Does leaving the fan on all the time cost a lot and is it worth it?
It depends on the speed of the fan and size of the motor. The faster the speed, the more wattage it consumes. Imagine the gas peddle in your car, drive 50 km/hr for 10 hours, you'll consume less than if you were driving 200 km/hr in that same 10 hours. (forget counter wind, friction, hills, traffic, etc)
Most furnace motors can be run at a fraction of the speed to circulate the air to prevent hot or cold areas and removing odors or moisture as well, depending on how the ducting is run.
You could be safe to assume running a fan motor at high speed would consume about 800 watts (8x 100 watt light bulbs for ease of understanding), and low speed may consume 200 watts. So to calculate, you need to know your KW/hr pricing. If it is $1.00 / KWhr, it would be 1000 watts (1 KW) consumed in 1 hour = $1.00. So you have 800 watts during 1 hour, it would be $0.80 for that hour, 200 watts on low speed for 1 hr = $0.20. So over a 24hr period, would cost you $4.80, 7 days = $33.60, 365 days = $1752.00. But that is with $1 rate per hour, which is not accurate as most rates change during daylight, nights, weekends, etc.
Best to program something to be off during peak rates, run at 40% during the cheapest times (nights and weekends)
Hope that helps.
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