Tuesday, August 21, 2007

Low Cost Cooling

A common hot weather strategy, especially for people living in big old houses without air conditioning, is to open windows at night when it is cool out, close them in the morning.

It should be straightforward to automate the procedure, using windows or vents that can be set to open when the temperature outside is cooler than the temperature inside, close when it is warmer, with fans to increase the airflow when desired. I would expect both the capital cost and the operating cost of such a system, used to replace or supplement air conditioning, to be trivial relative to the cost of air conditioning itself.

Yet I do not think I have ever seen such a system. Have I missed it? Or is there some non-obvious problem with the idea?

[The reason I have not been posting is that I've been travelling].


At 6:16 AM, August 21, 2007, Blogger Brian Dunbar said...

It should be straightforward to automate the procedure, using windows or vents that can be set to open when the temperature outside is cooler than the temperature inside, close when it is warmer, with fans to increase the airflow when desired.

I have a big old house, constructed in 1890.

I am not sure but it feels like the cost would exceed the benefit. This could not be a 'one size fits all' solution so each house would be a custom job. Thus, expensive and hard to sell.

At 6:50 AM, August 21, 2007, Anonymous Anonymous said...

It's not just relative tempatures about which you have to worry, but also the outside breeze.

At 6:51 AM, August 21, 2007, Anonymous Anonymous said...

Shortage of Maxwell's demons?

More seriously, Brian Dunbar's explanation sounds plausible to me. You couldn't just stick a box in one window and have it do the job. You'd need a mechanical effector for every window you wanted to include—and you'd have to have effectors for several different types of windows: those where you open a latch by turning it and lift the window, those where you press the latch down instead of turning, those where you insert a crank and crank the window out, and possibly others. You'd also need two temperature sensors, one mounted inside and one outside, though the sensors and the CPU would likely be the cheap parts of the system. And you'd need to have cables or wireless connections throughout the house. Installation would be nontrivial compared to dropping a box into a window, so not many people would go for kit-based systems, and you'd have to pay for a few hours of a handyman's time on top of the hardware.

On the other hand, aren't small home air conditioners fairly cheap?

On the third hand, perhaps some clever entrepreneur could market these as a "green" alternative approach to home comfort.

At 7:04 AM, August 21, 2007, Anonymous Anonymous said...

I'm currently designing a house, construction to start next year. We're using passive cooling systems instead of air conditioning; one of the methods is to have automated windows hooked to thermostats -- if the temperature is in the right range, and the temperature difference between inside and outside is in the right direction, high and low windows will open automatically to induce airflow and cooling.

So, good idea.

At 8:40 AM, August 21, 2007, Blogger Alex J. said...

Hard to retrofit automatically opening windows.

My house has an attic fan. You turn it on, and it sucks hot air from the house up into the attic and then outside. You open windows the conventional way to let outside air in. The fan is covered by a vent that closes by the force of gravity and opens from the force of the fan's suction. So there are no other motors.

It occurs to me, that my house could use a basement fan that works on much the same principles. Instead of drawing air from all of the windows. Cool air could be drawn from the outside through a large vent into the basement. Air would then flow up through the house to the attic fan.

The basement air is much cooler than in the rest of the house, and this setup would take advantage of that. The air down there is a bit musty too, and this might be a problem.

At 10:05 AM, August 21, 2007, Blogger Leif Wickland said...

If you have a reasonably new furnace in your basement, you already have a basement fan. Block off the cold air return and open up the air filter box so that the intake to the furnace's blower motor is the air of the basement. Close off the vents in the basement. Switch the fan to run continually and not heat the house. I've been very successful keeping my house under 81 with this strategy on 105 degree day.s

At 10:31 AM, August 21, 2007, Blogger Matt Brubeck said...

There are solar-powered exhaust fans and attic fans that use indoor/outdoor temperature sensors.

At 11:16 AM, August 21, 2007, Blogger David Friedman said...

Brian argues that each house would be a custom job, but I don't see why. Surely you could have a unit that fits into a window, just as a window air conditioner does now.

Anonymous raises the issue of a breeze. To take account of that, have two units with fans, one on each side of the house, communicating via bluetooth or WiFi. One blows air in, one out, and they decide which does which according to which side of the house the air pressure is higher on--evidence of which way the breeze is blowing.

At 11:31 AM, August 21, 2007, Blogger Patri Friedman said...

Fitting things into windows tends to introduce leaks, which reduces the effectiveness. And there are many different size windows. And it's probably tough to get the aesthetics right. To do a good job, I think you'd want to build it into the window. I'm surprised no one has done it, windows are expensive enough that it wouldn't increase their price much (relatively).

I think the cheapness of air conditioners and electricity is probably part of the answer.

Also it's worth noting that attic fans, which someone else mentioned, are a similar technique which is fairly common. The fans turn on when the attic reaches a certain temperature, and vent the hot air to the outside. But they are more bang-for-the-buck, because the attic accumulates the hottest air in the house, so you are getting rid of more heat than just a room window.

Does Dave have a quote for how much his system costs?

At 3:08 PM, August 21, 2007, Blogger Brian Dunbar said...

Brian argues that each house would be a custom job, but I don't see why. Surely you could have a unit that fits into a window, just as a window air conditioner does now.

I see what you're getting at, now. I had thought you were writing about a system that is buried inside the walls.

Power would be an issue, but a minor one. You could control the system with a wireless system.

