Here on West Coast, thanks to the low humidity, the nighttime temperature can be significantly lower than the daytime temperature. The difference can be as much as 30 degrees on the Fahrenheit scale. I try to take advantage to cool my house and reduce how much the A/C has to run by opening a few windows at night.
So last night I as I was getting ready for bed, I noted that the outside temperature had dropped about two degrees below what the thermostat is set to. So I cracked a few windows and proceeded to go to bed. And I left the A/C set to “on”, figuring it wouldn’t actually cycle on anyway. But apparently the area around the thermostat was warm enough that the A/C actually did cycle on a few minutes after I went to bed, and since I was already in bed and didn’t feel like getting up and turning it off.
So my question is, was having the windows open while the A/C was running in this scenario a good thing, or a bad thing? When I was a kid it was drilled into me that you never leave any doors or windows open when the A/C is on, because it lets warm air in and runs up the power bill. But in this case it was actually slightly cooler outside than it was inside. So was that actually helping the A/C do its job faster? Or, since heat flows from warmer areas to cooler areas, I wonder if this maybe set up an airflow where the warmer air inside the house flowed out the open windows as the A/C blew cooler air into the room.
In general it would fall in to the ‘bad thing’ column. If the AC is running and the windows are open, you really are letting the cold air out, or rather, you’re letting the warm air in.
It sounds like the temps between outside and inside were close enough that, like you said, a warm pocket of air hit the t-stat and it turned on for a few minutes. So, not really a big deal. I’m also assuming this isn’t a Nest or other smart t-stat that can cycle the air on if the humidity gets too high (even if the temp is low). That could have caused it as well.
Also, remember that whatever you’re using to monitor the air immediately outside your windows isn’t going to be calibrated the same as the t-stat. So, your t-stat is set to 70 and whatever you use to determine the outside air is also 70, might be off by a few degrees. The t-stat might read that air as 72 as it enters the house. Not a big deal in general, but in that scenario it could cause the AC to come on for a little while.
The walls and physical objects in your home were warmer than the outside air, so your thermostat was affected by that. It creeped above the set temperature to activate the AC even though the air coming in was below that temperature. THere is a lag time until the building itself cools down enough. I always shut off my AC when the windows are open, as I expect it to take a while for the inside temperature to actually drop.
BTW, it never drops to the outside temperature exactly but stays a few degrees warmer since it will take a while for the structure to lose its heat.
The air temperature as it exits your A/C unit is cooler than the target temperature for the room. Even if the outside air is cooler than the target, the A/C will probably cool faster.
The A/C unit is not bringing new air into the house. It is recirculating it, so there is no positive pressure to push air out the windows if you open them. Also, heat flow is different than air flow. If you have a cross-draft, then you will get fresh air moving in one window and the air that your A/C just cooled going out another window.
I would just leave the windows closed, or open them and turn the A/C off.
Also common on the East Coast with high humidity. I doubt the humidity’s got much to do with it.
If it’s cooler out than in, running the air conditioner is a waste of money and of energy. A lot of them will cycle to some extent even if the temperature’s within the set temperature range, depending on the settings – the fan and/or a dehumidifier may be running intermittently. I don’t think it’s a disaster if you forgot to turn it off; but ordinarily when the temperature outside drops below the inside temp, I’d open the windows and shut the air conditioner off. Which it sounds like you usually do anyway.
It is neither a good thing or a bad thing. It just means that you are running your AC when you should not need to, so you are wasting money. If I open up the house at night I turn up the set point on the stat a few degrees so the AC will not run. Then I open the windows and start the whole house fan. If you do not have a cross wind, fans in the windows, or a whole house fan you are not going to get much cooling from the night air with windows open.
Depends on the arrangement of the windows. Open downstairs windows from the bottom, and at least some of the upstairs ones from the top, if possible. Open windows on as many sides of the house as possible, and on multiple sides of each room if possible. The place will cool down faster if there’s a wind, but it’ll cool down over the course of the night even if there isn’t, if the windows and window arrangement were designed for it.
Of course, if the house was build after and with airconditioning, or if it’s a middle apartment, the windows may not be designed for it.
thorny – likes her old house, and goes to open a lot of windows – locust
If the outside temperature has already dropped below room temperature, and it will continue dropping throughout the night, then I think it’s a fine way to do it. I do this in the spring and fall when the temperatures here in Ohio aren’t crazy nonsense. Basically, If it was near 80 during the day, and 70 degrees outside when I go to bed, opening the windows and running a fan won’t really cut it. The heat still soaking through the dark brick walls and from the crappy attic insulation can’t be cooled by mere 70 degree air, so it’ll be uncomfortably warm trying to fall asleep even if it does eventually cool off near sunrise. That’s no good if you’ve already soaked the bed in sweat, yuck.
