a/c fan idles forever?

daninthedirtJune 12, 2012

My 3-year old indoor Lennox a/c system works great. Temperature swing continues to be more than 20F. But in the last few days of high temps, it's started doing something I don't recognize. In between cooling cycles, when the compressor is off, there is a faint rushing/humming sound coming out of the air handler. Sounds as if the fan is running slowly. Constantly. There is a very small amount of air (ambient temperature) coming out of the vents, so that's probably indeed what's happening. Why is this? I hear this long after the compressor has cycled off. I would expect that when a cooling cycle is complete, and the heat exchanger has warmed back up, the fan would eventually shut off entirely. I can cycle power on the whole unit, and the idling fan comes right back on.

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It is probably a feature to get all the cooling out of the coil. How long does it run

    Bookmark   June 12, 2012 at 1:22PM
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Yes, thanks. That's what I was thinking, that the fan stayed on after the compressor stooped, until the heat exchanger was warm. But it stayed on forever!

But I now see that it may be user failure. Someone (not me) set the thermostat fan control to "ON" instead of "AUTO". I find it odd that this makes a fan go very slowly, instead of just turning the blower full on. That's why I didn't pay any attention to the thermostat.

    Bookmark   June 12, 2012 at 1:36PM
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You probably have an ECM or variable speed blower motor.

    Bookmark   June 13, 2012 at 8:25AM
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Yep, turns out, after reading the literature on it, that it's a Lennox G60V with a variable speed motor. That variable speed let's it ramp up gradually to cut down startup noise. But what I find a little surprising is that when the thermostat has the fan "ON", the blower runs at very low speed. One would have guessed that when you set the fan to "ON', you'd really want it full ON.

    Bookmark   June 13, 2012 at 8:54AM
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What thermostat do you have? Some thermostats allow you to run the fan constantly at different speeds. You may be set up to run at the slowest speed. If you are going to run the fan all the time, then the slowest speed is the best option. In this mode it will use less power, produce less noise, and allow the air to circulate throughout the house.

    Bookmark   June 13, 2012 at 9:19AM
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The thermostat is a Braeburn 5300. As far as I know fan="ON" doesn't give you any choices about the speed. So I presume it's just wired to keep the fan on at the lowest speed when at this setting. That is, I don't think this thermostat is aware that the blower can run at different speeds.

    Bookmark   June 13, 2012 at 9:33AM
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