HVAC Talk - Watch out for Short Cycling

It is starting to warm up in the South and we're all starting to crank up the A/C again. So I thought I'd briefly mention a huge efficiency killer with A/Cs...Short Cycling



Short Cycling is when your A/C comes on for a short period of time, cools the house and quickly turns off. Then a few minutes later, it is back on. Usually this means that your A/C is over-sized, but different outside temperatures can cause different run times. Here is why short cycling is something you want to avoid:
1. Especially in newer systems, the coils can take awhile (up to 10 minutes) to get fully cold. If your system is running for 3-5 minutes and then turning off, you aren't getting the full benefit of the compressor.
2. A/Cs are meant to be able to run for extended periods of time. Quick on/off cycles cause unnecessary wear and tear on your system.
3. A/C units remove humidity better when the coils are really cold. When the unit short cycles, the coils don't get cold enough and the humidity in your home isn't removed. With higher humidity, you generally don't feel comfortable and thus end up lowering the temperature, which costs money. Not to mention potential mold problems.

Now generally there are only 2 things causing short cycling, 1 of which you can easily control. First off, short cycling usually means your HVAC unit is oversized. This is why the experts always recommend against oversizing. This is hard to fix because it involves replacing the whole unit. The second is that the "sway" on your thermostat is set too low. The sway is defined as the number of degrees above your set-point before the A/C will turn on and the number of degrees below your set-point before the unit will turn off. So if your sway is set at 1 degree and you set your thermostat at 74, the A/C won't turn on until the temperature reaches 75 and it won't turn off until the temperature drops to 73. The higher the sway, the bigger temperature difference you have to get used to. But the lower the sway, the more likely your unit is to short-cycle.

Your programmable thermostat often has a place to adjust the sway. To find this out, check out the user manual that came with the thermostat. Some newer models may not have the ability to adjust the sway, but instead may have a place to set cycles per hour (or CPH). This tells the system not to turn on and off more than x times per hour. So if you set your system to a maximum 6 cycles per hour and it goes through 6 cycles in the first 35 minutes, the thermostat will stay off regardless of temperature for the next 25 minutes. This will likely cause the temperature to rise above the set point, which will allow the system to run longer the next time it turns on. This will ensure that your humidity is being regulated and ensure that your house is being cooled in the most efficient manor possible. Check out these features on your thermostat and make sure your are using your system efficiently.