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How to choose the right ceiling fan (other than the color)

Everyone has a “style” in their home, and glancing at the number of pages of ceiling fan choices in any online hardware or home decor store, there is a ceiling fan for every style!   You’ve got modern, traditional, glam, rustic, minimalist, and everything in-between.  Knowing the characteristics of the best-performing ceiling fans will make your choices easier within the style and color you like, so take a minute to check out these tips.

The object of a ceiling fan is to circulate air, so air flow should be near the top of your list.  Airflow is measured in cubic feet per minute, and although some manufacturers like to give measurements of velocity (feet per second or meters per second), they are not the same.  Airflow is what “mixes up” the air in a room and breaks up stratified air (thermal layers) so that your air conditioning system becomes more efficient.    The most efficient ceiling fans are High Velocity Low Speed (HVLS) types; these are the ginormous fans that you may see in a warehouse store or sports arena.  They typically measure 6 feet in diameter and larger, many topping 20 feet or more.  Surprisingly, they move the most air with the least amount of energy, but you do need a high ceiling so that each fan is between 20-25 feet above the floor.  If you aren’t looking for such an industrial size fan, Aeratron makes the most efficient ceiling fan in normal sizes.  Their fans incorporate several designs that help them to achieve this efficiency:

• Blade shape: Blades can get super-fancy, but the most efficient are oar-shaped, or in Aeratron’s case, with a “winglet” to reduce drag.

• Motors: the more powerful the motor, the more efficient the fan.  The most recent developments are “DC” or direct current motors, which can be adjusted to an infinite number of speeds.  These are electronically controlled and are typically quieter, smaller and lighter than AC (alternating current) motors.  

• Number of blades (Less is More): Did you know that the lower the number of blades, the better the efficiency?  More than two blades just cause more turbulence and do not move more air.  

• Slower is better:  Operating a ceiling fan at a slow speed continuously is better than higher speeds.

• Having a downrod (an extension that lowers the body and blades of a fan away from the ceiling) is critical to a fan’s ability to move air.  Even a 3” to 4” downrod increases efficiency by approximately 40% over a flush-mount fan (one that hugs the ceiling).  Check out downrod specifications below. 

So, after giving all this information, there is one number that could guide you to the most efficient fan in your style preference.  It’s called cubic feet per minute per watt (CFM/W).  Basically, it’s analogous to gas mileage for fans–how much air it can move per watt of energy expended.  Although 75 is the minimum to make a fan efficient, the higher the better, and the most efficient fans have a CFM/W over 400.   Here’s a list of the most efficient as compiled by EnergyStar.gov.

So, now that you think you have found the most efficient, stylish fan in your budget, we just wanted to make sure you know the following interesting information:

Ceiling fans don’t actually cool a room.  Say whaaat?  That’s right, ceiling fans don’t cool a room because they can’t remove heat, the way an air conditioner can.  In fact, because ceiling fans expend energy and a portion of that energy produces heat, they actually add heat to a room.  The cooling effect you feel is the wind chill effect, which is the temperature which a person feels with respect to the wind.  (cielowigle.com)  According to Consumer Reports, using a ceiling fan, can make you feel up to 4° F cooler, and you can save 3 to 5 percent on air-conditioning costs for each degree you raise the thermostat, so using ceiling fans and raising the thermostat setpoint can theoretically save you 12-20% in air conditioning costs.  Pretty cool!

Remember the rule above about the less blades, the better?  Well, an improvement on a two-bladed ceiling fan might just be a ceiling fan without blades.  There are actually quite a few products that advertise to be “bladeless”, but in reality, the blades are enclosed in a casing (enclosed fans, by the way, are great choices for kids’ rooms and near bunkbeds, to avoid the chance of getting fingers or toys caught in them).  One of the true “bladeless” fans is the Exhale Fan, which features a stack of spinning discs modeled after Nikola Tesla’s bladeless turbine.  Instead of shooting the air straight downwards like traditional fans, Exhale fans move air at a 45 degree angle, which accomplishes a lot: it creates a vortex and de-stratifies the temperature layers of the room, more effectively mixing the air.  The fans themselves move over 5,000 cfm of air, which is pretty amazing since they have no “blades”!  No blades also means very little noise.  In addition, they are preferably mounted directly to the ceiling, eliminating the use of downrods.

