I live in metropolitan Houston, Texas (note: high humidity), and this concerns a home I recently purchased. I have a heater (furnace) and blower unit installed in the attic, which is typical for many homes in Texas.

When I bought the home, I noticed the blower in the attic would always run even if I set the thermostat to off on the wall. I assumed there was something wrong with the compressor or thermostat, but something told me to look in the attic, and then I found this on top of the furnace/blower (I guess it is a combo unit).

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Here is a close-up:

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The Honeywell switch (I believe it is part of this system, and model number W8150A1001) was previously set to “On,” but then I set to “Override,” and I assume this enabled the wall-mounted thermostat to override that switch, because now the blower will not run if I set the wall-mounted thermostat to “Off.” Problem solved, or so I thought. While I am savings bundles on my electric bill, I am wondering if there was indeed a reason the previous owner had it set to “On” so that it ran all the time. After all, the switch says “On” offers “Optimized Ventilation.” As the switch is currently set to “Override,” do you think this might enable mold growth due to inadequate (i.e., not optimized) ventilation? Should I switch it back to “On”?

  • What model is the ventilation controller? Feb 16, 2017 at 1:55
  • @ThreePhaseEel—W8150A1001.
    – user56530
    Feb 16, 2017 at 2:05
  • What's the comfort level in the home, since you changed the setting? If it was more comfortable before, turn it back on. If you don't notice a difference, then you can probably leave it set the way it is. Some folks like to continuously circulate the air, and it can sometimes increase comfort. However, as you've seen, that comfort comes at a cost.
    – Tester101
    Feb 16, 2017 at 13:20

2 Answers 2


Below is the text from the user manual. Hope it is helpful. I have the same unit in Florida, also high humidity.

On (optimal ventilation) – W8150 ventilates, based on control settings.

Override – W8150 runs ventilation continuously.

Off (Remote Only) – W8150 supplies ventilation only when there is a remote call.

If the ON position is causing the blower to run continuously (mine does not), then recommend to set your unit to OFF, unless you always want fresh and humid air which is what OVERRIDE is giving you.


My understanding is that the fan will run high, depending on the pressure between your filter and the unit. If you change your filter regularly, then it won’t actually run all the time, but it cycles on and off so you wanna leave it in the on position. Where I live, it is recommended to turn it off if the temperature drops below 20°, so you don’t dehumidify the house and make it to dry in the house.

  • This isn't a speed control ("fan will run high"), but a control of when the fan will run ("Always" or "on demand").
    – FreeMan
    Oct 3, 2022 at 12:15

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