So I set up a fan on one side of my apartment blowing out, and then open a window or two on the opposite end to allow the cooler air in. I do have the advantage that the window with the fan is in the far opposite corner from where the a/c units are, so the cool air comes in near the living room a/c and warm air goes out through my bedroom closet to the sunroom. The a/c in my bedroom and living room keep running a ways into the night, but as the air outside gets cooler and the heat soak tapers off, then the a/c units eventually stop cycling since that cool air from the windows is enough to keep things comfortable. If I kept the windows shut with the a/c on, then they would run much longer into the night, if not all the way through until morning. If I have to have the fan blowing in to my bedroom because of wind (which is rather rare overnight here), then I’ll turn off the a/c in the living room since I’m not sleeping in there.
That’s my observation too. The heat-soak effect is too tough to overcome by merely an OAT marginally lower than indoors, even with a fan and/or cross breeze. The OAT needs to be at least 20 degrees cooler or more for it to extract the heat from the house…inside and outside.
My central A/C unit is very effective, but it is of a design where it’s not recommended to run it if the OAT is below 60F so as not to cause premature wear to the compressor. At times of the year where it’s still hot in the day ( or hot enough, coupled with the sun to want to run A/C ) but cooler at night, While I’m at work ( 2-11 PM ) I have to set the A/C to a lower temperature so as not to come home to a house that’s heat soaked, but too cool outside to run A/C for the reason stated above. When I get home and if the house is a stuffy feeling 77 or 78, opening up windows and patio doors to, say a 55 degree night and moving it around with a ceiling fan and breezes only cools the place very slowly. In fact I may be able to get the air cooled and moving to where the thermostat in the house “says” it’s 70 or so, but it doesn’t feel it because all the surfaces and contents of the house are still heat soaked and throwing off heat. Sure by sunrise it’ll even out, but at bedtime it’s still too warm. Can’t sleep in that.
Full disclosure: I am quite heat intolerant.
That seems excessive, but if you’re in the desert then maybe I can see that. Those hours could be a factor too, in that your system is shut off when it’s needed most, so the heat just builds and builds. I’d never leave the a/c completely off in a situation like that, but turned up 4-5 degrees.
In my case, with uninsulated 12" brick walls, outside temps in the low to mid 60s is still adequate to cool the inside to the low 70s. That’s more like a 10 degree delta. The box fan in my window moves a lot more air than the a/c units which only produce 50-55 degree air at best. Typical air conditioners generally only operate on at most a 20 degree delta, so I’m curious how yours is able to keep up if open windows and fans with a similar temperature spread can’t.
Granted, ceiling fans don’t really help circulate the air between inside and outside, they just prevent vertical stratification and help you feel cooler because of the breeze. Maybe you need a whole-house fan, or at least a decent box fan in one window. In most places it seems like the nighttime is just too calm, so you either need fans or the chimney effect from a multi-story building to help induce airflow.
Brick Bat. a delta of 20 degrees is excessive to cool a house. I can cool my house some with a delta of 2 to 3 degrees. But I have a whole house fan. A celling fan will only move the hot air around. If you want to cool your house it will be necessary to remove the hot air from the house and pull in cooler outside air. If the house is 77 to 78 and OSA is 75 it is possable to drop the temp near the 75 degrees. The faster the hot air is replaced the shorter the time to drop the pressure. I set my sta t 75 degrees. I switch off the AC when the OSA is 74 or below. The temp of the house does not increase. But as the OSA drops so does the house temp. If the low point of the OSA gets down to the low 60s the house temp in the mornings will be withing 4 degrees. So 60 will cool the house to 64. In th emorning I wqil have to wear a sweater.
I’m not sure leaving the air on in this situation wastes much energy, and it might cool things off faster. I do this in my car on a hot day; I open the windows but also turn on the air, then after a few minutes I pick one or the other.
You might consider investing in an attic fan, if your layout allows. My parents’ house in Southern California doesn’t have central air. But at night they can open all the windows and turn on this powerful fan that sucks cool air in from outside. It works surprisingly well.
I actually have one. I run it in the evening if it’s cool enough, but I don’t like running it all night when I’m sleeping because I don’t like the noise. I’ll also open the windows and run it in the morning while I’m getting ready for work to cool the house and delay when the AC needs to run.
Cool! Yes, it’s really loud. Also IIRC you live somewhere in the Fresno area, which means often even the most powerful fan won’t suffice. I’ve only been there in the spring and that was quite warm enough for my liking, thankyouverymuch!
On high speed mine makes too much noise for sleeping, but mine is a 2 speed fan. When I go to bed I put it on low speed. If you can afford it and if your house can fit it get the fans with the 36 inch blade. A 4 bladed 36 inch fan will move a lot of air with little noise. Lower RPM.
Also more important than getting the cooler air in is getting the really hot celling air out.
Getting the hot air off the top of the room will make more differance than just adding cool air to the room. In my house the 2nd story return air was at floor level. 2nd floor was at least 10 degrees hotter than 1st floor. When I remodeled the master bathroom I was able to modify the return air. I now have two return air registers one above the other. In the winter I open the lower register and close the top one. In the summer I open the upper register and close the lower one. I also restrict the 1st floor low return air. By getting the hot air off the top of the living spaces I have changed the effectiveness of my AC. The 2nd floor now stays about the same as the 1st floor. If I put the fan from auto to the 2nd floor will be 1 degree lower than 1st floor.