Furnace Fan Manual Override Switch Wiring - help!


Old 08-02-14, 11:08 AM
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Furnace Fan Manual Override Switch Wiring - help!

Hi All,

I have an older furnace (lennox 80's era) that I am using with a NEST thermostat.
I have the Honeywell Limit Switch which appears to run off of LINE voltage (110v).
I don't have A/C, so the house gets really warm in summertime.
I would like to be able to turn on the blower fan only to circulate/filter the air to help cool the house off faster at night.
I am wondering how I would go about wiring the NEST into the limit switch?
The current wiring to the furnace is just 2 wires (heat call only) and so there is not already a wire for the fan - I would need to add one in.
I know the NEST runs off 24v, so I am thinking I need a transformer in there somewhere - but I cannot seem to find a suitable transformer for 24v to 110v online for this purpose. Multiple searches come up with products that just don't look right to me.
I am no expert, but do have enough knowledge to do some damage.
I can take some pics of the circuit board and post them if necessary. Maybe there is already the ability built into my furnace to add a fan control wire?

Any help is greatly appreciated.

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Old 08-02-14, 01:32 PM
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Your best bet is to run another wire to the stat and land it on G on the stat. If you can post the model number of the furnace we can go from there.
Old 08-03-14, 11:24 AM
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Hi skaggsje!

Thanks for the quick response. I found the model number for the furnace, it is:
Lennox G8-105-1 (from picture of sticker inside furnace - see attached)Name:  photo 4.jpg
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I also took a pic of the honeywell limit switch Name:  photo 5.jpg
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The junction wiring box (no circuit board here, just wires to the limit switch, fan, and gas valve) Name:  photo 2.jpg
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Here is the gas valve: Name:  photo 1.jpg
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And finally a little black box attached to the junction box with the green wires which are connected to the gas valve. There are a couple wires from the junction box which feed into the black box (is this a relay?).Name:  photo 3.jpg
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Hope this can help provide more clarification. I do have a circuit tester/meter so I can verify voltages and amps if necessary to confirm any wiring.

Thank you for your help!
Old 08-03-14, 11:56 AM
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Welcome to the forums.

There is no G terminal or fan relay in this furnace.
Do you see a transformer mounted anywhere inside ?
I'm thinking the G8 is so old that it's a millivolt system.
Do you currently have the nest connected and working on this G8 furnace ?

Is this furnace still in good shape ?
Have you had the heat exchanger cleaned and checked for cracks recently ?
Old 08-03-14, 12:20 PM
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Hi PJmax,

I took pictures of everything other than the fan motor, so maybe the black box is actually the transformer?
I had the home inspected and the furnace cleaned when I first moved in and I thought I remember everything checking out fine. I have a fancy CO detector just in case. It hasn't had any readings whatsoever for CO for over a year since I installed it.
It's an older home and I am planning on demolishing and rebuilding in a year or two so no sense swapping out at this point.
Yes the NEST works perfectly with the system but it only works for heat calls, nothing else.
Ultimately I would like the ability to run the blower only through the nest because removing the cover and pressing in the white button (manual fan switch) and then turning it off again is a real pain!
I would settle for a SPST switch mounted on the outside of the furnace in the meantime!
Should I do a current wiring diagram of the setup so you can see how I might be able to do this?
Old 08-03-14, 04:13 PM
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A picture of the wiring diagram would be of great assistance.
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