A variable speed fan motor was originally, and still is, designed to eliminate the discomforts caused by ridiculous on/off part-time heating and air conditioning.  We use it for the comfort it provides.  The fact that it saves energy is a welcome bonus.

I don’t make big fuel savings claims for several reasons. Lets say you have a 10 room home and two of those rooms are bone-chilling cold in the winter or hot and muggy in the summer. You have a variable speed fan motor installed and those rooms get warm in the winter and cool in the summer. You look at your energy bill and maybe you saved a couple of dollars. Or maybe you saved a lot. Maybe it cost you a couple of dollars more. The important point is that you don’t have to retreat from very expensive real estate because your heating/air conditioning system is uneven and can’t keep up.

A variable speed fan motor, when running at half-speed, (which it does most of the time) will use about one eighth the electricity that it takes to run the same motor at full speed. In other words you can run the variable speed fan motor eight hours for the same amount of money that you can run the same motor for one hour at full speed.

If your focus is strictly on energy savings, then turn the system off and freeze in the winter and roast in the summer. Or buy a tent. You’ll save scads of money. Don’t buy the Variable speed fan motor for energy savings alone. Buy it for comfort and healthy indoor environmental quality.

Residential air conditioning is a complex subject with definite and consistent rules. There are two components that must be removed from the air. These are heat and moisture. They are of equal importance. One of the air conditioning rules is:

  • The faster you run the blower, the more heat and less moisture you remove from the air.
  • The slower you you run the blower, the more moisture and less heat you remove from the air.

With a fixed-speed on/off, part-time blower, there is a definite limit to how slow you can run the blower. A variable speed fan gives you the best these two choices by covering them both.

When the air conditioner compressor first comes on, a variable speed fan is running at a slow speed. That catches the latent (humidity) load and really pulls moisture from the air. At the same time, the cold air conditioning coil reduces the temperature of the air going over it. This immediately increases the blowers speed and and sends the cool/dry air into your home. The speed of the blower stops increasing somewhere before it reaches full-speed. This is where it has found a balance between the humidity load and the heat load. A variable speed fan doesn’t let go of the humidity load until it is down the drain and out of your home. Another interesting thing happens in Alabama, when homeowners install their first variable speed fan, they get worried because the blower seems to be running too slow. The reason for this is that a variable speed fan has found that the air is full of moisture and is busy helping the air conditioning system to remove it. When indoor relative humidity runs in the 75% range, there is a lot of moisture for a variable speed fan to work on. When the homeowner checks back the next day, the temperature of the air coming from the registers is cooler and moving faster and the indoor relative humidity is in the low 50% range.

It is disturbing to see the direction that the air conditioning industry is taking. Utility companies will kick-back a couple hundred dollars to the manufacturers if they can save a little electricity. So, what is happening? The pressure is on the equipment manufacturers to speed up their blowers. Remember, the faster the blower speed the more heat and less humidity is removed from the air. (some water is always removed) The thermostat reacts to temperature. Remove the heat and the temperatures drop. The thermostat shuts off the air conditioner and blower before enough moisture is collected. So you save electricity. This is false savings because the homeowner has to set the thermostat to a lower temperature setting in the attempt to get rid of that hot muggy feeling. Because the thermostat is set lower, the air conditioner runs longer. Thus more electricity is used. The resulting lower temperature, combined with not enough humidity being removed, causes the home to feel cool and clammy. That moldy sock smell shows up  as well.

The result is that speeding up the blower to ridiculous levels has produced what we consider is an uncomfortable situation. This might work in a laboratory setting, where nobody has to live all day. We don’t think it works in the real world. Also, the high-speed blower strips moisture from the air conditioning coil and blows it down the ductwork. (mold?)

Here are facts that have not been taken into consideration. The slower the air moves across an air conditioning coil, the colder that coil is and the more moisture it collects.  The faster that air moves across the coil the warmer it becomes and the less moisture it collects.  The colder the coil’s surface is, the better water will stick to it.  The warmer the coil is, the less water will stick to it. Combine high air speed with a warm coil and you have water shooting down the ductwork.  A variable speed fan solves most of this problem by limiting the air flow until the moisture is removed from the air.

During heating season a variable speed fan tames that high speed off/on blower. You only get the amount of air coming from your registers that you need.  Air is always circulating. Always at a comfortable speed for the temperature being delivered. Your electronic air cleaner, filters, humidifier, UV germicidal lamps, and any other added equipment is always working at peak efficiency.  Temperatures throughout your home become balanced. Drafts down stairways are eliminated.  That cold draft when the blower first comes on is gone forever.

Everything attached to your heating/air conditioning system relies on the blower being on. Once the blower shuts off, so do your filters, electronic air cleaner, humidifier, ultraviolet (germicidal) lights and anything else connected to the system.  When the blower comes banging back on, it kicks up dust, mold spores and anything else lurking in your ductwork. When this material gets back into the high speed air stream, it might end up in the best filter of them all, YOUR LUNGS!!

Mold cannot live without moisture. Many off and on fans blow hard enough to spray moisture from the air conditioning coil and down the ducts where it supplies the moisture needed to grow mold.

When a variable speed fan is operating, the average blower speed is much slower. This allows more time for UV lights to kill mold spores. Your filters or electronic air cleaner and UV lights operate about three times more efficiently. And, a super important point is that during air conditioning, much more moisture is removed from your home and it is much less likely to be blown off the A/C coil and into the ductwork where it can provide a great environment for mold.

These are the fan laws.

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 1 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: 1/2 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 variable speed fan 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 a variable speed fan 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.

This proves that a variable speed fan saves energy! Simply put, you can run a variable speed fan, for eight hours at half speed for the same cost as running it one hour at full speed.



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