No power to thermostat after the A/C stopped blowing cold air

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  #1  
Old 09-29-10, 09:39 AM
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No power to thermostat after the A/C stopped blowing cold air

I came home to a warm house. Honeywell digital thermostat read 82deg. I went to reset the circuit breaker and cutoff switch outside the house by the compressor hoping that a reset may fix things. Waited 5 min and turned everything back on. The thermostat went blank. I took of the thermostat to expose the back wires. I connected R and Y but nothing came on. I don't think I'm getting the 24V to the thermostat. I swapped thermostats with another one in the house (house has 2 a/c units) and it turns on, so the thermostat panel itself is fine, just no power at the original location. I also checked the safety overflow switch at the air handler unit and it's bone dry in there, switch is not activated.

What should I check at this point. Planning to use a voltmeter to check the thermostat wires for 24V. If there isn't, what else should I check. I want to see if it's something simple like a blown fuse or transformer before calling someone. There's a circuit breaker switch or fuse on the air handler unit itself.

House was built in 2006. Two A/C units are cooling the house. The larger unit (#1) is not working. Smaller unit (#2) runs fine. A/C compressors and air handler units were new in 2006. Units are about 2 years old because the house was vacant the other two years.

Thank you
 
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  #2  
Old 09-29-10, 09:46 AM
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You are on the right track...very likely a fuse or transformer (or a fuse for the transformer). Normally up around (or in) the air handler unit. Find where your t-stat wiring goes into the AH and track it down.
 
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Old 09-29-10, 01:30 PM
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I'm an amateur to all this but can follow directions pretty well. Any more specific details as to how to check the transformer or fuse and how to replace if needed, thanks. And if those are ok, what would i need to check next, thanks
 
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Old 09-29-10, 02:14 PM
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Should be easy, use a multimeter, check the transformer, one side should be 120V the other side (small wires) should be 24V, if not, there is your problem.
 
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Old 09-29-10, 04:48 PM
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I took a picture of inside the air handler. The wires entering and exiting the circuit breaker on the left side of the first pic is 240V (i thought it was suppose to be 120V). Any how, I opened up the transformer in the second picture. I'm not getting a 240 or 120 voltage on the black and red wires that enter the transformer as I should. And of course not getting 26V on the blue and yellow wires exiting the transformer. So my transformer may be ok but since I'm not getting any input voltage at the transformer.... Where should I check next. The control panel? And where else






bigger view of AH if you click thumbnail
 

Last edited by ysae8952; 09-29-10 at 06:59 PM.
  #6  
Old 09-29-10, 07:05 PM
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Are you measuring red to black on top of your breaker and then red to black on the bottom of your breaker? (want 240VAC measured this way)

Looks like moisture is getting to the breaker to me.

If you read 240VAC red to red and black to black, when your breaker is on, you need a breaker.
(Should read 0VAC red to red, or black to black when power is present and breaker is on)

In this example I have remove a wire from a disconnect and measured 240VAC black to black.
 
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Old 09-29-10, 07:08 PM
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Originally Posted by Houston204 View Post
Are you measuring red to black on top of your breaker and then red to black on the bottom of your breaker?

Looks like moisture is getting to the breaker to me.

If you read 240VAC red to red and black to black, when your breaker is on, you need a breaker.
Yes I measured black to red on top of the breaker and got 240 and black to red at the bottom of the breaker and also got 240. If I switch the breaker then I'll get 240 at the top and not at the bottom so I assume the breaker is working. It does look corroded. but it's dry there with no moisture
 
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Old 09-29-10, 07:19 PM
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Remove power and trace power from the transformer to the breaker.
Any harnesses in between the transformer and breaker?
Broken wire connections? Signs of heat?

Looks like black to orange is where we want 240VAC on that transformer. (not black to red which is for 208VAC applications)
 
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Old 09-29-10, 07:40 PM
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Originally Posted by Houston204 View Post
Remove power and trace power from the transformer to the breaker.
Any harnesses in between the transformer and breaker?
Broken wire connections? Signs of heat?

Looks like black to orange is where we want 240VAC on that transformer. (not black to red which is for 208VAC applications)
Which ones my power? Am I tracing the red, black, or orange from the transformer. Or red or black from the breaker?

Should I check the relay. I can swap it with the other air conditioning unit that is working.

And to respond to post earlier, black on top to black on bottom gives 0 and red to red gives 0 at the breaker

BTW I swapped control panels with the working air conditioning unit and it works fine. So the control panel is not defective.
 
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Old 09-29-10, 07:48 PM
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At your transformer, measure for 240VAC orange to black with power present.
If 240VAC is not present orange to black (@ transfomer), remove power and trace orange and black back to the breaker. What interupts this circuit?
 
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Old 09-30-10, 05:25 AM
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Originally Posted by Houston204 View Post
At your transformer, measure for 240VAC orange to black with power present.
If 240VAC is not present orange to black (@ transfomer), remove power and trace orange and black back to the breaker. What interupts this circuit?
Tstat's got power now and display is on! Not sure what happened...The control panel was left unplugged one night (too late to reassemble last night). This morning I plugged the control panel in to test the orang and blk wires, and after plugging in the panel and turning the breaker on, there was power to the AHU. The green light on the control panel came on (This didn't happen before when everything was hooked up earlier on). I tested and got 240V on blk and orange and 26V on the blue and yellow output (which did not happen earlier, all wires were 0V and no power earlier on). Checked the tstat, and there was power. Shut everything off an put everything back together in the AHU.

I used to turn power off at the main circuit breaker panel by turning the A/C breaker off (power to tstat) first then then the AHU breaker switch. To turn on, I'd turn AHU on first and A/C on second.

But this morning I switched the order and turned A/C main breaker on first and AHU main breaker on second. I'm not sure if the order of turning these on or off matters. The system may have gone into some sort of safe mode, I don't know.

All I know is that the problem all started when I came home to a working tstat and AHU blowing air but no cold air. I shut everything off A/C switch first then AHU second. Then turned back on AHU 1st and A/C second and this was when the tstat had no power.

Maybe someone knows what could've happened. I want to thank everyone who helped me out with this. Never worked on A/C's in my life, and now I've learned some things. thanks.

I'll probably be making a new post soon which gets back to my original problem of everything running but no cold air. I'm going to check when I get home from work to see if it's cool in the house or not before starting a new thread. thanks

ysae8952
 

Last edited by ysae8952; 09-30-10 at 05:45 AM.
  #12  
Old 09-30-10, 03:40 PM
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I suspect a loose or dirty wire connection. Possibly in a wire harness or one of the wires at the breaker.
 
  #13  
Old 10-01-10, 08:49 AM
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Originally Posted by Houston204 View Post
I suspect a loose or dirty wire connection. Possibly in a wire harness or one of the wires at the breaker.
Good to know thanks, I appreciate your help
 
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