Furnace Blower Fan on 100% of the time - Good or Bad or it depends? I was at a dinner party last night and two guests described how their HVAC reps told them to run their furnace blowers 100% of the time.  The HVAC reps reasons for the recommendation, as relayed by the dinner guests, were: 0 members like this I've found most blower motor failures are due to unbalanced blower wheels (bearings go bad) or overheating (windings burn up). Both caused by excess dirt in motor/blower wheel. Thanks for giving your furnace/motor experience John. One issue I have with the 100% furnace fan run-time is that it does not provide 'fresh air' but recirculates the indoor air. This practice would certainly require a increase in air filter replacement. And yes seems to me to be an energy waster. Anyone hear the argument that motor start up amps are huge when cycling, so better to leave the motor running and you could 'save energy'? Sounds like a similar argument we've heard about lighting.

Any electricians want to chime in on this one? Thanks for commenting and offering your experience. I'm with you about constantly learning something new. It's never ending in our fields! I also agree with you about poor duct design and installation. It is a root problem for many issues seen. As for running the fan on 'auto' or continuously 'on', from an energy perspective I tend to believe that 'auto' would be more energy efficient over time regardless of whether an individual fan is variable or not (I'm not comparing a variable speed vs a conventional motor here just the fan setting). 15 seer 4 ton ac unitWisconsin's Focus on Energy recommends setting fans to 'auto', see link here. air conditioning units gautengThey offer commentary on common assumptions about running blower fans continuously. 2000 jeep cherokee a.c. compressor removal

But Focus on Energy's take contradicts what my friends were told by their HVAC reps. Focus on Energy's assumption #3 really speaks to your point that running the fan for temperature stability really masks other issues such as poor duct design, leaky envelope areas, inadequate insulation, etc. This is just plain wrong. Standard furnace blowers use about 500 Watts and setting them to "ON" for the whole year vs. AUTO wastes about 3,000 kWh/yr. This is a tremendous waste of energy. In addition, in cooling season the fan adds significant heat to the home directly and indirectly if the ducts run outside the conditioned space.These are the fan laws. So, don't listen to the hype put out by some of the manufacturers who claim they invented them and produce the only equipment that they apply to. DEFINITIONS: S = Speed = RPM = Revolutions Per Minute. CFM = Cubic Feet Per Minute. W = Watts = Electrical unit of power. HP = Horsepower = Mechanical unit of power. 1 HP = 745 Watts = Conversion of electrical power to mechanical power.

Fan Laws: CFM2 = CFM1 x RPM2 / RPM1 or CFM is directly pegged to rpm. If RPM is cut in half then CFM is also cut in half. If 1,040 RPM produces 1,200 CFM then 520 RPM will produce 600 CFM. HP @ S2 = HP @ S1 x (RPM2 / RPM1)3 This demonstrates that the horsepower required to turn the fan is related to the cube root of the speed change. Or if the fan’s speed is cut in half, then the amount of air delivered is also cut in half but the Horsepower required is only 1/8 of the original Horsepower required. OR, a fully loaded ½ HP fan motor running at 1,040 RPM and producing 1,200 CFM will only require 1/8 HP to deliver 600 CFM at ½ speed of 520 RPM. This demonstrates that the power required to turn the fan reduces a lot faster than the reduction in CFM being delivered. Watts2 = Watts1 x (RPM2/RPM1)3 or Watts2 = Watts1 (CFM2/CFM1)3 This demonstrates that the electrical power required to turn the fan drops by the cube of the speed change. Or drop the speed (RPM) in half and the Power (Watts) required is 1/8 the original power required.

Our example: ½ HP fully loaded fan motor running at 1,040 RPM and delivering 1,200 CFM requires 745 watts/2 or 372.5 Watts. Cut this motor’s speed in half and you cut the air delivery in half but the Wattage required is 372.5/8 = 46.6 Watts. Therefore, a fan motor must run 8 hours at half speed to use the same amount of electricity as it would running at full speed for 1 hour. Now lets bump this up-against reality. Lets assume that our example fan is in an average furnace. On an average winter day it runs about 1/2 of the time. It therefore uses 372.5 watts / 2 = 186 watt-hours of electricity. Now we install a FanHandler that runs the fan full time. Now lets say there is a call for heat 3 times during that hour and the fan reaches top speed three times for 3 & 1/3 minutes each time (which it probably won’t) 10 minutes per hour = 1/6 hour x 372.5 watts = 62 watts and the other 50 minutes it uses 5/6 of 46.6 watts = 38.83 watts for a total of 38.8 + 46.6 = 100 watts per hour.

For a savings of 86 watt-hours. And the home is comfortable, the air cleaner or filters are working full-time at much higher efficiency. Monthly cost for the FanHandler equipped fan at $0.10 per KWH = 86 watt hours x 24 hours x 30 days per month = 61,920 watt-hours or 61.9 KWH x $0.10 = $6.19 per month. If you ran the full-speed fan round the clock, it would cost 372.5 watts X 24 hours = 8.9 KW = $0.89 per day X 30 days per month = $26.00 per month. If you ran the full-speed fan ½ the time it would cost $13.00 per month. You can use your electricity costs and horsepowers etc. to do the comparisons. There are some Permanent Split Capacitor motors being installed in equipment today that are designed to meet a price criterion. These motors will not follow the fan laws and will growl and rumble at low speed. They will not follow the fan laws. They will not reduce amp draw with speed reduction, or the amp draw reduction will not be in line with the fan laws. ECM motors are generally of much higher quality.