cold a/c died

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  #1  
Old 07-07-03, 05:17 PM
yalinluo
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Unhappy cold a/c died

Central a/c with programmable t-stat. T-stat says the system cool is on and fan is auto but nothing happens. The unit worked fine until some time last night. Already checked the following: main circuit breaker (not tripped), outside disconnect box next to the a/c unit (fuse seems OK).

It's hot in here, please help!

TIA,
Yalin
 
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  #2  
Old 07-07-03, 06:37 PM
Ed G
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We will need much more information. Is the fan that circulates air in the house running or not? Is the unit outside running or not? This will give us a starting point.
 
  #3  
Old 07-07-03, 08:29 PM
yalinluo
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Like I said, nothing happens. Neither the outside unit nor the blower fan inside would come on. I tried this: setting the system to off and setting the fan to on, the fan wouldn't start at all. I am more convinced now that something went wrong with the furnace in my attic. The worst scenario could be a dead motor but I am hoping that it's just a blown fuse.
Thanks,
Yalin
 
  #4  
Old 07-07-03, 08:59 PM
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Yalin:

You will have to be able to use an electrical test meter to be able to toubleshoot an electrical problem.
 
  #5  
Old 07-08-03, 07:06 AM
yalinluo
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I got in the attic and looked around. I have two unit. One downstairs and one for upstairs. And the upstairs unit is broken. The furnace is made by Duncan in SC. The install manual is by the furnace side. According to the manual, it uses the Honeywell ST9120C timer (chart here: <http://hbctechlit.honeywell.com/tech...0s/69-0644.pdf>). I check the 5amp pull-out fuse, it's fine. I also traced the voltage to the transformer (~110v) but I did not get any output voltage. Is it supposed to have 24v all the time? I did not have time to measure the voltage at my other good unit but I sure can hear the humming noise from it. If I turn off the input power, I can also hear the clicking sound from my good unit. But my bad unit is all quiet. I also noticed that the transformers on the two units are different, which makes me thinking that one of the units must have replaced the xfmr before.

I switched the two Tstat control units and the used to be upstairs one works just fine now downstairs. I had a hard time to find the reference on how to check the voltage at the Tstat. Can any one help me out here?

I also could not find any link that allow me to search the text for past postings if there is any.

I would appreciate your help.

yalin
 
  #6  
Old 07-08-03, 12:49 PM
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Yalin:

Just make sure the primary voltage to the transformer is supposed to be 120 v and that you test the voltage between the two terminals on the transformer and not to one side of the cabinet.

There should be 24 v present at all times.
 
  #7  
Old 07-08-03, 02:38 PM
yalinluo
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Greg,
What should I look after if I do not get the 24V reading at the transformer? Can I assume the transformer had gone bad or do I need to look at other aspects?
By the way, the input voltage reading was someway around 120.
TIA,
Yalin
 
  #8  
Old 07-08-03, 05:11 PM
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Yalin:

If the transformer is 120 volt, you have 120 volt at the primary side and no voltage at the secondary the transformer will be bad.

Rare exception: There are a rare number of transformers that have a replaceable fuse. The fuse is quite obvious. It sticks partially out of the case and looks like a glass automotive type.

Change the transformer.
 
  #9  
Old 07-09-03, 08:19 AM
yalinluo
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Greg,

It's offical, the transformer has gone bad. I hope this is the only problem.

I am having a hard time to find the "proper" transformer. I called a local a/c service company. They have one with 115 input volt. I was told that it's rated the same as my current one and all furnaces use the 115 volt transformer. I also noticed that the transformer on my other good unit is 115 volt. I was also told that the city power supply here is 121 volt, which is kind of an odd number to me. Any way, I need your opinion on this.

