Saving on Air Conditioning

Hi all,

I have a rule that triggers at 5:30AM to cool the house down to 72 degrees if the high temperature for the day will be hotter than 85 degrees. After cooling, I set the thermostat to 78 degrees for the rest of the day. (Our high temp lately is in the mid-90s)

My rational is… I can cool the house down more efficiently early in the morning, rather than having the HVAC kicking on and off throughout the hot afternoon. This seems to work and the HVAC doesn’t normally come back on until the early evening. (I figure this may also save on wear and tear on the HVAC unit)

I’d like to hear what others think of this idea?


If your AC truly does not come on again until early evening you’re probably doing it right.

I’m no expert, but from the metric crap ton of reading i’ve done i feel I’m at least somewhat qualified.

In general, a properly functioning central AC system can bring a house down 1 degree F per hour (of course there are lots of conditions attached to that). That being the case, it takes AC a lot more time to cool a house than to heat it, which makes it a bad idea to leave AC off all day. An exception would be if your AC really only starts up again in the early evening. As long as it’s remaining comfortable for you running like that, I see no issue.

If it takes all day long from morning until evening for your home to heat up from 72 to 78 with an outside temperature in the mid 90s, then you must have an extremely well insulated home with amazing windows. This is not something that would work for most homes. Personally, I am uncomfortable above 70, so I could never live under such conditions :slight_smile:

That said, you are likely making it work harder when it does come back on. All day long it may not be getting hot in the house, but the humidity will certainly be rising regardless. When the AC does come back on, it will have to suck that humidity out of the air. Also, come evening, if you’re turning it back down to 72 or something, now you’re wasting a ton of energy with constantly taking the house down every evening.

Thanks Matt,

Excellent thoughts. My house is not the most efficient, but we do have a ton of shade. We are okay during the day with the temp set to 78. At 10:45PM, I do have the upstairs drop to 76 degrees for sleeping. :slight_smile: The humidity is definitively a consideration. Thanks for the feedback!


Thank you Scott for the great info! -Mike

If you want to go “all in” on crazy you can start looking at motorized dampers or vent misers (failed product of the mid 2000s?) and make a DIY zoning setup. I started my trek down this path about a year and a half ago when we had our twins. I thought it was ludicrous to heat/cool the entire house just to keep the nursery warm ( i was used to keeping the heat off at night even with the house dipping into the 50s way into the night - on extremely cold nights). At first I had an oil filled radiator for their room but I think it cost as much to run that for their room as it did to heat the house.

So, remembering seeing motorized vents by a company called Keen I thought about doing that. But, at 80 dollars a vent (i think) I couldn’t swallow it. I had just just dipped my toe into the arduino pool about this time and I thought maybe I could use a servo to open and close a regular vent. Well into researching that and how to make it all work I found vent misers. My prayer was answered. Motorized vents for like…10 bucks a piece on ebay. All i had to do was get an arduino to open and close them on demand.

18 Months later and now most rooms have their own temperature sensors, setpoints, and motorized vent to regulate temperature. Another arduino (esp8266) controls an 8-relay board that calls for heat/cool (including cutting the hi heat stage) and selects a different fan speed on the furnace based on how many vents are open.

The house can be 80 degrees while the nursery and master are a cool 74 at night. So…there are options for you :rofl:

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That’s awesome Scott. I don’t think I am that ambitious. Haha. I’m glad to see that you are regulating your fan speed. I have heard that closing vents can put a strain on the HVAC unit. Thanks for sharing. Congrats!



Would you be able to share the details of your setup in a new topic?
Could you include the arduino’s code, hardware used, hacks required?

Sure, let me work on it tonight or tomorrow.

Brilliant. Thanks

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