Possible Backfeed Issue?

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Old 10-11-15, 06:35 AM
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Possible Backfeed Issue?

Hello, hoping I can can get a bit of help with an issue that arose. I have had an Aprilaire 600 with the Humidifier Control 60 wired to my furnace for several years now and have had no issues. That just changed after I attempted to add a Honeywell switch to the plenum. Not sure exactly what is wrong but now humidifier control keeps calling for furnace blower to run and is overheating humidifier control board.

Preparing for winter, I decided to add on a wood furnace tied into plenum of gas furnace. To keep plenum from overheating I added in a Honeywell L4064B to call for furnace blower when plenum reaches a set temp. Supplying 120v to Honeywell then transforming that power down to 24v I tied it in to C and G. When I first tried to manually switch ON the Honeywell the blower kicked on as expected but after a couple seconds the Aprilaire board started lightly smoking then the furnace fuse blew and furnace breaker popped.

I then disconnected the 24v (C and G) coming from the Honeywell, changed fuse, reset breaker and tried again. The Aprilaire called for blower then after a bit started smoking again. I disconnected Aprilaire from G on furnace hooked the Honeywell switch back up to C and G and tested. It works fine.

I took board out of humidifier control and checked it. Looks like one of the varistors are bad now. So, now I am not sure what I did wrong and am hoping someone can give me some insight. I am guessing that having two lines to G caused a backfeed to the Aprilaire Control board? If that is the case, what can I do in order to keep both items hooked up?

Thanks for any help.
 
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Old 10-11-15, 10:53 AM
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Welcome to the forums.

Your problem was that you have two transformers competing with each other. That doesn't work.

You don't need a second 24v transformer to do what you want. Run a single 2 conductor cable from the L4064B to your furnace control board. Connect that cable to R and G. When the new stat senses heat it will short the R and G and turn the blower on.
 
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Old 10-11-15, 11:43 AM
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Thanks you for the quick response. Figured I would be waiting awhile.

Went ahead and did as you mentioned. That was much simpler than what I had done. I pulled the brass jumper from the L4064 though I wasn't sure if I was supposed to. Looks like the L4064 still works at it should. When I manually flip it ON it does kick blower on. But....

Tried to kick fan on through the thermostat and I get nothing. Triple checked all connections at furnace. Why would that stop working?

Also just tried turning on A/C then heat. Neither kicked blower on. When I called for heat it didn't even attempt to fire up.

Disconnected the L4064 from furnace and I still get no blower from thermostat. Hooked the two wires back up and the L4064 does kick on blower in manual.
 

Last edited by DoinItAlone; 10-11-15 at 01:27 PM.
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Old 10-11-15, 03:49 PM
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Try putting the stat in FAN ON mode which is the same thing as connecting R to G.

There should have been wires already on the R and G..... you left them connected also... correct ?

Actually.... now that I'm thinking..... you currently route the G from the thermostat to your humidsitat.... correct ? In that case your stat is probably not passing the G signal back to the furnace.


You probably have wires on G and Gf on your h'stat. They may both be green. Remove them from the h'stat, connect them together and then try A/C and heat.

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Old 10-11-15, 03:49 PM
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I finally figured it out knowing it had to be something simple. Had forgotten that that thermostat G ties in to humidifier before going to the furnace. Since I had removed G coming from humidifier the thermostat was unable to send the signal to the furnace.

Thanks again PJmax. I did not realize I could wire it with the 24v from the furnace. That did simplify and tidy up the wiring. I appreciate the help.
 
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Old 10-11-15, 03:52 PM
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I had disconnected G since humidifier control is constantly sending a signal to furnace. I separated G coming from thermostat from the humidifier and wired it directly to the furnace. Now I need to replace the humidifier control since I messed it up.
 
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Old 10-11-15, 04:01 PM
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I added the diagram as you were writing but appears you have figured it out in your own.

In my opinion.... you are better off and not allowing the humidistat to turn the blower on when there is no heat as the humidity transfer without heated air doesn't justify running the blower.
 
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Old 10-11-15, 04:08 PM
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So leaving the humidistat hooked up the way it is now (without G) will not effect how it operates? Other then replacing the humidistat I should be good now.
 
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Old 10-11-15, 07:34 PM
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The humidistat will work fine without a G or Gf connection. It uses the W connection to know when there is a call for heat to activate the humidifier.
 
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