Turning off AC when unoccupied

JakeCK · July 2023

Had this show up on my phone. This really feels half baked.

Opinions?

https://www.pbs.org/newshour/amp/science/analysis-does-turning-the-a-c-off-when-youre-not-home-actually-save-electricity

EBEBRATT-Ed · July 2023

That's a question that can't be answered. Down south you probably start it in April or May and don't turn it off until October. In New England there always on and off

mattmia2 · July 2023

depends on how long you're gone and what the temp and dewpoint of the outdoor air is

Hot_water_fan · July 2023

It seems the takeaways that: 1. Longer setbacks save more energy and 2. Modulating equipment benefits less from setbacks are exactly what I would expect. Don’t you think @JakeCK?

JakeCK · July 2023

From their research, they found that turning it off while away almost always saved energy over all. Which I agree with. A structure can only absorb or lose so much energy, and the closer it gets to equilibrium the slower the movement will be. We all know and understand this. But total energy consumed is only a part of the total picture. There is the comfort side of it, and I'm sorry after sweating in 90f weather all day I want to walk in and see penguins chilling in my living room tbh. If the system is anywhere near being sized correctly it won't catch up quickly when it's real hot out. But also things like time of use rates, solar production, and grid demand.

Hot_water_fan · July 2023

Ah. the article wasn’t a book !

JakeCK · July 2023

Hot_water_fan said:

Ah. the article wasn’t a book !

It is billed as an analysis. It even says so in the title. If one is going to offer up an analysis it should be thorough. Sure turning it off until you get home will some some energy, up to 11% they quoted. But if every Tom, ****, and Harry in the state turn on their AC at the same time between say 2 and 4 pm and it has to run non-stop for hours to catch up... Well what happens when they implement rolling black outs to prevent a grid collapse? Gonna be saving a lot of energy then. Lol

Same idea with time of usage rates. If the price per kwh is double for the late after noon vs mid day, that energy you are saving is going to be much more expensive. Which brings me to solar. If one has solar panels and no batteries it's better to turn the house into an ice chest while the sun is shining then to try to cool it after the sun is past its peak and you're pulling it all from the grid. These are not exactly fringe scenarios.

Jamie Hall · July 2023

The physics -- and economics -- are just the same for air conditioner set backs (set forwards? Whatever) as they are for heating set backs.

ethicalpaul · July 2023

That cleared it up, Jamie! No room for debate now!

Hot_water_fan · July 2023

Ha true @JakeCK. I am awaiting your report. I think they simplified things so they could answer one particular question

JakeCK · July 2023

Jamie Hall said:

The physics -- and economics -- are just the same for air conditioner set backs (set forwards? Whatever) as they are for heating set backs.

True, now let's apply the same temperature swing as suggested in the article. Who would be willing to do a setback of 15 degrees from say 68 to 53f and back up over an 8 hour period. Even when the air temperature gets back up to 68 that house is gonna have a chill for hours afterwards. Now turning off the AC and letting the house go from 75 to 90 sounds just as crazy.

JakeCK · July 2023

Hot_water_fan said:

Ha true @JakeCK. I am awaiting your report. I think they simplified things so they could answer one particular question

But they didn't even mention those other considerations. They just said it still might depend. It's at a minimum misleading, at worst it's going to cost someone a lot of money if they adopt that strategy. And since PBS is a source a bit more trustworthy then some no name internet website it carries a bit more weight and influence. But hey I guess we're talking about 2023 here where click bait and ad revenue is paramount and editorial integrity is non-existent.

Growing up our electric provider was a co-op and they would often tell everyone to leave the AC on during the day and if needed to use only small setbacks. They didn't advertise it as a way to save energy, they explicitly said it was to reduce strain on their grid and level demand out.

Hot_water_fan · July 2023

@JakeCK I will need to double check, but I’m pretty sure very few Americans are on time of use plans. Not saying that’s good or bad, but it’s not really on most minds. I agree, if the grid is at risk, the utility should incentivize to prevent that. On the hottest days, my utility actually encourages setbacks and pays handsomely for reductions during those peak hours on those days, which happen once or twice a year.

JakeCK · July 2023

Hot_water_fan said:

@JakeCK I will need to double check, but I’m pretty sure very few Americans are on time of use plans. Not saying that’s good or bad, but it’s not really on most minds. I agree, if the grid is at risk, the utility should incentivize to prevent that. On the hottest days, my utility actually encourages setbacks and pays handsomely for reductions during those peak hours on those days, which happen once or twice a year.

Those set backs are not during the middle of the day while people are gone at work but late afternoon when the grid is strained the most. Basically exactly when this article is suggesting you turn your ac on to save energy.

Hot_water_fan · July 2023

It’s from 2-6, similar to the study I’d say

HVACNUT · July 2023

They briefly touched on humidity, but that's what I would be concerned with most. I think constant normal cycles would do a better job than leaving it off for extended periods.

Big Ed_4 · July 2023

It comes down if you want to save 10% and wait for your comfort level .... My two cents

JakeCK · July 2023

Hot_water_fan said:

It’s from 2-6, similar to the study I’d say

Your utility wants you to run the ac during 2-6 or to set it back? Running it 2-6 makes absolutely no sense that is peak time, why would they encourage that and drive more demand on the peakers?

clammy · July 2023

I’m a set and forget it and leave it till it’s cooler out and then off ,I follow this for both heat and ac and everytime I get my utility bill gas or electric they state I’m in the top tier of efficient homes in my neighborhood . My home is built in the 40 s and is not the most highly insulated or weather sealed home in my hood . Is this all bull that I’m in the top tier yeah I have a low temp system using a mod con and a 24 year old r 22 2.5 ac system .
Over the years I used set back on both systems and found for the lowest bills the set and forget has been my ticket to the lowest possibly energy consumption . I figure the only way to get it really any lower is to just shut it off and either sweat or freeze so I’ll stick to the set and forget. I do have to agree on the controlled rolling black outs they would love to have control of our comfort while they’re setting comfortably . Example x mas weekend and nys announcing possible rolling black outs , yeah no black out but lower permissible voltage and some issues w ecm blower motors funny when the announced the threat was over Monday the ecm blower issue ceased to occur . There’s your answer to the government and going all electric it put them totally in control ,I really don’t like that though being the nys governer wasn’t having issues w her heat . Fire away
Peace and good luck clammy

Turning off AC when unoccupied

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