Thermostat faceplate removed - yet fan operates

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  #1  
Old 09-10-20, 03:39 PM
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Thermostat faceplate removed - yet fan operates

This brand new experience as of late last nite is:

1. Air handler fan always running, even with switch set to AC/Heat off.
2. Yes, the fan is set to auto
3. Its a Honeywell CT3600 programmable with good batteries. 4 wire setup, Red, Y, G, W. Electric heat.

What I have done is to remove the t-stat faceplate and stare at the incoming wires. With NO jumper cables connected, the fan runs when main power is on. I do have AC when jumping R > Y. Cooling is not a problem. Its too hot to run the heater, so that has not been tested.

Is that relay up in the attic in the air handler shorted to the ON position? I guess it could have happened overnight, but I thought it would have to get SOME signal from the t-stat backboard.
 

Last edited by handyandy2; 09-10-20 at 04:04 PM.
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Old 09-10-20, 09:24 PM
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Electric heating uses relays called sequencers. They are relays that allow the element to start heating before the blower starts. They also keep the blower running until the heating element cools. They can get stuck with the fan on.

I'd need a make and model number off your air handler.
 
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Old 09-13-20, 09:04 AM
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Smile

Well Pete, a huge thank you, and you were spot ON!. It was the sequencer (what I called a relay), pic attached. While a bit blurry, the pencil is pointing to sooty deposits from the contacts arcing. I didnt notice as I poked at it if it was arc'd shut or not. The thermostat went back on the wall, and cycles just fine. I threw in the old sequencer (replaced in Dec) when I was diagnosing slow-blow fuse problems with the heater and replaced the thermocouples which I diagnosed a being problematic. The root cause (back then) was the disconnect switch "heating up" and blowing the fuses due to heat, and not over-amping of the unit.

Getting back on topic, the sequencer was replaced at the same time in Dec, tho it didnt exhibit any issues, but for $30 it seemed good insurance to avoid trips into my hated attic space.

The unit is a BDP/Carrier, Product 961AN0042010F16 Model 961-042-010-A

Case closed. Again...thanks for pointing me in the correct direction!!!





 
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Old 09-13-20, 09:45 AM
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You're welcome. Good job.
 
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