User: zompus

zompus

August 8, 2023

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Depends on a few things: Temperature difference between morning and afternoon: the temperature lift on the air conditioner is lower when it's cool outside which makes it more efficient than when...

I'd love to see some math about how much it might be worth having your AC cool your house excessively in the hour or so before sunrise and then keep it off for hours, when it would theoretically be less work for your AC to cool the house than in the full sun.

Depends on a few things:

Temperature difference between morning and afternoon: the temperature lift on the air conditioner is lower when it's cool outside which makes it more efficient than when it's warm. How much more efficient depends on the temperature difference. As a rule of thumb, it's about 1-2% more efficient per degree F of lift, so about 10-20% more efficient when it's 70 degrees out vs. 80 degrees out.
Humidity: if you try to cool your house below the dewpoint you will spend some energy dehumidifying the air vs. just making it cooler. So your energy spend to get that next deg F down might be more than if it wasn't so humid out.
How insulated your house is: heat is going to infiltrate your house. The bigger the temperature difference is between the inside and outside of your house, the faster heat transfers, in a linear fashion. Insulation will slow the heat transfer, but it's still a linear relationship. So pre-cooling your house to a lower temperature will eventually result in more heat transfer that has to be removed by the AC, compared to keeping it at a higher constant temperature. How insulated your house is affects how long you can ride out the temperature rise. This is what your HVAC guy was referring to when saying to set it to a temperature and leave it there.
Does your utility rate vary: if you are on a variable rate based on time of day, you can capture some cost savings by running your AC when your electricity is cheap vs. when it's expensive. However if you are on a fixed rate then it doesn't make a difference.

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