Sizing your fan is very important.  The larger your room, the greater the diameter of the ceiling fan you need to circulate air effectively.  This sizing guide is very simple to use, and when in doubt, it’s best to go one size larger!   The reason for this is that it’s more efficient to operate a large fan on lower speed, than to operate a smaller fan on its highest setting.    

Choosing the correct downrod length is very important.  It’s not common knowledge, but if you have a standard 8’ ceiling, you should have a short downrod on your fan (3”).  When I purchased my home, even though all of the ceilings are 8’ or more, 6 out of 7 ceiling fans were flush-mounted (mounted directly to the ceiling).  Flush-mounting is only recommended for ceilings lower than 8’, and a flush-mounted fan moves approximately 40% less air!   So, before purchasing a ceiling fan, measure your ceiling height and check this handy guide.  If a fan accommodates a downrod, you can always adjust the length by purchasing the right downrod separately.  Basically, you’ll want the fan to be at least 7’ from the floor and 8-9’ for optimal airflow.

LED lights continue the savings.  If you are going for efficiency, it makes sense not to squander savings from the fan by using incandescent light bulbs.  Many high-end fans now have permanent LED bulbs and LEDs that can be adjusted for color in cooler or warmer tones. 

What is Inverter Technology and what does it do for my Air Conditioner?

To start, let’s review the way conventional air conditioners work.  You set your preferred temperature, and when the room gets too warm, a thermostat sensing the higher temperature will cause the air conditioner to come on.  When the room is cooled properly, the air conditioner shuts off.  The lag in turning on and off varies by unit, but it can be as much as two-three degrees:  at a set point of 72 deg F, the air conditioner doesn’t come on until the room warms up to 73, and doesn’t shut off until the room is cooled to 71 deg F or lower.  In the case of inverters in air conditioners, they can keep the room at an even 72 all day long by running continually with lower power input.  The best way to explain them may be the example of cruise control in a car.  If you try to keep the car at a steady speed, you’ll need to depress the gas pedal when you see the car decelerating, and let up when it goes over your target speed.  Letting the car’s computer keep the speed is actually more gas efficient, because it will sense speed fluctuations sooner, and meter the gas pedal more accurately than you can!  Here is a pictorial by Panasonic:

Once again, the power of computers is making our lives better by modulating the power consumption of the unit and avoiding those annoying temperature swings.   But wait–there’s more!  With only a 2-3 degree temperature swing with a traditional air conditioner, why does it feel like one minute you need an iced drink, and the next you need a sweater?  It just doesn’t seem like that should happen with only 2-3 degrees. The answer lies in the humidity of the air.  Temperature fluctuations cause the relative humidity (RH) to fluctuate, which makes a big difference in comfort levels!   Humidity gets taken out of the air as it passes over the evaporator coils, and this is an important function of the air conditioner, because as you cool the temperature, RH will go up.  If you don’t have a non-inverter air conditioner sized just right (having the optimal compressor power and refrigerant for the size of room), it will run for too long or too short, causing the RH to fluctuate:

• Too large of a unit means it will run for short cycles, cooling the room quickly but not running long enough to remove adequate moisture from the air.  
• Too small of a unit means that it will run for long periods of time and humidity will go down, but jump right back up when the unit shuts off.

The inverter air conditioner not only keeps temperature constant, it can also keep humidity lower and constant.  Here’s how that works:

• Back to physics class:  There are two kinds of heat: sensible and latent heat. Here is an excellent page to understand more, but basically sensible heat is related to changing the temperature of a body (in this case the air), and latent heat is related to the phase change of the body (in this case changing the water vapor in the air to liquid so that it will drain away).  
• Constant speed on/off systems are good at removing sensible heat to lower the temperature of the air.  However, they don’t do such a great job at removing latent heat (humidity) out of the air.  This diagram shows why: it is a diagram of how much energy is needed to push water into a different phase.  The long flat part at 100 deg C (212 deg F) shows how much energy is needed to change water from liquid to vapor, and once it is a vapor, the same amount of energy (970 BTU, or British Thermal Units) is needed to condense it back to a liquid, without even changing the temperature!

