Why don't air conditioners use cold outside air?

I live in a place where it’s hot during the day and relatively cool (colder than I want to keep the house) at night. During the day, the outside temperature is much warmer than the inside temperature (maybe 90 vs 75). At some point during the evening, the outside temperature becomes colder than the inside temperature (often going into the low 60s or even high 50s). So if I just keep the windows shut and let the air conditioner do its thing, it will end up working hard to cool down the hot air inside the house, when we could just open a window and have a fan blow in the cool air from outside.

My question is, why don’t air conditioners allow doing this automatically? And furnaces, for that matter. It seems that if the outside temperature is close to the desired inside temperature (or overshoots it, as the air blown by the AC or furnace does), it should just suck that in with a regular fan. It seems like it would use much less energy. Yes, I can (and do) manually open windows and put up fans, but if my HVAC system could automatically manage it (including figuring out the optimum times to switch over), that would be much nicer. And also I wouldn’t have to worry about leaving windows open when I leave…

the outdoor air couldn’t be too humid overnight.

every night i draw cool night air into the house and close the windows in the daytime. the house stays cool through much of the day with no air conditioning. doing the windows is just a thing to do when the evening temp gets cooler than indoors and and in the morning when the outdoor temp is warmer than indoors.

Too damn complicated.
Also, it’s not enough to just “suck air in” - you need to exchange air. Air conditioners are explicitly designed to be as closed as possible - there is no inside/outside air transfer. To design a machine that did both would:
a) Make the system that much bigger and more complicated.
b) Create more opportunities for inefficiencies (due to air leakage).
c) Require much more area than a typical A/C (You would need at least a window-sized opening for both inlet and outlet air).

Also, it’s pretty easy to just open the windows at night and use an Attic fan - that’s what my folks do.

Most the window or wall type air conditioner units (as opposed to a central A/C system) that I have experienced had a little lever that you could pull to let outside air into the air current if you wanted to. The OP is just envisaging that process being automated. It does not seem to me that it would be all that complicated to implement.

Besides, people do incredibly complex things and spend large sums of money in the name of energy efficiency. They install monstrous solar arrays on their roofs. They buy ugly and overpriced cars just because they’re hybrid. They put in geothermal heat pumps. I’m not sure I completely buy the complexity or cost argument here.

There is an air conditioning unit that basically does this. It’s a heat pump that at night (when both the air is cooler and the electricity is cheaper) builds what is essentially a giant block of ice inside of it, and then during the day uses that stored spent energy to cool the house.

Most modern furnaces have a ‘fan only’ setting. If you open the windows, fresh air will be drawn into the return air vent and then recirculated into all the rooms with open registers. As mentioned, most window shakers have both a closed and open circulation selector, and many have a fan only switch that you would use with the open setting. Even the crappy unit in my RV has that option.

What you are describing is common with building HVAC systems. It is sometimes refered to an economizer system. to work properly requires two fans. One the supply air fan and the other the return air or relief air fan. And also works better with speed drives on the fans. The system will have outside air dampers and return air dampers. The system looks at the outside air temp (OSA) the supply air tem (SA), and the return air temp (RA).

Lets start with a cold building and cold outside. As the building system starts on a cold morning with the building low. The outside air dampers will be closed and there relief fan off. Return air will be pulled from the building and blowned across the heater and to the building spsces. As the building temp increased the heating will decrease. As the outside air begins to increase the outside dampers will begin to open and the return air dampers will begin to close. At some point the relief air fan will have to start to remove air from the building. At some point the outside air dampers will be fully open and the returnair dampers will be fully closed. Starting around maybe 76 degrees the outside air dampers will begin to close and the return air dampers will open. Close to 80 degrees outside the building should again be on full return air.

I would guess if a complete AC system costs around $10,000 to include a good ecconomizer system would be another $5000, to $10,000. And would require more room and duct work.

I know a guy in the trade who built an eccomomizer system in his house as he built it.

