2-story house and 2 AC units need a balanced approach

[ChippyTX]

I have 2-story house and 2 AC units (4-ton for 1st floor and 2 ton for 2nd floor... each AC unit independent of each other ).

2 Bedrooms, Game room, 1 bathroom, and large storage closet (AC'd) Upstairs. 2nd-Floor Game room is open to the stairs and entry-way on the 1st floor. Most of the 1st-floor open floor plan.

We are in south-central Texas. Outside temps June-Sept are routinely 96-103 every day.

As an energy savings approach, my wife tells my son to shut-off the 2nd-floor AC when he comes down. Of course the 2nd-floor temp rises over time and of course the warm air from the 1st-floor rises up to there too. Then, my son goes back upstairs to play video games and the temp is 86+. So he turns on the AC and it is set to 80-81.
Now my wife says the 2nd-floor AC is running non-stop and never automatically shuts off.
In addition, on the 1st-floor, my wife wants to set the temp to 84 or 85 (to keep the electric bill down). I have a home-office. Eventually, that becomes too hot (uncomfortable) for me, so I set down to 80-81. Then my wife sets the Temp back up to 85, then I set it down to 80.

I know this scenario sounds silly. I applaud my wife's interest in energy-saving and therefore cost savings. But, I need some additional information from all of you HVAC-Talk forum experts to help support my argument that maintaining a consistent balanced temp is more energy efficient and cost-effective than the repeated running the temp up and down or AC unit off and on approach.

2 Questions:
In the long-run is all this yo-yoing of the temp on the AC units actually causing the AC units to actually work hard and longer (using more electricity) to re-cool the house than if the 1st-floor and 2nd-floor temps were set to maintain some type of balanced temperature ?
If so, How can I achieve an optimized balance between the two floors, given that most of 1st Floor, Stairs, and 2nd-Floor Game Room are open to each other?

Thanks,
ChippyTX

[classical]

Short version letting the upstairs temp elevate is causing the downstairs system to work harder. Set the upstairs at your comfort set point and leave it alone.

[ChippyTX]

Thanks for quick reply...

Can you give me a bit more explanation?

I know one way to cut the cost of my electric bill is to cut off both AC units. But, in this Texas heat that is not practical.

So, I'm trying to learn and understand the Air Conditioning principles associated with operating my two independently controlled AC units on two separate floors that share a significant amount of shared air-space. These two AC units have separate thermostats, air handler units, and separate return-air ducts.

If the warm air rises (and collects) to the 2nd floor when the 2nd-floor AC is off. How does that make the 1st-floor unit work harder? The 1st-floor AC unit return-air duct is on the 1st floor (and I'm thinking presumably not directly drawing that 2nd-floor hot air back into the 1st-floor system).

If I run both systems all the time, should they both be set to the same temperature? Or, should the 2nd-floor unit be set different because it will normally catch more of the rising hot air?

[tnhikr44]

ChippyTX,

Not to sound like a jerk, but either you can afford to keep a reasonably even temperature throughout your house or you cannot. If you can, set the T-stats at the same temperature and let the systems do their thing. You may have to make a degree or two adjustment to find the happy place, but after that just leave the T-stats alone.
If you really cannot afford to maintain a reasonable temperature (at what cost comfort?) then keep fiddlin' with your temperature control. Place the fans on continuous run. Turn the ceiling fans on high. Use the envelope your electric bill comes in to wipe the sweat off of your face.
Here in central Georgia our summer highs are around 100 degrees. My T-stat is set at 72. That is where we are comfortable. If it ever gets above 75 in our house our system ain't working. Coming home to a comfortable house is worth every dollar I pay. What are you willing to pay for comfort?

[beshvac]

Thanks for quick reply...

Can you give me a bit more explanation?

I know one way to cut the cost of my electric bill is to cut off both AC units. But, in this Texas heat that is not practical. That's why it's sometimes called "comfort cooling"....if you really want to save money shut them off

So, I'm trying to learn and understand the Air Conditioning principles associated with operating my two independently controlled AC units on two separate floors that share a significant amount of shared air-space. These two AC units have separate thermostats, air handler units, and separate return-air ducts.

Set the thermostat and forget it

If the warm air rises (and collects) to the 2nd floor when the 2nd-floor AC is off. How does that make the 1st-floor unit work harder? The 1st-floor AC unit return-air duct is on the 1st floor (and I'm thinking presumably not directly drawing that 2nd-floor hot air back into the 1st-floor system).

Your downstairs system was designed that the area above it was conditioned, when you cut off the upstairs...the heat load on the house is the same but its only being made up for by the downstairs....it runs longer.

If I run both systems all the time, should they both be set to the same temperature? Or, should the 2nd-floor unit be set different because it will normally catch more of the rising hot air?

I set mine the same.....my wife likes 74....up or down...