HVAC help needed - buzzing furnace

F

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hey guys - I know there are a couple HVAC techs on here.

We have a mid-eighties forced-air NG furnace and probably much older A/C unit. Haven't had any issues before this (owned the house since '10) and it's been tested/inspected three? times.

When the fan is on (via switch at the thermostat to the forced on position), there is a fairly loud buzzing coming from the side-mounted transformer? box, it's not coming from inside the furnace.

With heat on, no noise, heat is normal (only tested...
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trixter192

trixter192

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Sounds like you have a fan center that's pooched, or there's a short in the low voltage cooling electrical circuit.

There's probably two types of failure, either the relay is defective or not closing, or transformer failed. If you have ~27VAC between RC and and Y, then the transformer is probably fine. If the relay is open style, bugs like to crawl into them and prevent the plunger from closing the contacts.

If I were there as a tech, I would just replace the fan center if I carried one. I would seriously consider replacing a 20 year old furnace, ac at your discretion.
 
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ooooookkkkaaayyyyyyy, so I guess I'll memorize this and say it back to the tech that is dropping in tomorrow lol :)

Thanks so much...I think this is a wee bit more technical than I can handle. I was hoping for a 'whack it with a hammer' response ;)
 
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Thanks again [MENTION=1730]trixter192[/MENTION] oh yeah, FYI the furnace is actually 30+ years old, not 20.

update - with several solid smacks, the fan centre quiets down and doesn't affect the function.

AC does turn on, I just didn't give it enough time after testing for the heat. We ran it for about 1.5 hours last night and again today for about an hour.

Pressure test on the A/C is good, fan is blowing hot air out the top of the exterior unit and cold air inside of course.

The tech said the fan centre is the culprit in this case, $390 installed (plus today's service call). He of course said he cannot guarantee what he will find inside (i.e. red tagging is a possibility with this old of a furnace).

We're going to get a few quotes for a furnace replacement...don't think it makes sense to put another $400 into this one. Probably will keep the A/C for now.
 
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