already? There is 120 at the emergency shutoff switch, which then hooks into the transformer. Should I be able to get 120 from the other wire on the 120 side of the transformer where it plugs into the little board thing in the back? (It's a/c so it shouldn't matter which of those black wires goes to the power at the switch, right?)


There is no 24 volt coming out the other side of the transformer. :(

should have 120 in.

Is the emergency switch you mention the door switch that pops out?

Customer: replied 4 years ago.

Yes, the door switch. Sorry.


Transformer reads 120 in and nothing out. Aaargh. I have to go up to Chicago for the day tomorrow and won't get to mess with this again until Saturday afternoon. So it might be early next week before I reply to this again. I'll get you more money if I need to--I very, very much appreciate the help. Until then, have a good weekend.

Yep the transformer is bad.

next time get a transformer with a small breaker built in.

It has a button or rocker switch that trips instead burning out transformer

Customer: replied 4 years ago.

I could not find a transformer with a breaker in it, so I just bought another 120 -> 24 volt doorbell one from Menards. I also bought 50 feet of new thermostat cable.


Installed new transformer. Furnace fan now works when thermostat is turned on. Still nothing out of the air conditioner, even with new wire run directly from Y and C (out the basement bathroom window) and to the unit.


I took the thermostat off, and if I short R and G, as expected, the house fan comes on. If I short R and W the heat goes to start. If I short R and Y I get a small spark and nothing else happens.


Is that transformer thing supposed to be getting warm? The last one was hot after it had sat all day with power to it--the new one is getting warm as well when the power is on.


I read 24 volts both behind the thermostat and in the furnace where the wires come back from the thermostat.


I'm running out of ideas. Hopefully you are somewhere cool today having a drink of your choice and not responding to these messages. :)


Thanks again for everything. I'm about ready to just call someone at this point and make it their problem. :)



Ok this is looking like a bad contactor. This is what the 24v 2 wire feeds in the unit.

Customer: replied 4 years ago.

Are those pretty generic as well or something that is more specific to each unit? I think that was replaced a couple of years ago, actually (that's the thing where the guy didn't tighten down the hot wires coming from the breaker panel tight enough and one of them shorted out....). That thing has a LOT of wires coming out of it.


Sorry to keep hassling you with this.



It looks worse than it is.

It is a 2 pole switch. 220v sits on one side waitng to cross the bridge. The bridge opens and closes with the 24v on the thermostat wire.

It is a generic part and can be replaced one wire at a time.

When I first started in hvac I just pulled one wire at a time and put on the new contactor

Customer: replied 4 years ago.

Okey doke. I'll find one on line that looks similar and go ahead and order it. Talk to you in a few days. Enjoy the rest of your weekend. :)

Would you like a cheat to get cooling in the meantime?

Customer: replied 4 years ago.

Ordered a 30 amp contactor and a 40 amp contactor, because I couldn't tell by the one that was on there which one I needed.


Short answer: it is running, and you're awesome.


Long answer (if you care): When the knucklehead that came and looked at it last was here, and charged me $145 to tell me that there wasn't power to the outside unit, I took apart the flexible conduit and found that indeed, both hots had melted through or otherwise corroded. I assumed this happened because both were loose and twisting around in the current contactor. I took the time to put an outside box on the wall, put in a 30 amp breaker, run the old wires to the box, and new wires from the box to the unit. The guy told me part of the problem was that the breaker in the house was 50 amps and the unit only needed 30 "and that extra power probably blew it."


So it worked just fine with the 30 amp breaker outside until our recent problem. Tonight I put in the 30 amp contactor because, again, I didn't know which to use. Put it together, shorted the wires behind the thermostat, and the unit turned on briefly before the breaker OUTSIDE (30 amp) flipped.


The contactor that had been in the unit was 40 amps, not 30 amps.


So I took out the breaker outside and just wire-nutted (technical term) the red to the red and the black to the black, then replaced the 30 amp contactor with the 40 amp contactor. Thermostat back on, flip to cool, turn the temp down, and VOILA!


It's been running for 20 minutes now. Once this 90+ degree heat goes away I'll probably put a shutoff back outside, but do 40 amps instead of 30. For now, I'm just going to let 'er run.


Thanks again so much for all of your help. And yes, swapping out the contactor was a piece of cake.


You rock, Billy. My family and I thank you.



I am very glad you got it going!

Especially in this brutal heat.

Take care,