A differential thermostat

[wesalexleft]

Im not sure if this is what it would be called, and the Internet didn't turn up what I was thinking, so I thought I'd ask here. Does anyone know of an inexpensive thermostat that would measure two temps and then switch on a fan if the difference between the two temps was greater than a set value? My garage is air conditioned when I'm going to do work inside, but I'd like to hook up a fan to bring in outside air when the temps warrant it. My main problem is the fast temp rise when a hot car is pulled inside. I would love a fan that would tun on if the difference in temps is over a set amount, but then turn off when the temps equalize. Anyone ever seen anything like this?

 

[Steevo]

My last house had a "smart vent" that worked like that.
If it got cooler outside than inside, and you had the inside temp set lower than it was currently, it would blow outside air in through the A/C system.
It was great on summer nights.

 

[bdamico]

maybe i'm missing something -- don't most digital thermostats have a swing setting that can be adjusted-- i'm fairly certain my filtrete does

 

[Sureshot]

I have thought of putting two thermostats in series. One inside one out. The inside one set to come on at 75 or more(AC mode) and the outside one to come on at 72 or less(heat mode).

Not a true differential but close.

 

[wesalexleft]

I was thinking more for one that could be plugged into an outlet to operate a box fan in a window, a vent fan installed in the ceiling, etc. That way, when a hot car was pulled into the garage and the temp started rising quickly, the fan would bring in cooler air until the two temps equalized. and then shut off. I imagined something that would have a wireless unit for the outdoor temp, and would switch the accessory on and off depending on the settings.

 

[pseudorealityx]

If you've got A/C in the garage, why would you want to blow unconditioned outside air into the space?

 

[wesalexleft]

Living in Memphis, our high temp today was 84--a nice spring temp here. During the summer we will be in the high 90's and very humid. Add a hot car, and WOW. It's midnight now and the garage is 83 with the windows closed. I don't run the AC full time, but only when I'm going to be in the garage for a quite a while, or to chase out the humidity during summer. Through the week would cost a fortune for a little benefit. I'd like an "auto" fan and then just the AC during major stints. Normally, after work, the temps start dropping just as the car gets parked and I watch while the garage hits 90's while outside is high 70's. My thought was let mother nature bring it down mostly, then supplement with AC. I'd leave the door up, but then I have mosquito issues. I HATE Mosquitos!

 

[tylernt]

wesalexleft are you into electronics? There's a little programmable thing called an Arduino that with the right expansion boards can do whatever you want. It's a bit technical, but let me know if you want more details.

 

[VHF]

I wonder if a differential controller designed for a solar hot water heater would work? These have two temperature sensors, one at the solar collector and one at the storage tank. When the temperature differential exceedes the threshold (for example, the collector is 15F hotter than the tank), the differential controller turns on the pump. When the temperature differential drops to say 4F, the controller switches off the pump.

Some differential controllers are designed for hardwiring. Others are plug-in with a recepticle into which to plug the pump. Some have fixed thresholds, others are adjustable.

Basiclly you want to use a fan to transfer heat out of your garage when the garage is hotter than the outdoors.

P.S. It is 34F and snowing today in northern Wisconsin!

 

[Grumpy365]

http://www.ledclocks.com/controllers/DSD2_Specs.pdf

http://www.ledclocks.com/controllers/DSD2DifferThermostat.html

 

[pseudorealityx]

Any 'fan' that is going to cool off the garage 'quickly' when it's 90 and it's 70 outside is big enough that you might as well run the A/C. A hot car has a LOT of thermal mass. It's exactly the reason it will spike the garage temp up so quickly.

You would have to have the fan, as well as an intake louver, ideally, on the opposite side of the garage. If it's 90 inside and 70 outside, then you will need 800 cfm to provide *1* ton of cooling.

And since your worst humidity conditions are going to be when it's 80 outside, but with a wet bulb in the low to mid 70's, using a dry bulb to control this ventilation fan is a poor solution.

 

[wesalexleft]

Thanks for all the input. I was thinking of using a window box fan, but I understand that it may not be enough air flow to do much. I have an old attic fan, but didn't want the opening in the ceiling for fire suppresion reasons. I'll have to kick it around some more. I did like the two-thermostats in series approach. Seems cheap enough to use as a control. Most of the "specialty" ones look expensive. I had seen the Arduino in looking for bluetooth garage door control. Those look fun to experiment with.

 

[tylernt]

A thought just occurred to me -- you can get a cheap twist-dial timer switch that replaces a normal flip switch, and counts down between 1 to 8 hours. You could install one of those and just give it a twist when you pull your car in.