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Over the last couple of winters once in awhile I would notice that my electric furnace wouldn’t kick on when the thermostat would click on to the temperature setting. I don’t remember how I figured this out but backing the the thermostat down, turning the fan to -on- for a minute then turning it back to auto, running the thermostat back up till it clicks back on, it would come back on and run no problem. This would happen 3 or 4 times a season. This winter it happened and it would take running the fan for 3 or 4 minutes before it would work again. I replaced the thermostat twice and it seemed to work……. For about a day. Same thing is happening (also see “weird side note” below)

The blower motor was replaced last spring by a technician and we couldn’t get it to do the malfunction again at the time.

It’s an older furnace, in a 1995 manufactured home. I’ve always kept a clean filter.

Here is a weird side note: when I replaced the thermostat the first time, the furnace/fan would work in auto mode but was blowing loud and fast like it would in fan-on mode. I switched out that thermostat to the same model as before but a new one and it worked all day then back to the original problem. Now it will work only if I run the fan in fan-on mode for several minutes then switch it back to auto and run the thermostat back up. And cross my fingers. It’s Christmas Eve and the next week is going to be really cold.

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  • Can you post the wiring diagram for your furnace please? Dec 25, 2021 at 16:40
  • Thanks for your reply. I added the diagram to the original post.
    – Jason
    Dec 25, 2021 at 22:05
  • Can you not set "fan on" mode while in heating mode on your thermostat for that matter? Dec 25, 2021 at 23:09

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I suspect the controller board in the furnace has a loose wire somewhere. Until you can get someone to look at it you can try getting the thermostat to run the fan with the heat, often called an "electric heat" setting.

Furnaces will commonly use different circuits for starting the fan motor depending on the thermostat settings, there will be a circuit for continuous fan operation (often at a lower speed), a circuit for heating, and possibly another circuit for cooling. With the fan always on the fan will already be spinning if there is a need to heat and so bypasses whatever electrical issue there may be in spinning up the fan for heating. It could be a failing motor bearing or capacitor, those can show as intermittent problems too. Replacing the motor doesn't mean you got a good motor to replace a bad motor, maybe you got a lemon.

What might be a fix, at least temporarily, is to program your thermostat for "electric heat". This will be a setting in an electronic thermostat, or a switch or jumper hidden inside a mechanical thermostat. Electric resistance heating often relies on the thermostat to signal turning on the fan, not a controller in the furnace. There's a long explanation on why this is that I won't go into (and another long story on how I figured this out) but in most thermostat manuals it's often called an electric heat setting and not explained what it does. Some manuals will explain what this setting does, or call the function something else. Either way if the thermostat can run the fan but the furnace can't then telling the thermostat to run the fan with the heat is a way around this, again at least temporarily.

I find it odd that you say the fan blows loud in "fan on" mode, that should be the low speed setting in a dual speed fan. The installer might have some wires crossed, this may be another sign the furnace controller is bad, or again a lemon fan motor.

Addendum:

The fan running fast in "fan on" mode may not be so odd after all, after giving this some thought. The thermostat will control the fan for the air conditioning and that will be at a higher speed than heating, selecting "fan on" at the thermostat is closing the high speed fan circuit. Whether you have electric heat or fossil fuel heat doesn't change the function of the "electric heat" setting in a thermostat, that's just telling the thermostat to close the fan circuit with the heat. If there is air conditioning that relies on the thermostat to run the fan then the fan control wire will be wired for high speed. Even though "electric heat" selected in the thermostat could be running the fan at high speed during heating this would be preferable to not running the fan at all. I offer this option as a temporary fix to get through a night, weekend, or whatever to avoid an emergency service fee from HVAC repair services. Just leaving the fan on all the time is another option but that will introduce noise, and a bit of a chill with the air moving and the furnace is not running. On more than one thermostat I've seen this "electric heat" setting is a switch behind the cover, just pop the cover off, flip the switch, pop cover back on. This is simple, requires no tools, and easily reversed after repairs are complete.

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  • When the fan is on it’s quite loud. Not in a bad bearing or bent shaft loud way, just blows stronger. When switched to auto and all is well and the thermostat kicks on its considerably quieter. This is bad or not right? Not much of a HVAC guy as you can probably tell.
    – Jason
    Dec 25, 2021 at 22:18
  • @Jason -- putting the fan ON on your unit puts it to high (cooling) speed, so yeah, it'll be louder than if it's on low (heating) speed. As to the fault? I suspect something (dodgy capacitor?) is causing the blower motor to struggle to start on low speed Dec 25, 2021 at 23:10
  • If it’s the capacitor all i can say is man, I hate it when my wife is right.
    – Jason
    Dec 25, 2021 at 23:27
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It might working as designed.

When the thermostat calls for heat, the fan doesn't start immediately because it would be blowing cold air on your neck. When the furnace has warmed up, THEN the fan starts.

Conversely, the fan my run for a few minutes after the heat call is satisfied to utilize any remaining heat in the furnace as is cools down.

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