TIA,
Yalin
 
  #10  
Old 07-09-03, 09:09 AM
bigjohn
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First go to a parts place, much cheaper than if you buy one from a service company. You can buy a transformer with multiple input voltages [usually 120, 208 and 240] and you just select the wires you need depending on what voltage you have. The other wires you tape off. Try to find a transformer that has a resetable circuit breaker on the SECONDARY side. That way, if there is problem in the wiring or controls, you won't fry another transformer, you'll just trip the breaker on the transformer itself. Now, class is in session. Besides the voltages on the transformer, you also need to pay attention to the VA rating. The VA rating gives an indication of the transformers' wattage capacity. For your system you want no less than 40 VA, 50 VA would be better. Don't worry about the difference between 115 and 121 volts. Your air handler will most likely be in the 120 volt range or the 240 volt range. Be sure to check it before you wire the new transformer.
 
  #11  
Old 07-09-03, 10:08 AM
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Yalin:

bigjohn pretty much said it all.

All I can add is to be on the lookout for a dead short on the 24 v side that could have taken out the transformer.
 
  #12  
Old 07-09-03, 08:48 PM
yalinluo
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Bad news. I seem to have a bigger problem. I replaced the transformer(120 VAC/24 VAC, 50 VA rating). The fuse (on the ST9120C control board) is blown out like crazy. I blew out 2 5-amp fuses and the last one I put in is a 10-amp and still no good. If I turn on the fan only, it works. As soon as I switched on the cool, it blew my fuse out. I am thinking of calling the a/c service already. What is you guys' opinion?
TIA,
Yalin
 
  #13  
Old 07-09-03, 09:36 PM
hvacman
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you may have a bad coil on the contactor in the condenser unit. or a dead short in the control wire to the condenser. turn off the power to the condenser and cisconnect the low voltage wires at the contactor. if you have an ohm meter you can check the coil, it shoiuld show some resistance, they all vary. also while you have the wires disconnected at the condenser, disconnect the other end of the wires at the air handler in the attic and with the meter check the wire out...it should read open or infinity on the resistance scale..it it doesn't then your wire is bad...also check this same wire to ground as well to make sure it isn't grounded some where......good luck

by the way don't use any fuses over the 5 amp rating.you will destroy the circuit board and that can get expensive
 
  #14  
Old 07-11-03, 03:17 PM
yalinluo
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Unhappy

Still no air. The service person felt light-headed after staying in my attic for about 15 minutes. He will come in early tomorrow morning

I measured the resistance for the pair of 24V wires to the outside unit: R to ground (1003), W to ground (1003), R to W (0,short). That is not very encouraging.

Here is my question regarding the wiring. I may have messed up the wiring on the 24V side of the transformer. Could that have caused any new damage to the board or the wiring?

Have a nice weekend.
 
  #15  
Old 07-12-03, 07:44 AM
yalinluo
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Smile

Yes, my a/c is back! The paire of control wires to my out side unit has gone bad. The 24V ground wire (white) shorted with the hot wire. The gentleman showed me the voltage readings, they are both 24V. He told me it's hard to replace the wire, which would require to break the dry wall of my house. He instead cut off the white wire and wired the 24V common to the 120V common. I was wondering if there is any risk by doing so.

I would like to thank GregH, bigjohn and hvacman for your insightful opinions and prompt responses.

Thanks again,
Yalin
 
  #16  
Old 07-12-03, 08:02 AM
bigjohn
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Glad to hear that you're back in business. You're lucky it didn't fry the board. $$$ I'm just curious, did the tech run the furnace thru a heating cycle to make sure all is ok with the board? You don't want a surprise when the weather turns cold again.
 
  #17  
Old 07-13-03, 11:14 AM
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Sometimes it works

Connecting the white wire to the voltage side was really not needed. He could have just zipped a screw to ground. The only thing is that in both instances this works only with grounded transformers. By the way, the fuse that is in the board is proitecting the secondary side of the transformer and the board, so that it doesn't burn up requiring you to replace it. Never put a larger one than specified on the schematic, most installations have a spare wire wrapped around the outer covering of the wire in the event of a shorted wire...I take it this had no spare wire...
 
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