Setting Thermostat for Central AC in a Two Story House

Update: the principles stated here apply to heating system in a two story house as well.

We did not pay much attention to our central Air Conditioning system as long as it cooled the house until an incident occurred recently during the repair of our roof.

The Incident 

During the replacement of our roof in early May, the workers punctured the AC lines for both units of our AC system. The roofing company had to hire an Air Conditioner Service to fix the AC lines and we did not have AC for two days. It happened to be the hotter days in early summer - the second floor  in the house reached 87F in the late afternoon and early night, the first floor's temperature was 82F.

When the AC technician finally came, his tests confirmed that both units did not hold pressure and there must have leak somewhere, and he quickly located the locations of the punctures -  where I observed long nails going through  AC lines' insulation rubber wraps in the garage attic. Because of  the punctures, first of all, the brass AC lines needed to be fixed, and the Freon in the system were 100% lost and needed to be refilled , and thirdly the system was open, the dryers on the AC lines needed to be replaced.

After fixing the AC lines on the garage roof, he updated his office about the price for his work. Initially it was ~ $400+Freon for one unit, now his invoice was  ~ $1400 for everything. A dispute between Roofing company and AC service ensued - the roofing company refused to pay, and the technician was packing his tools before the repair was completed. I had to assure the technician that he would be paid and I called my roofing company project manager. The dispute was eventually resolved.

The AC finally functioned properly at 2pm that day.

The Observations

Because of this repair, I paid special attention to the AC operation. I expected that AC would idle most of time as it has fresh fully loaded Freon and outside temperature was at low to mid 80s.

The next day afternoon, while I was cooking, I noticed that second floor AC was on all the time while first floor AC was idle. Since second floor is hotter than first floor, I mentally gave the second floor AC extra time to idle.   The second floor AC was still running after dinner, without ever idling. That was more than two hours! Later the up-stair AC finally idled, but only for a couple of minutes.

With this type of cycling, the second floor AC should break much faster than the first floor AC. In fact that was exactly what had happened - the first second floor AC was replaced 7 years ago, while the original first floor AC is still working.

I decided to fix this problem.

Setting Thermostat for Central AC in a Two Story House

The first thing I did was to check AC manufactures' websites. I did not find any relevant information. Then I did a wider search on the internet. There are a lot of complaints about central AC system for two story houses - second floor warm, first floor cold; second floor AC was on all the time, first floor was idle most of the time .....not really any solutions -  one common suggestion was to rework ducts to balance the cold air flow.

One fact that dictates the mechanics of the air flow is natural thermal convection - hot air goes up, cool air goes down. In addition, the second floor is closer to the roof, and has more heat goes into it. This was observed when our AC was down, the second floor was 5 degree warmer than first floor.

After some thinking, I believe that I found the source of the problem - we set the first floor and second floor at the same temperature. When the two floors are set at the same temperature, the first floor would reach the set cool temperature faster than 2nd floor, as the second floor AC continues to work, the first floor temperature would rise - just slightly - the warmer air would rise to second floor, the air at setting temperature in second floor would drop to first floor. Due to this convection, the first floor will be at setting temperature or slightly lower than it; the second floor will be just slightly above setting temperature. This leads to the fact that first floor AC idles for long time while second floor AC just can not idle despite the fact the thermostat shows that the temperature is at setting temperature already. The reasoning is validated by physical observations - when both floors AC were set at same temperature, both floors would reach cooler afternoon/evening temperature from day time higher setting after some time, the first floor AC would then idle, but second floor just would not idle.

So the solution to "Setting Thermostat for Central AC in a Two Story House" for optimal AC working cycles - keep the house comfortably cool while increasing AC idling time, is to set first floor temperature a couple degrees lower than second floor.

The outcome of this setting was immediate that evening - after the first floor temperature was lowered by two degrees from second floor temperature, the second floor AC quickly idled.  Afterwards, I timed the AC cycles - the second floor AC would run 10 minutes, and then idled for 43  minutes - I observed two cycles; the first floor AC idled 2+ hours, and ran 7 minutes before idling again (observed only 1 cycle).Note that outside high was at mid 80s that day.

We programmed our thermostats according to "first floor temperature a couple degrees lower than second floor" for all time periods. Though AC run time and idle time change according to outside temperature, I consistently observe the same trend as I observed when I first tested the setting. It works!

With second floor AC idles regularly, and added 4 new wind turbine vents on the roof, I expect our electricity bill to decrease this summer - Save Money and Save Energy.

Note - in winter, when heat is on, you still need to "first floor temperature 1 ~ 2 degrees lower than second floor" for all time periods.  Same principle here as for cooling.