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So I live in an rental with a forced air furnace. The thermostat that controls it is an old honey well that uses ancient analog components. It works well during the winter for heating, but unfortunately I realized with summer coming on it has no option for a fan mode. Pulling the thermostat off the wall I found that the wiring only has the red live wire and white heat wire running to the furnace.

After googling around I figured out that basically the thermostat is a fairly dumb component that just connects the red 24v to different input wires to active different heating/cooling mechanisms. Following this logic then I figured I could go to the furnace and rewire the connections to furnace board such that the white wire input instead connected to the fan terminal. I did all this and then replaced the thermostat to a rocker switch so I could easily flip it on and off. So far the fan is humming along nicely and things are cooling off.

My question is this, since I cannot solve the problem correctly by running more lines and getting a updated thermostat (I figure my landlord wouldn't like me doing impromptu wiring) is this a safe long term solution?

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    Are you (and your landlord) OK with a wireless thermostat/interface module pair as an upgrade from your existing thermostat?
    – ThreePhaseEel
    May 18, 2020 at 3:19
  • Also, can you post a make/model number for your furnace, or its wiring diagram for that matter?
    – ThreePhaseEel
    May 18, 2020 at 3:33
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    that's basically what a fan on/off on a dumb thermostat would do, so it shouldn't be a safety issue.
    – dandavis
    May 18, 2020 at 12:46
  • "(I figure my landlord wouldn't like me doing impromptu wiring)" unless you've already spoken with the landlord and received approval for what you've done, it seems a little late to be worried about this.
    – FreeMan
    May 18, 2020 at 13:31
  • @Freeman I mean it's easy enough to undo everything I've done up to this point. But adding wires falls into a category of making changes to the property, if I made a mistake in wiring and caused damage (short circuit, fire, etc) I would be liable.
    – Chris Copeland
    May 18, 2020 at 19:28

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Can this be jumped safely, YES. Many furnace controls have the fan only option but as you found the wires did not extend to the thermostat. If you were able to use the 24v to pull in the fan relay and not activate the gas valve what you have done would not be a problem with normal low voltage thermostat wiring and would be hard to be unsafe at 24v.

Prior to installing any wiring I would get permission from the land lord. I have installed toggle switches on furnaces in the past for this same reason, to have a fan option, I did not consider pulling new 5/7 conductor wire to the thermostat as that would have doubled the cost of the job. The furnace was conveniently located, so it was no big deal to just flip the switch.

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