If it's hot out during the day, and by 8 or 9 it cools off, and I open the window, is it true that the humidity from the outside will fill my house and then next morning, maybe 9 or 10 when the AC is turned on again, it will have to run more or longer to keep the house cool?
Enough to cost measurable money during the hot/humid summer? A dollar a day. 50 cents, 2 dollars?
It would be one pretty large 2nd floor window in a townhouse with basement. (Of course the basement is never hot.)
google HVAC psychrometrics
If it is humid outside it will become humid inside. Better off just keeping it closed and maintain. If it is dry outside then yes, not a big deal and you can enjoy the outside fresh air. You may be able to find a chart for the best dew point. Above X keep closed, below X open up.
The more humid the air is, the more energy it will take to cool the house down the same number of degrees. Then it depends on how much cooling the house gets by opening the windows over night, how humid the air is, etc. For most of us it doesn't matter because when you need AC it's also humid and if you open the windows, it's going to be humid and uncomfortable.
I don't like wasting my money or society's resources, the oil to run the power plant to make electricity, but otoh I like hearing the sounds of outdoors and I like the breeze, even if it's actually less than what the AC makes or what a table fan would make.
So I'm willing to spend some money to be able to open the window at night. If it's 2 cents a night, that would be easy to decide, 25 cents is not bad, a dollar or two each night would be harder. So I'm trying to get an idea of how much money it would actually be.
I've opened the window in the past, knowing that it may be humid out but it was too cool at night to turn the AC on, until of course the next morning when the sun started to heat the house and then it might take twice? as much running time to keep it at the same temp as without the humidity . I didn't worry much about tomorrow.
I used to wonder why, when you open the front door on a freezing day, all the warm air in the hall didn't fall out, but it doesn't in the 60 seconds the door might be open. This is different of course because it has all night, but the humity won't rush in all at once. And while humidity is entering the bedroom, some is going to other rooms and some is going back out the window again.
Actually, FWIW, in practice relative humidiy goes up when it gets cooler at the end of the day, even though the absolute humidity stays about the same, because cool air will hold less humidity.
Yes, opening the windows when outside humidity is high WILL cause the humidity in the house to go up and could cause the AC to have to run more to make it comfortable the next day. If there is a haze or dew the relative humidity is high. The "absolute" humidity may not be higher than it was during the day because air holds less humidity as it cools. It WILL still be higher than the inside air if you have been running the AC throughout the day
I doubt it really makes that much difference either way. If it is that cool outside, the AC won't be running that much anyway. The real answer is the difference between the latent heat you bring in with the windows open (humidity) and the sensible heat your house generates at night. If you have a smart meter, look at the PoCo web site for time of day usage. Sensible heat (BTUs) will be power consumed (KWH) x 3400 With the windows open, you get that sensible heat removed for free or for what it costs to run a fan. For us, it is just an opportunity to get some fresh air in the house and smell the jasmine blooming. I never thought about the money.
I have to think about this stuff.
It's hard to turn my back on a lifetime of thriftiness. Have to convince myself the money and national resources are not that much.
There are two kinds of waste: Wasting money, but there I mean that time someone else gets the money, maybe even someone nice. Wasting stuff, like oil. If it's waste, that means either no one benefits or the benefit is substantially outmatched by the cost.
And we don't have jasmine but we do have honeysuckle for a few weeks. I don't want to miss that.
.. it's Micky ... he/she likes to ask ridiculous questions. John T.
I am as cheap a bastard as you will find but the actual economics of turning the AC off at night and turning it back on during the day seems to be lost in the wash of my electric bill. If you have a smart meter and your PoCo has a decent web page, you can do some science. Keep a log of the nights you left the AC on, nights you turned it off and balance that against the degree days on those dates and see how you do. For us, the 2.5 amp "whole house fan" running non-stop might get so close to the AC cycling intermittently on a cool night that we won't see a difference. My standing load from things that run 24/7 (based on the lowest hourly usage recorded) seems to run around 1kw give or take but that is the smallest increment the dashboard records. It might be less than that. It is still 3400 BTU/hr, about like a small space heater running all the time.
I don't think it is a ridiculous question but there are too many variables to give a precise answer. We asked that question ourselves and I did do some research. If you do have a smart meter, see if your PoCo makes that data available on the web site. Do your own research.
sounds complicated, but it's too late anyhow. I have no AC now and need to decide before it's installed whether to get a wifi thermostat.
The advantage they push, that if you forget to turn down the heat or AC before you leave home, you can do it remotely, is contraindicated for me, because IF I cand do it remotely, I'll always forget, but I never did before.
But what would be good is could turn off the AC from my bed, which is when I usually think to open the window. Woudln't have to go downstairs and wake myself up.
I don't expect a precise answer to certain questions, just discussion and opinions, like I got here. Well worth asking.
These days if I was starting from zero on an AC system I would look at mini splits that have a thermostat in each area with an air handler. Then it makes it very easy to zone the parts of the house you want to cool and the SEER on some of these mini splits put whole house systems to shame. We have a whole house system that I keep at a fairly high temp and a mini split in the bedroom that my wife can put on the "meat locker" setting at night. That alone saves us a lot of money. Sometimes the bedroom unit may be the only one on. You do get a remote with these so you can control it from your bed. Another advantage for a retro fit is they don't need ductwork. You just run a line set and a small cable. The power goes to the outside unit. I also have another mini split in the addition that usually gets no HVAC at all unless we are using that area. (usually when the kids are here)
That's a good idea. At night I like it cool so I turn it down 4 degrees to 73 at bed time. It is really only the bedroom that needs it.
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