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The fan on my american standard furnace surges. slows down,

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the fan on my american...
the fan on my american standard furnace surges. slows down, speeds up, slows down. etc. does not run at a steady speed.
Submitted: 2 years ago.Category: HVAC
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Answered in 38 minutes by:
11/4/2015
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,152
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello, the problem is in the electronics, either in the motor end bell, or more likely in the speed signal output from the printed circuit board.

If you can attach a clear well focused and well lit, close up photo of the wiring diagram glued to the inside of the blower service cover I can be more specific, use the paper clip icon at the right end of the tool bar for that or the '+attach files' link to the left of your send button.

I will mark it up with trouble shooting instructions.

Tell me a lot about your technical skills with electricity etc, so that I can coach you most effectively.

We can go from there.

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HVAC Technician: Phil, Mechanical Engineer replied 2 years ago

Thanks, ***** ***** it has the error codes printed on it... are you seeing any of those in form of blinking lights on the printed circuit board.

That is not the furnace wiring diagram however it is much larger 10 x 11" or so probably in the same area that you found the error code tag that you just posted. the wiring diagram will have dozens of wires drawn it and will show the blower, and gas valve, and printed circuit board (PCB, or IC)

I can also use a photo of the printed circuit board where all of the thermostat wires terminate it is about 6 x 8" or so, with a lot of plugs attached and some black boxes on it.

We can go from there.

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HVAC Technician: Phil, Mechanical Engineer replied 2 years ago

Hello again,

there are two plugs that plug into the blower motor, the largest one has 8 pins on it, follow that wire away from the fan motor until it attaches to the blower speed control electronics module then follow these heat stress test instructions.

Use a hair drier to slowly and gently warm the card to 120F(warm to the touch but not hot and not over140F)... if you get any change in behavior that means the card is bad...

A micro crack has opened in the printed circuit at a solder joint, or inside its CPU in that case .... or a short circuit has developed as the card expanded when you heated it.

Constant expansion and contraction as these electronics heat and during their duty cycles over the years is what wears these cards out.

It is also a good idea to replug all of the connections in the unit,that burnishes any oxidized terminal connections and will sometimes solve the problem, especially at the printed circuit card and in any sensor molex connectors. Do that with the power off to the unit of course.

Be very alert of any changes in motor behavior as you warm the card up. Let me know what you discover, we can go from there.

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Customer reply replied 2 years ago
I wasn't having much luck on my own, so I had a tech look at it and said it was the control board. he put in a new board and the problem persists. recommendation now is that the problem is in the motor. its a 2 stage, variable speed furnace, 80 afue. recommendation is to replace the furnace, as the variable speed motor is very expensive. Furnace is 15 years old. Any thoughts/recommendations?
HVAC Technician: Phil, Mechanical Engineer replied 2 years ago

Hello again,

Its a close call. The life span of the furnace however is in 15 to 20 year range. The furnace itself is worth about $1,290 wholesale, a new motor will run very near that amount.. when you add in the printed circuit card you will be spending more on the furnace than the furnace is worth.

However if the the technician had been a bit more skilled, he would not have mis diagnosed the problem. So he owes you so to speak.

Tell him that in light of the situation that caused you to waste money on a printed circuit card. that you didn't need .....you are now in a tough spot, ask him if he can give you a real good deal on a new furnace to make up for it, so that you will not feel that you wasted all that money on repairs.

A good deal on replacing that furnace with an equivalent high efficiency furnace would be under $3,000... that would be a good deal. The company most likely tries to sell those for $4,000 or more.

Thanks for letting me work with you. When we are finished,if my answer has been polite, professional, and thorough, please locate the 'rate your expert button' and click one of the ratings .

If you still need help let me know!. Rating will not increase your charges, nor close the question, but is how I earn my living. Thanks again.

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Phil
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Phil, Mechanical Engineer
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