Connecting old Lennox furnace to Nest

Old 05-06-15, 12:57 PM
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Cool Connecting old Lennox furnace to Nest

Hi All,

I have an older 80's era furnace (Lennox) with only 2 wires to the thermostat. I understand this is HEAT ONLY. I can remove the door to the furnace and turn on the FAN in the summer to help move the air around, but this is a pain to do.

I would like to wire it up to work with my NEST thermostat, so I can simply have the nest turn on the furnace fan only periodically to move the air around, and possibly schedule the fan to come on through when the temperature rises so it doesn't get so stuffy in the house (older 1947 bungalow).

I have found wiring diagrams for the L4064B, and I am pretty sure I need a 24V transformer (already purchased) to convert line voltage 120V to 24V to the NEST, but I am lost as to how to physically wire it up so I don't blow up my NEST, or more importantly the furnace fan or short circuit stuff.

I can run some new low voltage wires to the location of the NEST to enable my wiring setup if necessary.


Any help is greatly appreciated!
Old 05-06-15, 01:06 PM
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The Honeywell L4064B is just an AC switch for the blower fan.

You may already have a transformer in your furnace unless it's a standing pilot furnace.
Post the model number of the furnace.

You may ultimately need what is called a fan center. It's a combination relay and transformer in one package.

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