I know that you can use a timer to have the block heater start whenever you want but what if you could rig up a system where if air temp dropped below a certain point a curcuit would open and start the block heater?
Similar to a thermostat, really...
You would save even more electricity this way.

is that they usually can't be set below about 55F, but if you had a permanent mounting place, the type with the bimetalic spring operating a mercury switch could be mounted at an angle instead of level in order to change its operating range, although that would require some experimenting. I wonder what kind of thermostat kicks on the big ventilating fans in my brother-in-law's dairy barn (Minnesota)? -- they are set just barely above freezing, I think.

With a home-heating thermostat, of course, you need a low voltage transformer to power the thermostat, and it has to send its signal to a relay to handle any serious wattage at 120V.

In doing control systems many years ago, there was a company called Omega that had a set of manuals that showed you how to make about any temperature control circuit you can imagine with their modular pieces. It would take a themocouple mounted in a strategic location, connected to one of their simple units, then use the output of the unit to trigger a relay to turn on the heater. No big deal.

I have been in software for a long time now, but I'll bet a search for Omega Temperature Control would home you into a wealth of information.

Good luck,

There are plugs (little square boxes that you plug into an outlet) that you can get preset to turn power on when it's below 45 or 30 degrees etc. They also turn power back off at a predetermined temp. I can't remember where to get them, might look in a hardware store. I had one that would turn an electric heater on in a greenhouse when it got below 40, it turned off when the temp got over 45 I think

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k

Get a Grainger catalog. Buy a "line voltage thermostat". They are made in a wide variety of temperature ranges. They usually handle 15 amps 120VAC. You need to buy one for heating mode. You could mount it on the car or at the outlet. I think they're about 50 bucks or so. You'll have to wire it in, but the directions make it simple.

Fred

Right direction and not a bad price but think they could sell a lot more if they'd drop the min temp another 20F.

http://www.infraredheaters.com/apt2.htm

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1982 300TD - 36K mi
1993 Infiniti J30t - 61K mi

waybomb is pointing out something that should be paid close attention to; 15AMPS!

Electric resistance type heaters require lots of Watts. Don't pick up something that was designed to turn on a radio, you need something with some serious power capability for the block heater.

Good luck,