Posts Tagged ‘thermodynamics’

A Thermo Problem – Need a “Rule of Thumb”…

July 4, 2012

Happy 4th!

I have a question for you thermodynamics wizards – I need a rough answer – a “rule of thumb”, so don’t get carried away with precision and nuance considerations. Here is the problem:

What is the “degrees per hour economy” to set my AC thermostat up when I leave the house? If I will be gone for 10 hours, and I set my thermostat on my AC up to save electricity when I leave the house, and then set it back again when I return, how many degrees up must I set the thermostat to save money on my electricity bill?

The intuitive answer is “…any period of time with a higher thermostat setting will save electricity.”

But, wait a minute…  The AC runs at a 100% duty cycle to cool the house from the higher “away” setting to the cooler “occupied” setting.  There is a cost just to transition from the “away” steady state to the “occupied” steady state temperatures that may exceed the cost saved maintaining a warmer interior temperature for a period of time.

There is a thermal gradient at the perimeter of the house – exterior walls, foundation and roof that transfers energy from outside to inside during the summer. A house’s thermodynamic behavior also has a characteristic analogous to inertia – everything in the house absorbs thermal energy, so you must do more than cool the air – you also must cool a portion of the physical mass of the structure to transition between steady states or maintain a state. The significant consideration to frame the problem and then solve it is therefore to specify differential between the inside temperature and outside temperature for the “occupied” and the “away” states, and specify the time in the “away” state. The time in the away state also includes the time for the structure to “relax” – the zero-cost time when the house warms to the “away” state.

So, you “thermo-heads” out there – I know of two of you reading – what is the energy use relationship that helps me decide the minimum away time to bother with and the critical thermostat setting necessary to save energy? There ought to be a simple rule of thumb…


I have a rule of thumb from Oncor:

Set the thermostat up for your two story home in summer 2-3 degrees – just 2 or 3 degrees… for an unoccupied period of 8 to 10 hrs. The rationale:
– The home warms about one degree per hour and cools two degrees per hour (deg F).
– Warm air rises (from the first floor to the second) – more than 2 or 3 degrees F and the upstairs AC compressor will attempt to cool the first floor, too – you can stress it to failure.
– the refrigerator also may be over-stressed to cool when the ambient exterior (the home interior) is more than 40 deg F refrigerator / 60 deg F freezer warmer.
– wood floors can dry and warp and crack.

Just 2 or 3 degrees… I am banking on 4 or 5…