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. ac power supply priceThe photo below shows how my thermostat is set, and that's how I recommend you set yours, too.top rated home hvac units The number one reason you don't want the fan to run continuously is that your air conditioner is probably oversized. main components of an ac unitSince 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. The way to do this would be to install John Proctor's Western Cooling Control, which Martin Holladay wrote about at Green Building Advisor. This Thermostat Setting Can Cost You Money and Make You Sick How NOT to Use Your Heat Pump Thermostat

Do Programmable Thermostats Save Energy? The Blower motor on my AC unit turns off at random times while the outside unit continues to run which is causing the coil to freeze up. The blower won't come on for 30-45 minutes after the AC has been turned off, but after that it seems to turn back on fine. I have been trying to track the problem down for the last couple months, but it literally only happens every 2 or 3 weeks. Last night it stopped again and I tried to just turn the fan (from the thermostat) on without luck. I took a look at the blower and it seemed to turn just fine. I put my hand on the back of the fan motor and it wasn't hot to the touch. About 45 minutes after it turned off the blower came back and it was working normally. What could be causing this problem? I ended up calling a buddy of mine who is an HVAC tech and he pointed me in the right direction to check the fan wire on the back of the thermostat. I checked the wire and it was just hanging in the terminal.

I'm pretty sure that was the cause of the problem. It makes sense because there wasn't anything wrong with the blower motor itself. The fan wire on the thermostat was in fact the problem. I fixed the connection and it has worked fine for the last 3 years. I have a ruud rheem heat pump, I had this same issue. The unit was installed in 2006. After going through the typical steps of checking air filters and electrical components on the unit, I found the wiring from the thermostat for the blower control to the control board was rusted in the wire nut. I cut the wires and stripped them back and reconnected the wires and the AC unit is running with out any trouble now. Remember to keep it simple. HVAC service companies love these type of calls that an experienced tech can track down quickly. If you have a basic understanding of how circuits work you can fix this easily and save a bunch of cash. My central AC unit decided to stop on the hottest day of the year. It would start and run normally for a few minutes and then the blower would shut off.

After checking the breakers, thermostat and overflow shut off (did not have one) I saw something about a blower motor capacitor. This was easy to get to and replace and my last hope before waiting for someone to come fix it. The capacitor cost $4.04 with tax and had my AC working again. Needless to say I was beyond happy. I hope this can help others with a similar problem. Check your drain lines for water run off. Make sure they are clear and then check the float for the excess water shut down reset them Also check your ground. I just ran across an intermittent blower issue (cycling on and off constantly during call for heat from the thermostat) that was due to a poor ground connection on my 120 volt to 24 volt transformer on an old Janitrol furnace. The ground side of the 24 volt half of the transformer had 2 wires connected to it. The good one went to the mounting screw of the transformer. The bad wire went to the circuit board. It was bent over perpendicular just above the connecter, and many of the strands were broken.