Heating Blower won't start on "Auto"

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rc0467

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Hi,
I only recently noticed that my blower does not kick in when on Auto. When I switch the thermostat to ON it works. I have had service tech in for other things in the past so I don't know if something they did caused this, it's always been like that, or something is broke. I tried changing thermostats to no avail. It's a oil boiler with hydronic coil fed air handler. There are two air handlers in the house (up and downstairs) but only the upstairs has this issue.

I have limited electrical/electronics knowledge. I do have a multi-meter and know how to use it.

Any ideas?

Thanks,

Bob
 

Jadnashua

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Since the blower does turn on, the fan itself and that control relay that powers it, at least on those contacts, are good. When in auto, many furnaces rely on an internal temperature sensor to turn the fan on once the heat exchanger starts to heat up to prevent blowing cold air prior to that. But, each company has it's own technique. Some of them are just a delay after the burner turns on (and some also have a delay after the burner stops to extract the heat you paid for). Without a brand and model number, it's really hard to tell.

There is almost always a wiring diagram on the inside of the furnace (at least when new). They can be somewhat cryptic unless you work with them, but look for the fan and work your way back to 24vac control. It will likely go through a couple of things. Or, find where the ON signal comes from your thermostat, and see how it connects. There will be a second path that does that on AUTO...that's the one you need to follow.
 

rc0467

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Since the blower does turn on, the fan itself and that control relay that powers it, at least on those contacts, are good. When in auto, many furnaces rely on an internal temperature sensor to turn the fan on once the heat exchanger starts to heat up to prevent blowing cold air prior to that. But, each company has it's own technique. Some of them are just a delay after the burner turns on (and some also have a delay after the burner stops to extract the heat you paid for). Without a brand and model number, it's really hard to tell.

There is almost always a wiring diagram on the inside of the furnace (at least when new). They can be somewhat cryptic unless you work with them, but look for the fan and work your way back to 24vac control. It will likely go through a couple of things. Or, find where the ON signal comes from your thermostat, and see how it connects. There will be a second path that does that on AUTO...that's the one you need to follow.

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Hi Jim,
Thanks for the post. I went through the system today and pulled the schematics and parts list to post them here. I was thinking that it could be the sensor that detects heat is available that could be defective. If it never goes closes then neither does the fan. The "ON" position would bypass that sensor/relay and therefore work. Would this be the FDR (Fan Delay Relay) on the diagram? Thinking that I could test by jumping it which would eliminate the delay but should prove the theory for which I could pickup a replacement part. The schematic is the last image attached.

Any additional comment would be appreciated.
Air_Handler_Label_Upclose_2.jpg
Air_Handler_Manual_Cover_1.jpg
Air_Handler_Full_Relays_1.jpg
Air_Handler_Parts_List_1.jpg
Air_Handler_Functional_Parts_List_1.jpg
Air_Handler_Schematic_Full_1.jpg


Thanks,

Bob
 

Jadnashua

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If the red to the PCB comes on when it is calling for heat, and all of the other connections are making good contact, it appears that PCB is bad. The relay is operating, otherwise, it wouldn't turn the fan on with the thermostat's control. Hopefully, another set of eyes can confirm this, as with most places, electronic parts are not returnable.
 
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