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I have a White Rogers 1E78-144. My house has heating only and no AC.

https://climate.emerson.com/documents/ie78-non-programmable-thermostat-instructions-en-us-4209010.pdf

The current wiring only enables the fan to run when heat is required, even if I force the fan switch to ‘on’ and set heat to ‘off’. If I set it to cool or off, the fan won’t run no matter what.

In other words, if I want the fan to run, it must come with heat, which won’t happen if it’s warm enough in the house.

Is there a way to bypass the heat and just have the fan on?

My current thermostat wiring

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  • Is running a new thermostat cable in the cards? Aug 18, 2020 at 23:09

3 Answers 3

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Your thermostat cable coming from the wall appears to have only 2 wires. Therefore the thermostat can only send one command: everything on, or everything off. (This is very common in older systems.)

In order to get separate fan control, you would need at least three wires, but at the other end of the cable, the furnace would also need to be wired for this option.

To determine whether this is possible, examine your furnace and update your question with photos of the wiring diagram (if you can find it) and how the thermostat wires are currently connected to the furnace's control board.

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Yes you can but the easiest way is to add a switch at your air handler. I have done this when a new set of wires was impractical or the unit did not have the capability. Installing a switch to turn the fan on at the furnace a standard snap switch rated at 2x the motor fla and wired across the contactor is what I have done. This turns the fan on but the heat only turns on when the thermostat calls for heat.

Example manual fan on there is no controll voltage to the gas valve that comes from the thermostat. Fan running. It gets cold at night fan is running the thermostat commands the fan on but no problem it already is that command turns the gas valve on and it heats as normal. Turn the bypass switch off and the furnace operates the way it always has.

In years past I have put this switch in my home, my moms home and several that were remodel jobs. Works great and cost about 8 dollars for the switch and wire.

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You can turn the gas off to the furnace and then turn the heat on. The fan will go on but no gas so no heat. Not a solution if you want to run the fan often but I just needed to remove cigarette smoke from an apartment so I needed to circulate the air for several hours then done. Then have to relight the pilot.

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    Basic concept sounds fine. But "relight the pilot"? Most furnaces (at least in the US) have used electronic ignition - no pilot - for decades. Oct 5, 2021 at 15:02
  • In the UK too, boilers with pilot lights have been against building regs for some years now.
    – Chenmunka
    Oct 5, 2021 at 17:17
  • Most furnaces that only accept a two-wire control are extremely old -- anything made in the era of widespread electronic ignition (DSI or especially HSI) will have a fan override input (G wire connection) Oct 6, 2021 at 0:09

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