It's been hot here in the Atlanta area lately. Just three weeks ago we were wearing jackets with low temperatures in the thirties Fahrenheit, and now we're getting up above our summer design temperature every day. That means that a lot of air conditioners are running as everyone tries to keep cool. One common question I get from homeowners is: Should I put the fan in the on position? Most thermostats have two settings for the fan - auto and on - and some well-meaning HVAC techs tell their customers to put the fan in the on the position to distribute the cool air in the house better. Let me back up a bit here. For purposes of this discussion, your air conditioner has two components: the part that cools and the part that moves the air. You adjust the part the cools by changing the temperature setting on your thermostat. You can affect the air movement by setting the fan to auto or on. When the fan is in the auto mode, the air conditioner moves air only when the cooling part is running.

When the thermostat senses that the house has reached the setpoint, it shuts off both the cooling and the fan. When the fan's in the on mode, the fan keeps running continuously, even when the cooling part is turned off. One of those settings is better than the other. The photo below shows how my thermostat is set, and that's how I recommend you set yours, too. The number one reason you don't want the fan to run continuously is that your air conditioner is probably oversized. Since your AC is responsible for doing two jobs, cooling and dehumidifying, moisture from the air condenses on the cold evaporator coil when it runs. An oversized AC will shut off after a short run time and leave a lot of water sitting on the coil. By running the fan after the coil warms up, a lot of that water will evaporate and be put right back into your home. So, putting the fan in the on position compounds the error of having an oversized cooling system. Your house will cool down quickly without dehumidifying well.

Running the fan continuously makes the dehumidification worse and your home less comfortable. Update (9/8/14): See my actual data on how setting the fan to the on position raises the humidity. Even in a dry climate where dehumidification doesn't matter because there's little moisture in the air, there are other reasons not to leave the fan in the on mode. First, it costs more because you've got that fan using about 300 watts while it's running, and you're probably not getting much benefit from it. Good HVAC design in new homes will obviate the need for this if you're running the fan because of air distribution problems. [See addendum below regarding dry climate fan operation.] Another reason to use the auto mode is that if you have leaky ducts outside the building envelope, it can add to the air leakage of the house. Depending on the type of duct leakage, you may end up with higher infiltration (air leaking in) or higher exfiltration (air leaking out). In either case, you add to your cooling load and make the home less comfortable.

Here's another really good recommendation, too. If it's just too hot out, go find yourself a nice waterfall to cool off in, like I did this weekend. Addendum (6/2/13): If you're in a dry climate, running the air conditioner for a few minutes after the compressor shuts off can help cool the home more efficiently. You don't want to run the fan continuously 24/7, though, because that will use more energy than is necessary for the extra cooling and have the duct leakage problems mentioned above if ducts are outside the enclosure. non window unit air conditionerThe way to do this would be to install John Proctor's Western Cooling Control, which Martin Holladay wrote about at Green Building Advisor.how to wire outdoor ac unit Do Programmable Thermostats Save Energy?central ac ratings consumer reports

Fan motors are the unsung “heart” of HVAC equipment. They work unobtrusively and quietly for years, often under tough conditions. They don’t need a lot of love, but they do need periodic attention and it’s up to you to provide it. The cost of overlooking regular testing and maintenance of motors can be an HVAC system that grinds to a halt, so be sure to check the motor on every maintenance and service call. When you arrive on a jobsite in which a unit’s fan motor is not working or the high-limit switch has tripped, the first thing to check is power to the motor and/or unit. If there is proper voltage, check the low-voltage control circuit. Check for proper voltage at the transformer and check for a fuse in the low-voltage circuit. Make sure the circuit breaker is not tripped (or the fuse is not blown). If the circuit breaker is tripped (or fuse is blown) or if the transformer is bad, it could indicate a short in the motor. In that case, check the windings in the motor before turning the power back on.

To check the windings of a motor for an open or a short, you’ll need to measure the ohms. If the unit has a 120V motor, it will most likely have three or four colored wires (black, red, yellow, and blue are common), a white wire, and two brown wires. Do a resistance check between the white wire and each of the colored wires. The higher the resistance, the lower the speed, with each color representing a different speed: i.e., four colored wires, 4-speed; three colored wires, 3-speed. You want to see a resistance reading. If you get a zero reading that means the motor winding is shorted and may be the cause of the tripped breaker/blown fuse. If you get an infinite reading (overlimit or OL on most digital meters), that indicates an open motor winding. If either of these conditions exist you will have to replace the motor. When checking the ohms on motor windings, many technicians have difficulty determining an open winding vs. a shorted winding. But it’s really not hard to tell the difference between the two.

It should show a resistance. If it shows a zero reading it probably means the windings are shorted out. If it shows an infinite reading, that means the windings are likely broken or open. A good trick to remember this is that an infinite reading means the greatest resistance in the world. It’s like an open door leading to infinity — in other words, an open winding. Meanwhile, a zero reading means the load is taking a short cut around it — in other words, a short. That’s just a simple trick to help you remember that a zero reading means a short, and an infinite reading means an open or broken winding. Just because a motor is not running doesn’t mean the motor is bad. If the power is correct and the motor is neither shorted nor open, check the capacitor (that’s where the two brown wires go). A capacitor helps the motor run and gives it more torque. If a motor doesn’t have the torque to turn the blower wheel or the fan belt, it won’t start. So the capacitor plays a big role.

The capacitors on most of residential blower motors are very small, so technicians tend to overlook them. You should discharge the capacitor before handling it. Using a capacitor tester, make sure the microfarad reading is within 10% of the rated capacitance on the capacitor. It will be a number listed in uF or mfd, depending on the manufacturer. If the reading does not match the rating, replace the capacitor. Checking 240V motors is slightly different because you will have two hot wires and most will be only single-speed. But you still ohm-out the windings to check for open and shorted windings and check the capacitor the same way. Finally, when doing a routine maintenance call on a unit, always check and clean the motor. One of the biggest problems with motor burnouts is dirt. The main thing to tell homeowners is to keep everything clean. Most of the furnaces in heating climates are located in a laundry room or a basement. In those rooms, lint and dirt kicking up from inside the house can block the motor holes and make a motor overheat.