Monday, June 19, 2017

The Poor Man's Air Conditioning

It has been very hot in San Jose the last few days. One solution is air conditioning, which we have and sometimes use. The other is to open the windows in the evening, when it cools off, close them in the morning. Sometimes–last night, for instance–that cools the house faster than air conditioning. And it doesn't show up on our electric bill.

I have long wondered why there isn't an automated version available, or if there is and I haven't come across it. It would be easy enough to have an insert for a window that was controlled by a simple thermostat. If the inside temperature is both above your optimal and higher than the outside temperature, open to let air in, if not close. A few such inserts should do, automatically and more reliably, what we do by hand. For an improved version, provide the insert with a fan--that uses some power, but much less than an air conditioner.

Does anyone know of a commercially available version of such a system for home use? Alternatively, am I missing some reason why it isn't as good an idea as I think?


At 8:35 AM, June 19, 2017, Blogger Ken said...

Something akin to this has been done in commercial HVAC for decades now. Not sure why no one has tried to adapt it to household systems.

At 8:49 AM, June 19, 2017, Blogger Dustin said...

At 9:11 AM, June 19, 2017, Blogger Walter Guyll said...

Greenhouses often use similar devices, usually triggered by temperature.

At 10:18 AM, June 19, 2017, Blogger Rohan said...

The window would have to be powered in some fashion, and that is unusual for a normal home. Also, you'd have to avoid opening the window when it is raining or has high winds, which means more sensors. A third issue might be insects or small animals getting inside. That would mean a mesh is necessary, which ruins the view. Not all climates benefit from opening the window. If it's particularly hot or humid, you want to leave the windows closed.

Also, I think many people prefer the air conditioner to open windows, even if it saves a bit of money. In the end, it may just be the preference of the majority.

At 12:19 PM, June 19, 2017, Blogger David Friedman said...

"In the end, it may just be the preference of the majority."

The puzzle isn't why this is not something everyone does but why the product does not seem to exist, at least for home use. It doesn't take a majority of customers to make a product profitable.

At 12:36 PM, June 19, 2017, Blogger Baconbacon said...

We use exchange fans, simply window fans with two blades which can be controlled separately. You can have both blades spinning to blow air in, or out or in opposite directions such that air is exchanged fairly quickly. They also have temperature sensors that shut the fan off automatically when the room is cool enough.

This is ~50% of the way to your idea, I have longed for a brand that would sense the outside temp and automatically turn on when it was lower than the inside temp (providing it was above a minimum you set) and turn off when it was matched or higher.

A simple way to make this compatible with AC would be to build it into a window unit, where the window unit is used when the outdoor temperature is to high, and the exchange fan is used when it is low enough.

At 2:02 PM, June 19, 2017, Blogger Rohan said...

"The puzzle isn't why this is not something everyone does but why the product does not
seem to exist, at least for home use. It doesn't take a majority of customers to make a product profitable."

I don't understand. The product exists. See

I thought we were discussing why it never seriously took off.

At 3:59 PM, June 19, 2017, Blogger David Friedman said...

Rohan: Following that link, it has electric window openers but it doesn't seem to have them controlled from sensors comparing inside to outside temperature.

At 4:35 PM, June 19, 2017, Blogger Ryan J said...

Controlling things by sensors is just a YouTube video away. Raspberry pi/plc type deal.

At 8:34 PM, June 19, 2017, Blogger Rohan said...

Hmm, I can't find comparison sensors, that's true. I did find some guy's patent on something similar: Though that uses a wall fan rather than open/closing windows.

So maybe he'll make and market your desired product. :)

At 1:57 AM, June 20, 2017, Blogger EH said...

It is my general impression that the home temperature control market lags decades behind what is both technologically feasible and already implemented in the typical office building. When I will be in the market to settle down and buy a house, bringing its air and temperature control firmly into the 21th century will be one of my top priorities. My current house also requires me to make all kinds of compromises between the temperature, humidity, oxygen and noise levels; and frankly, I dont want to make any such compromise in my bedroom. But I expect to invest quite some tinkering to meet my standards, and to be disappointed by off the shelf solutions, because I havnt seen much of it yet.

At 1:54 PM, June 20, 2017, Anonymous Miko said...

> Alternatively, am I missing some reason why it isn't as good an idea as I think?

My first thought would be crime. Manually opening and closing windows ensures that the owner is home to defend the property at times when the window is open while an automated system fails to do this. As the system is aimed at the less affluent who statistically are more likely to live in high-crime areas, it's likely to be most unattractive to those who would otherwise have the most use of it.

