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How much energy is wasted for the continuous operation of two bathroom exhaust fans?

Why are the wired this way?

closed as unclear what you're asking by isherwood, Tester101 May 5 '17 at 14:24

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    A lot....whatever air you exhaust you'll bring the same amount of air back into the house...and then recondition, (heat or cool). – Lee Sam May 2 '17 at 22:03
  • How much energy is wasted depends on the wattage of the fans. Hardwiring them to "on" seems like a bad install to me - aside from the continuous noise and wasted power, I would expect it will shorten the fan's life. – CactusCake May 2 '17 at 22:03
  • Is it possible that you have a fan with a humidity sensing switch, for which the sensor is never getting dry enough to turn off? – CactusCake May 2 '17 at 22:06
  • The fan itself is a relatively low-power device. I don't typically see power usage noted, but links here and here indicate usage about 5-60 W. If you were to run the 60 W fan continuously all day, every day, for a year, you'd use 525.6 kWh of electricity. The cost to do that would of course depend on your area, but at 10 cents per kWh, it'd cost just over $50 per year. Two of them would thus cost $100. – mmathis May 2 '17 at 22:13
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    Are you asking because your current fans run continuously all day every day, and you want to know how to fix that? If so, edit the question to ask it that way. Or is this just curiosity, in which case it's off-topic. – mmathis May 2 '17 at 22:16
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I suppose it's at least half an answer, so moving from comments:

If the fans are wired to run that way your house may well be built to a high standard of air-sealing, and the fans running (possibly with a heat recovery ventilator involved) may be an important part of maintaining good indoor air quality - at which point the power to run them is not a "waste" and the heat/cold required to adjust the air they replace is far less than that required in an older house with random air leaks.

In that case, turning them off may cause major indoor air quality problems.

The above is the only case I'm aware of where bathroom fans are set to run continuously as a deliberate design part of the HVAC.

A mechanical vetilation requirement evidently was integrated into IRC circa 2012

As for the other half of the question, the linked document notes a minimum efficency for bathroom fans of 2.8 cfm/watt and a maximum ventilation rate for them of 90 cfm, leading to a maximum power draw (per fan) of 32 watts, or roughly 282 kwh/year. Or it could be as low as 31 kwh/year for the minimum sized bathroom fan (10 cfm) or even lower if the fan is more efficient than the minimum required efficiency.

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