I have been running some tests in my home, in severe winter weather, to reduce the elevated upstairs room temperature due to over heated first floor air rise to the 2nd floor.
My house design has a huge open two story atrium space in the middle area of the home. There is a large open balcony to the downstairs area. The space is easily 50% of the floor area.
The home has two separate HVAC (Heating Ventilation Air Conditioning) systems, one for each floor.
Heat rises. All heated air from the first floor HVAC system, easily rises to the second floor area. There is no need to operate the separate heating (HVAC) system for the second floor area.
What I do is run the second floor HVAC system in fan only intermittently (no heat) which cycles the system fan about three times per hour.
But the upstairs is still overheated by 5-7 degrees or more from the downstairs ceiling air which is well above the thermostat set point and rising to the upstairs area.
We have reversible ceiling fans in almost all rooms of the home.
Since the fans are reversible, I can move room air downward for cooling season, reverse and move room air upward for heating season.
What this does for heating is create an even temperature gradient in the room from floor to ceiling. It mixes the warm ceiling air with the colder air near the floor. This is efficient even heating. Without the fans running there can be up to a 10 degree difference between floor and ceiling.
That high temperature air at the ceiling is what was rising and overheating the upstairs area.
With all downstairs ceiling fans running 24/7 (all the time) the temperature upstairs is only 1 degree above the downstairs set point.
Overall it should (will) decrease operating costs. Less downstairs “heat on” time trying to replace heated air moving up to the second floor area. The air now migrating to the second floor is now very near downstairs set point temperature. Reducing overheating upstairs.
Air distribution science is much more complex than I can explain here. Basic premise is “heated air rises” and “cold air settles (falls)” It’s a basic law of nature and weather.
The “take away” here is that in homes with a huge open area (ground floor to roof) living space, constant mixing of room air using first floor floor ceiling fans (upwards flow) is an excellent operational mode in heating season.
It is also good practice to operate ceiling fans for rooms on the higher level for better comfort. But this will not correct the over temperature effect from heat rising from the lower level.
The heat setting for the entire home is controlled by the thermostat five feet above the floor on the lower level. The temperature control for the second floor of OFF. Of course it can be activated if and when needed.
It is absolutely not appropriate to operate cooling systems upstairs at the same time heat is operating at a lower open area. The two separate systems will be feeding each others power demands producing horrendous operating costs and energy waste. Both systems must be operated in the same mode. (Or one system OFF.)
Ceiling fans use much less energy as they are only moving (mixing) air. Their action is not adding or removing expensive heat energy.
I want to also mention, that in cooling season. The reverse often occurs. The upstairs cooling system when operated will sub-cool the downstairs for the same reason: heat rises, cool drops.
Generally the upstairs HVAC in cooling mode will cool the entire house except in extreme heat (here in Texas). I then use down-flow ceiling fan operation to keep air mixed. Also intermittent HVAC fan only operation for first floor.
This will eventually cause latent heat (moisture in air) to increase downstairs since the heavy cool damp air does not rise to pass thorough the upstairs cooling coil (which removes moisture) Eventually I must power up downstairs cooling to remove this “latent heat” when we are in the full cooling season.
As a well trained professional lifetime (60 years) HVAC expert, I know air distribution rules and fixes. Texas home HVAC is generally designed more for cooling, with return air grills in ceilings (each floor separate). All supply air grills (registers) are also mounted in the ceilings or high near ceilings on sidewall. No discharge at floor level. So heating comfort can be a compromise. (Heating works best when heated discharge air can entrain and mix with cold air near floor.)
Ceiling fans are not just winter decoration in Texas! 🙂