At 11:42 PM, June 20, 2017, Blogger David Friedman said...


The obvious solution would be louvres, which are probably easier to open and close than a whole window.

At 8:45 AM, June 21, 2017, Blogger Baconbacon said...

Louvres are poor insulators in general, any gains you make using them with this system would be at least partially offset by losses when your heat/AC is running on other days.

The window openings in Rohan's link are going to be poor (when compared to traditional windows) for air circulation.

At 12:56 PM, June 21, 2017, Blogger FlyingBeer said...

For family homes it's probably cheaper to build a single windcatcher type system that cools the whole house than it is to automate every single window in the house. Although I seem to recall at least one commercial building using automated windows that open at night, I can't find the reference, so I may be misremembering.

What you're talking about is an expensive device that can only be used in hot, dry climates in the summer at night. That's a very narrow range of potential buyers. Within that range there are a lot of cheaper alternatives before automated windows could be considered, and very few projects will have the resources to even consider such a method. Some cheaper alternatives:

At 11:11 AM, June 26, 2017, Anonymous Steve French said...

The Mr Money Mustache blog covered this topic a few years ago.

At 7:00 PM, June 26, 2017, Blogger David Friedman said...

Flying Beer:

I wasn't suggesting every window or close.

At 2:19 PM, July 05, 2017, Blogger Mike Sproul said...

Whole house fan.
End of problem.

At 1:42 PM, July 07, 2017, Anonymous Bruce said...

Rather than pumping the air in and out of the windows,m it might be easier to install separate vents. Bathroom fans and kitchen range hoods do this very simply. A few registers and it would look like the regular HVAC system.

At 12:46 AM, July 08, 2017, Anonymous Mercy Vetsel said...

I've wondered this too, especially with attic fans.

Mine is set to turn on when the temperature reaches 80 degrees and it makes a big difference in keeping the house cool, especially when we take other measures to reduce A/C usage, BUT it's hard to find the optimal setting since I really only want it to run when the temperature outside lower than the attic temp.

During the day when the sun is heating the roof and especially borderline A/C days (~80 high/~60 low), that's almost always the case, BUT there are a number of situations where the fan is wasted or even counterproductive such as really hot days or somewhat hot nights when the A/C bleeds into the attic at a faster rate than the sun warms the attic.

All I really need is a switch with one built in thermometer and a second one connected to a 30 foot wire that I can stick outside the gable vent. The switch would need to be smart enough to measure the difference in temperature and only trigger on specific conditions.

At 4:35 AM, July 09, 2017, Anonymous Mercy Vetsel said...

THIS is the device that I'm looking for, a "differential thermostat" switch:

This would solve my attic fan problem and could be used as a switch for a window fan with louvres to solve the original posted problem.

Considering how widely applicable this would seem to be, I'm surprised that it doesn't seem to be available for direct sale to consumers online.

At 10:50 AM, July 11, 2017, Anonymous Adam Craig said...

Hi David,

Yes, you can get such a system. It is exactly as you imagine; and the prices are reasonable. We have had several heatwaves here in the UK, and it was too hot for me to sleep. It was 22°C in my bedroom, 18°C is best for bedrooms.

I asked for advice from Rocburn in Essex, who make electronic window openers. They quoted me as follows:

1 x Kato 230v White With PVC Mounting Brackets - £108.95 + vat
1 x MB08 Proportional Temperature Control Panel Inc. Rain & Temperature Sensors - £141.85 + vat
Delivery - £6.95 + vat
Total - £257.75 + vat

This is for a very small window, but it would be effective. When I experiment with leaving the window open, the temperature drops by a few degrees. It includes the window opener and thermostat. It is wired into mains electric.

Noise could be a problem. The window opener would run for 10-20 seconds at 60db, which could wake me up. Silent window openers do exist, but they cost over £2,000. The security risk is acceptable for me because of the layout of my house.

£300 is a good deal, compared to installing air con. Much cheaper to run, too.

At 12:29 PM, August 22, 2017, Anonymous Mercy Vetsel said...

UPDATE: Here's a $30 solution that you can buy from Amazon:

I wasn't able to actually find that differential thermostat, BUT as a completely separate project I purchased a sous vide controller as part of my quest to cook the perfect pork ribs.

It occurred to me that this is a simple, if not ideal, solution to your problem when combined with a window fan. Just run the thermometer lead out the window and set the range of outside temperatures for which you want it to run, say 60 to 70 degrees.


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