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I have a '96 Trane YAC048... H&A unit that apparently went out somehow in the midsty of a power outage New Year's eve (power came back on and heat didn't!) I have tried thermostat on and off, gas valve on and off, removed outside fuse, and also rotated all gas flow valve inside unit on and off in various permutations with apparently no ignition of gas. With thermostat in a normal (heating) start-up position, fan will start and blow cold air for a few minutes before cutting off. What could be potential causes?

Submitted: 311 days and 3 hours ago.
Category: HVAC
Value: $9
Status: AWAITING EXPERT REPLY
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Accepted Answer

Check the circuit board inside the unit for a small fuse. Also, listen for the induction fan motor (small fan that supplies air to the gas chamber) to see if that starts up.

If the fan doesn't start, check for continuity of the fan motor with a multimeter. The power surge/outage could have damaged the motor windings. (make sure you disconnect power before checking the motor)

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Expert: Doug Eby
Pos. Feedback: 100.0 %
Accepts: 11
Answered: 1/2/2009

HVAC Technician

15 Years Navy, commercial, restaurant

Posted by Doug Eby 310 days and 15 hours ago.

Info Request

Did you find the problem?
Also, please leave feedback.
Thank you.
Doug

310 days and 9 hours ago.

Reply

Thanks very much Doug, for taking the time to answer my question.

With the outside power breaker bar pulled (power off) I checked "continuity" of a fan with about 3-1/2" diameter motor case that I had seen that draws air/exhaust off panel above burners, and it checked solid low 70's ohms (I suspect at least that fan is good).

 

As I had not had the panel off circuit board before, I did remove same to see if a "small fuse" jumped out at me from same. While I am sure it may be there among the many small items on the board, I could not read (with my admittedly bad eyes last night) anything that I could see with amps or milliamps rating and looked for sure like the fuse you are talking about.

However with the panel off I reconnected the power last night, and when the cold air exhaust fan/blower immediately started as had been happening when a heat signal should be sent by thermostat I also saw a blinking red diode light on the circuit board, that blinked steadily about one red flash per second for 3 or 4 minutes until the the fan blowing cold exhaust only cut off after a few minutes. Once the fan cut off, the steady "one per second" blinking of the same diode changed to a couple quick red blinks every couple seconds, with everything quiet or nothing happening on the unit (I suspect this is not good!) I did all the preceeding in the dark and at no time did I notice any glowing or sparking etc. in the burner area when power was on.

 

Strange thing, when I replaced the big outside power breaker this morning after seeing your reply (it did rain a little bit last night after I covered all parts of unit), the burners sort

of inexplicably fired up (this was first time since New Years eve outage and after all my "fooling with" same!) I did notice in the 10-15 minutes or so while the furace appeared to sort of run right that the roar of the burners did appear to "cut off and on" a time or two, when I would have expected them to run pretty much steady if everything was right, and the unit did cut off before attaining proper temperature. I'm wondering crazily if some moisture somehow may have caused it to try to run a little!

 

(Earlier incidentally I had checked at least the continuity of the mercury level switch in my thermostat which read no resistance across when flooded with mercury and though a little erratic on the meter little or no continuity when the mercury tilted off the wire end. Also, I noticed that while the fan was running there appeared to be a closed circuit or no resistance across the vacuum switch terminals, though that reading for some reason may have been at least a little erratic also.)

 

Again thanks for help.

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