Thermostat to help control ventilation intelligently?

June 21, 2012 1:07 PM

Is there a sensor device or thermostat setup that can inexpensively switch on a fan based on the difference between two temperature points (one inside, one outside)?

I have a gable fan. I use it at night to help flush cool air from outside into my house. I was thinking a smart way to do this would be if it would automatically go on if it were, say, 5 degrees cooler outside than inside. e.g. if it is 80 inside and 75 outside, stay on 'til either a) outside goes up to 80 or b) inside cools down to 75. And of course, if it is 80 inside but 90 outside, don't turn on at all. (Obviously you would want a few degree buffer so it is not switching on and off rapidly when the two temperatures converge, I believe this is referred to as hysteresis.)

Apparently, this is not a frequently requested task. The closest thing I've seen is expensive stuff originally designed for custom solar water heating setups, like this. (I have emailed to get a price, but my guess off-hand is S-P-E-N-D-Y!)

This seems like it could be a simple thing to accomplish with a few sensors and some sort of home automation scripting language, but I haven't gotten anywhere near that far tricking out my home.

Also, I am a little intimidated by relays, so the idea of something designed to simply switch on/off an attached household current device appeals, but maybe this fear isn't justified.

posted by BleachBypass to home & garden (13 answers total)

Something like a temperature differential thermostat, or is this the one you already found?
posted by caddis at 1:28 PM on June 21, 2012

Yep, that's the link above.
posted by BleachBypass at 1:34 PM on June 21, 2012

Here's one on Amazon for $92.
posted by caddis at 1:35 PM on June 21, 2012

Measuring temperature is a basic example project for a lot of inexpensive hobby microcontrollers such as Parallax Basic Stamps and Propeller products and of course the Arduino. Writing a little app to do the comparison and set an output would be trivial, although if you have no electronics experience the parts to get into it would probably cost as much as the one caddis found.
posted by localroger at 1:50 PM on June 21, 2012

Caddis, I've been having problems with terminology while I've been searching. (The art of googling = knowing the right words to google)

I believe this product is referring to differential in the "hysteresis" sense; ie, one can use the lower knob to adjust how wide the buffer is between shut-on and shut-off temperature (also known as "swing setting"), rather than the switching based on the input and difference between two temperature sensors.
posted by BleachBypass at 1:55 PM on June 21, 2012

You could program an Arduino to do something like this, using two thermistors and an AC relay (that allows the low voltage Arduino to control power to heavy duty things like fans) and a few lines of code that define what to do when temperatures are inside/outside of the specified range. You could even get fancy and do things like tweet new temperature ranges to the device if you had an ethernet/WiFi shield.

edit: Beaten!
posted by speedgraphic at 2:52 PM on June 21, 2012

I have one of these WIFI-accessible thermostats. You can't program it per se, but you can adjust all of its settings and read its indoor thermometer via a simple HTTP / JSON interface.
posted by qxntpqbbbqxl at 2:54 PM on June 21, 2012

Parallax currently has the Basic Stamp Activity Kit on sale for $29.99, down from the usual $69.99, and that contains everything you need except the thermistors and high level output, for which you could use the recently introduced Dual Relay Board Kit. So at the moment even if you have no electronics toys you could get everything you need for the project, including a basic tutorial on how to do such things, for less than the cost of the commercial thermostat.
posted by localroger at 3:37 PM on June 21, 2012

On re-reading, if you really have no electronics toys at all you'd also need a soldering iron for building the relay kit and remoting the outside thermistor. That would add another $20 or so. And, of course, a few evenings doing the exercises and learning how to do it, but a lot of us consider that fun :-)
posted by localroger at 3:42 PM on June 21, 2012

I believe this product is referring to differential in the "hysteresis" sense

I think you're correct.

Note that even an expensive temperature controller with 2 temp probes may not be capable of the fancy options you mention. I really think you want a programmable controller (Arduino, Stamp) which can look at the temp of each sensor and the program will tell it what to do.
posted by exphysicist345 at 3:44 PM on June 21, 2012

Here's one of those Arduino-based DIY attic fan controllers mentioned above.

You might want to search for whole house fans or components for them to see if any of them do what you want, since they are designed to exchange inside air for outside.
posted by mbrubeck at 6:15 PM on June 21, 2012

Thanks! I think I'll look at the Stamp kits and take the plunge (unless that Arduino post convinces me to go that route). Learning sounds fun and potentially has bigger project payoffs down the road than simply buying another off the shelf component.

mbrubeck - you nailed it. I actually want this to control a combination of two fans - one is my whole house fan, and the second is the aforementioned gable fan that acts as a "booster fan," reducing the static pressure in my attic and effectively allowing the whole house fan to exchange air much faster.

I always thought it would be neat to automate based on temperature conditions; though as I think about it, maybe I'll have to consider a actuated louvered intake vent if I want it to automatically trigger (no furnace backdrafting for me, thanks.)
posted by BleachBypass at 7:41 PM on June 21, 2012

You could easily set this up with an Arduino a Relay Shield and some temperature sensors.
posted by odinsdream at 6:34 AM on June 22, 2012