I live in Michigan and have a basic natural gas furnace (not high efficiency). The brand is ComfortMaker. The AC unit sits outside and the brand appears to be AG. I think they were both installed in the year 2000 (i bought the house in 2003). I have had no issues or professional maintenance to date (lucky me).

The furnace works fine in heat mode. I tried the cooling out this spring and nothing. What I remember is when I would kick on the air, both the outside unit would kick on and the blower on the furnace would kick on. If I remember correctly, they would both do it right when the thermostat told them to or within seconds after. Now I get nothing on both. Here is some more relevant info.

I went outside and took the panels off the electrical parts on the AC unit. Everything looked fine. No mouse nests or chewed wires. What I could see was the simple red and white wires (thermostat wiring?) going to what was likely a relay and a large capacitor and the big 240V wiring. That was about it. I wiggled some wires and checked some things out. Nothing obvious.

If the AC was bad, I guess I would expect the outside unit to not work but the blower in the furnace to work. The blower does not come one.

Ok, one more piece of info. Last fall i Upgraded our thermostat to a new Honeywell WIFI unit. I did it in November so this is the first time I have tried to use the AC since it was installed. The heating works just fine. To eliminate the new thermostat possiblity, I installed the old thermostat. Same behavior. Heat work just fine - everything is dead on AC.

Based on how I have described the behavior, I am thinking something is wrong in the furnace circuit board. I guess if just the AC was bad, I would expect the blower to still blow. Or, is it smart enough to know that if the outside AC is not working, to not turn on the blower in the furnace. I find that hard to believe given I see just two "control" wires (red and white) going out to the AC unit. My thinking is those wires simply tell it to go on and off.

Thoughts anyone? Thank you.

 

your right, the furnace does not know if the AC is running or not. Sounds like you are not getting a signal from the stat in cooling. Do have a volt meter ?

 

Yes, I have a volt meter. Remember, I hooked up the old thermostat with the same results. Let me know what I should test.

 

Remove the thermostat from it's mounting plate and set your multimeter to AC volts. Put one probe on the R terminal of the thermostat and the other probe on the Y terminal. You should read ~24VAC. If you don't, you may have lost the 24VAC from the transformer, either a bad transformer or (hopefully) just a blown fuse.

 

Ok, I did what you said and it recorded 0 zero volts. A couple things:

1. There was no wire to Y. I have a Red wire to R and and a white wire to W along with a blue wire to C. The blue wire was never hooked up on the old thermostat but is now on the new one for the WIFI power (I think).

2. When I touch the volt meter to R & W I get 24VAC.

3. Again, the furnace does work just fine.


Any other ideas? I am still thinking it is the circuit board in the furnace. Thoughts on that?

 

You have to have a wire connected to Y for the AC to work. Are there any spare wires at the stat ?

 

You should also have a wire on the G terminal. The G terminal controls the blower in A/C mode. Without a G and a Y connection between the thermostat and air handler (furnace), nothing will work in A/C mode: neither the outside unit nor the inside blower.

 

You need at least 5 conductor thermostat wire for this to work. With only R-W-C hooked up you have heat only.
Y is required. G is needed but can be jumped at the control terminal block
The board has little to do with the outdoor unit beyond providing control power.

 

Ok, duh, I can be an idiot at times. Thank you all for your feedback. Of course I need more then just white and red. Here is what lead to all the confusion. I have three wires connected at the circuit board in the furnace. On the circuit board it uses RED, WHITE and YELLOW. That is all that comes to my thermostat. One of the things leading to confusion is the YELLOW wire then is connected to a new chunk of wire about 3 fee out of my furnace and the YELLOW is connected to a GREEN wire. Does that GREEN wire come out of my wall to my thermostat - of course not. In the wall another chunk of wire is used and the GREEN wire is then connected to a BLUE wire. So color wise I have RED, WHITE and BLUE at my thermostat. The BLUE wire is actually connected to the YELLOW terminal in the furnace. Follow all of that???

So, my old thermostat had RED and WHITE connected to RED and WHITE and the BLUE wire connected to the YELLOW terminal. When I hooked up the new WIFI i got caught up in the WIFI thermostat saying you must have a "C" connection which I read in forums was BLUE. Hey I got a BLUE wire (forgetting it was connected to the YELLOW terminal on my old thermostat). So in November in Michigan I connected RED to RED, WHITE to WHITE and BLUE to "C". The thermostat worked and the WIFI worked. Wonderful. Again, it was November and I was just using heat. Obviously AC won't work in this setup and doesn't.

So I hooked up my old thermostat like it was and everything works. Ok, so nothing is wrong with my AC. How I thought I could get it to work in the WIFI thermostat was to run a jumper between the YELLOW and the C at the thermostat. Remember, the YELLOW (BLUE in my example) connected to the C (when I incorrectly had it setup) gave it power. So why not jumper it and steal that power. With YELLOW also connect at the thermostat everything should work right? Well, the WIFI works but the AC will not work in this configuration either. Anyone got an idea why that wont work (let me lay it out again. FURNACE RED to THERMOSTAT RED, FURNACE WHITE to THERMOSTAT WHITE, FURNACE YELLOW to THERMOSTAT YELLOW. Jumper at THERMOSTAT between YELLOW and C?

Thoughts anyone?

 

You are confusing the issue greatly with all the color changes.

You CANNOT connect/jumper any letter to C.
R, Y, W, G are all 24vac hot leads.
C is the common or cold lead.

You need at least 4 wires from the stat to the furnace.
R = 24v power to thermostat
Y = 24v power - stat to A/C condensor (You can jumper Y to G at the furnace for the blower)
W = 24v power - stat to furnace for heat
C = common to run wifi stat.

It is easier to replace the undersized cable with one with more conductors. You can also add a kit called fast-stat but it can be confusing to wire.

https://www.fast-stat.net/

 

Oh, Boy! Hot and cold electricity. I haven't read about that since I read the blurb for a super atomic electronic ignition supercharger in a J.C. Whitney catalog years ago. That blurb defined the difference between hot and cold electricity as being the difference between amperage and voltage although I don't remember which was which.

On point, The R (red) lead is one side of the 24 volt control power and the C is the other side. R is switched through the thermostat to W for heating, to Y for cooling and to G for fan operation. ALL of these functions are also connected to C for operation. The C connection at the thermostat is used to operate the electronics of the thermostat itself and has no other function. It is the C connection at the furnace board that connects to the other functions.