• So, when the system is set to only lower temperature, it does not continue to supply the needed energy to change the phase of the water vapor in the air to a liquid, to condense it out of the air.  It will simply stop cooling when it hits the temperature set point.
• Inverter technology uses a computer and variable speed fan to slow down the speed of the air crossing the evaporator coil.  When the air spends more time in the evaporator, more water vapor will condense out of the air, meaning more dehumidification occurs.    It also uses a variable speed compressor to better control the flow of refrigerant through the system, because slowing the fan alone with a standard compressor may cause the coil to freeze.  Since the system will be continually cooling instead of stopping and starting, a variable speed compressor allows for a small steady flow of refrigerant when the room is in a steady state (no one coming in or out or cooking, etc.), but also a ramp-up when someone opens a door or turns on an exhaust fan, etc. 

This is all great information, but what if you have a central AC system that’s only a couple years old, and very little budget to go making changes?  The good news is that it is possible to replace your standard single-speed blower motor with a variable speed motor.  Here are some facts about this type of upgrade:

• The term “variable-speed” applies to two different components in an air conditioning or heat pump system: the blower motor and the compressor. A variable-speed blower can be matched with a single-speed compressor, but a variable-speed compressor must always be paired with an adjustable-speed blower. (therma.com)
• Single speed motors in blower units are generally called “PSC” or Permanent Split Capacitor, while variable speed motors are called Electronically-Commuted Motors (ECMs) or Brushless Permanent Magnet motors (BPMs).  ECM is a trade name for BPM (BuildingScience.com)
• ECMs come in “3 flavors”: non-variable constant torque, variable constant torque, and variable constant flow.  (David Butler, energyvanguard.com).  If you are looking for better dehumidification, you may want to go for the Variable speed, constant torque model.  The Variable speed, Constant Flow models are the most expensive.
• ECMs are quieter and use less electricity than PSCs.
• Retrofitting the blower fan has the following challenges:
  • You’ll need to make sure the existing electrical service is compatible with the new ECM.  Although the resulting operating current draw should be less than the PSC motor it replaced, you’ll need to check that the motor nameplate ratings don’t require upsizing the supply circuit in order to provide the necessary electrical safety.  (Titus HVAC)
  • The new motor may require a different type of mounting, ie. a “belly band” vs. a bracket, and the technician may need to use parts from several conversion kits to install it. 
  • The Building America program retrofitted PSC blower motors with ECM motors at 8 homes in upstate New York in 2013 as part of a study.  The study did extensive screening of homes and systems so that only homes with the right size hp motors, the right size blower housing and cabinets, and the right type of motor mounts were chosen, making the install straightforward for the technicians (US Dept. of Energy).  According to Titus HVAC, ECM retrofit doesn’t just mean replacing the motor. It generally means replacing the motor, the speed control, and blower assembly. It will likely also require additional components like power cables, communication cables, and a power filter. It could even require changing internal options like line and/or motor fuses.  
  • There are several manufacturers for these retrofit motors.  
    • The most well-known may be Regal Rexnord with the Genteq Evergreen motors, also offered on Amazon (this company purchased the ECM trade name).  
    • US Motors’ “HVAC Rescue Motors” series includes bluetooth-enabled motors and some that do not need the aforementioned airflow programming, making for an easier install.
    • The Concept 3 motor used by the Building America Program is currently out of production.
  • A knowledgeable HVAC technician can help you with the decision and options; don’t be afraid to get several opinions!  Ask if he will be taking pressure and power readings before and after the install.  This is key to getting your new motor running right, or preventing an ineffective change to the system.  Here are two links for more information on pressure and power draw readings.
• Some states or power companies may offer rebates to retrofit your PSC with an ESM, such as this 2013 offer by Minnesota Power. 
• ECMs will save power on systems that are set to have the fan always “On”, such as when a Whole-Home Polar Ionizer is installed.  This type of purifier runs 24/7 and as such needs the fan to run 24/7.  

If your whole HVAC system is nearing end of life (10 years or more), you may ask about replacing the blower and compressor with variable-speed units, which will give the best energy efficiency available in forced air units today.  Some HVAC companies will try to sell what is easiest for them to install, or what their suppliers recommend, or what has always been done, but there is always room for improvement…keep searching and asking until you find it!