An another thing I have worked in many buildings with ecconomizer systems that were computer controlled. I still had to make adjustments for Spring-Fall, Winter, & Summer.

I do not think it is that complicated. Modern medium to high end furnaces and water heaters already have processors and feed back systems for efficiency. Active and Passive HRVs are becoming more and more common and will be code soon in many jurisdictions.

Make up air management is more important than ever with tightly sealed homes and hence the need for codes requiring complete interior air changeover on a regular basis. If you already have an HRV warming incoming air in the winter, I really don’t see it as that big a jump to automate it a little to up the efficiency of central air conditioning in the summer.

I would be surprised if this isn’t already available. Is there not an HVAC tech on this board?

I’m sorry sir, but you are incredibly uninformed. Think long term. Read up on the subject before you comment.

Hello. See above.

I sometimes wonder why so much money is sspent to save a little. 2 years ago I looked into solar for my house. I run on average over 900 KWH per month. I end up being over 400% of my life line rates. That means I am paying over $.50 per kwh. Not a full system but enough to get me to the lower rates. Acording to the solar company calculations I could knock on average $124 off my power bill for $16,000.

The numbers did make sence to me but people are putting in solar panels.

Just a nitpick - when it’s cool outside, the unit will have to work less hard to cool the inside air.

Re bolded part above: Why would the air inside your house be hot if you’ve been running the AC all day? I’m pretty sure that if I have my AC set to 75 and it drops to 65 outside at night my AC just won’t kick on (unless it’s humid maybe, I’m not sure). If I decide I want the house cooler than 75 at night I’ll just turn off the AC but leave the circulation fan on. That said, using the AC just to draw air that doesn’t need to be cooled/conditioned into the house seems way less efficient than opening all the windows. I could see the system you describe as useful for someone with a lot of allergies or who isn’t around to do the AC/window switch.

We installed a heat pump and a tankless water heater. Payback is four years, with continued savings until either unit dies or I do. How is that not cost-effective?

that’s true that those systems can cost money and appear complex. though there is a good payback on many of them.

solar hot water for domestic hot water will cost multiple thousand dollars. it could almost double the plumbing in a house. it could pay itself back in maybe 5 to 8 years. runs by itself. just get a good system installed by a good contractor. the usual periodic maintenance for your plumbing or heating would apply.

there are trade associations of providers of the energy conservation and alternate energy systems. there are also private and government programs where a consultant can help you plan a system; e.g. energy conservation groups might have consultants (not connected to any installer), for a small fee, help you figure what might work for you and its costs and payback.

The system you’re proposing would be seperate from a typical A/C system since typical A/C does not do any type of air exchange. It basically absorbs heat out of the inside air via freon, pumps the freon outside the house, then releases the heat outside the house.
Since there really is no such thing as transfer of cold, you can’t cool a house by bringing in the cold outside air, you have to remove the hot inside air. Plenty of homes have attic fans (since heat rises) that will try to blow the warm air out of the house. Cooler air will seep in from the outside which can be aided by opening windows.

Unless there’s a good cross breeze, on my house at least, I still need a fan pulling air in. Also, I’ve found, that if instead of having a fan pulling air IN, I have a fan blowing air OUT I get a lot less bugs coming into the house. Also, I’d imagine that over the long run my screens won’t get as dirty either.

fans exhausting will create a significant enough pressure difference that you can feel a breeze which is very noticeable in stairways and hallways. one or two fans can cool a house.

with a fan blowing air in you have all the air in the room that needs to be compressed before air will leave the room through the doorway and so on until you reach the windows that are open. in the end you lose a lot of air movement.

having fans blow on you can help if you are in the air blast.

One thing to do if you wish to use the cool air outside is install a Whole house fan. This can either supplement the air conditioning to lower costs, or replace it in some areas where you don’t get that many days of extremely hot weather. Note the size of the fan involved.