Another possible gotcha. It gets warm at 3:00 a.m. Windows open up, a cool breeze blows in .. just as a burglar strolls up. This isn't really a problem but the perception of a house opening up when it wants to is. Perception matters.

At 5:47 PM, August 21, 2007, Blogger Franco said...

Would be useful for NYC apartments with overactive radiators, too, though the resulting feedback loops might be bad.

At 4:59 AM, August 22, 2007, Blogger Mike Huben said...

You may not have seen such a system because it's not reliable: there's no guarantee that the temperature will drop low enough at night during a heat wave.

The other major benefit of air conditioning is reduction of humidity.

I have a 130 year old house, and while I open and shut windows to cool the house, I bought two $100 air conditioners to cool two rooms as refuges from heat and humidity. I run them only a few days per year here in MA.

At 12:16 PM, August 22, 2007, Blogger Jonathan said...

Here in Spain, we have air conditioning in the bedrooms only, and don't use it much because it doesn't work well! (It tends to overchill the room no matter how we set it.) We open and close windows quite a lot. But doing so manually isn't arduous; the incentive to install an automatic system seems low. Furthermore, many of our windows are actually French windows (sliding doors).

At 1:59 PM, August 22, 2007, Anonymous Anonymous said...

When I was in engineering school about 25 years ago, I did a project studying such systems. They were entirely possible (and in fact some had been built as demonstrations), but the cost was prohibitive. One key enabling technology has blossomed since that time: networking over power wires. You've got to run power wires to each window for the motor, so you use them for the control network also.

However, it still costs too much. The cost of networking has plummeted, and a 3-GHz PC costs much less than the TRS-80 8-bit computer I used in that project, but a motor in each window costs just as much as it ever did, and the cost of installation labor is higher.

I agree that it would cut the costs quite a lot if the idea became popular enough for window manufacturers to sell windows with the motors pre-installed. I think it would add less than $25 per window, which isn't much considering what energy-efficient windows themselves cost. The other cost would be to run power wiring to each window. That's a labor-intensive job, even in new construction. So, while not hugely expensive in new construction, it's not cheap, either. And finally, in most places you still need air conditioning because some summer nights stay warm. (Or at least cool down too slowly - I can't sleep in an overheated room, so it doesn't help much if the house cools down at 3 am when I've got to be at work at 8. And this is in northern Michigan.)

Brian mentioned burglars. Not a problem where I live (too many people believe in 12-gauge behavior correction), but I can see where it would be a big issue in cities. Having seen some surprising figures on how small an opening some men can squeeze through, I think you'd have to supplement the motorized windows with grills, and alarms to detect burglars trying to remove the grills. Urban and suburban dwellers may also have trouble with pollution, dirt, and privacy issues from open windows.

A more practical idea is to rig the central heat/air with high-volume controlled vents to the outside. You leave your windows closed, but the system detects when it is appropriate to circulate outside air rather than recirculating artificially heated or cooled inside air. A mass-produced system like this should only cost $200 more than a regular high-efficiency furnace & central air (that is, about the same as the building permits required to install a new furnace!), and I think in the climate here it would pay off in the first year. But I just don't see anything like that on the market...

At 10:05 AM, August 23, 2007, Blogger Les Cargill said...

Air conditioning isn't 100% about temperature. It is also about reducing humidity.

At 12:05 PM, August 23, 2007, Blogger Ajay Shah said...

On a related note, I've often fantasised about cars that "sweat" - about small nozzles that spray water on the exterior of the car so as to keep the car cool and thus reduce the energy required for air conditioning. I'm not able to do the calculations - might that make sense?

At 9:25 PM, August 23, 2007, Anonymous Anonymous said...

Maybe some people just don't like open windows. Like me -- I don't like opening my home to (more) insects and I'm kinda irrationally concerned with noise both coming in and going out. My windows stay closed.

At 4:00 PM, August 26, 2007, Blogger Mark said...

Another thing to worry about: rain.

At 7:45 PM, August 26, 2007, Anonymous Anonymous said...

And something else to worry about: anyone thinking of blowing cooler basement air upstairs should check their radon level first!

I think David's idea is a good one, except it should be integrated with a central exhaust fan, which is more powerful and efficient. The window unit could be a lightweight, louvered polycarbonate panel with a small motor and wireless receiver. Not a bad idea at all.

At 8:57 PM, August 28, 2007, Blogger Unknown said...

At the blinds store today, saw a set that opened and closed via remote control. So I bet we see David's system in a few years. My car can open all windows & sunroof with one click-why not my house?

At 4:35 AM, August 30, 2007, Blogger Steve_Roberts said...

Temperature-controlled window openers / closers are sold for domestic and probably commercial greenhouses for very similar reasons.

At 6:35 AM, September 02, 2007, Blogger Zendo Deb said...

Living in San Jose, you don't know about humidity. (Trust me, I lived in San Jose for 4 horrible years)

In Tampa it will often cool off at night, but the humidity will not drop below 50% (usually not below 70%) at night.

When I lived in Miami, they sold AC units with 2 settings - full AC and humidity control. They basically ran all the time - though in low - to keep the mold at bay.

And the Midwest isn't much better. Humidity means you want the AC on all night long.

At 1:53 PM, September 09, 2007, Blogger Hernan Coronel said...

Take a look at enertia.com, they've been featured on David Pogue's column in The New York Times.

At 12:41 AM, September 11, 2007, Blogger gcallah said...

"I don't like opening my home to (more) insects..."

There's a new invention called a "screen." You should check it out.


Post a Comment

<< Home