Can I use a programmable thermostat as a simple on/off timer?

[zubzub]

Can I use a programmable thermostat as a simple on/off timer at very low ambient temps?
I would like to control a proheat.  I have found a few dedicated 12V timers on line, but a I have a spare 7 day timer
  http://www.kijiji.ca/v-electronics/city-of-toronto/noma-programmable-thermostat/1013787075 
I was wondering if I could use it to time my proheat.....
Obviously I can use it to turn on/off the timer at reasonable temperatures, but as the low setting is 55 F., I'm thinking it will turn on anyway as  soon as I get it outside in the cold as a default.  Most thermostats made for residential have a low setting of 55 which I presume means they kick in no matter what at 55 to avoid freeze damage in the home.
I would like to do this ASAP and I seem to recall someone here saying I could use a thermostat to control a proheat, and I had only just considered the 55 cut in possibility.
BTW if I had a multimeter handy I could double check the low cut in, but I don't, so it is really just a presumption.

[sparkplug188]

You can wire a programmable thermostat to a contactor to control nearly any 120/240vac or 12/24vdc load.

Can I use a programmable thermostat as a simple on/off timer at very low ambient temps?

The answer to that question will depend on the capabilities of the thermostat.  Most programmable thermostats let you automatically schedule change the set temperature four times per day.  Some will let you choose "off" for the scheduled set temperatures-- others only let you set a numerical temperature (55-90).  I think you will need to experiment with a couple thermostats to find one that will do what you want.

If the thermostat doesn't work out, consider a basic home automation system.  I know you can get the results you want with a Z-Wave Vera Lite, temperature sensor and an appliance module.  ..or a simple timer would work, like you mentioned in your post.

[gumpy]

Not quite sure I fully understand what you are trying to do.

Most diesel burner units have a control board and temp sensors that turn them on and off to keep the coolant within a specific range around 180°F. Typically,
all you have to do is turn on a switch and the unit will heat up and shut itself off when the sensor says it's hot. Then, you use a thermostat to control the
circulation pump and heat exchanger fans inside.

I don't understand what the timer would be used for.

[zubzub]

I want to turn the proheat on when I am not around.  Proheat sells a programable timer for this but it is $$ and I was told I could use a programmable thermostat.  I could order something online just trying to use what I have at hand.

[gumpy]

Ok. So it seems you are trying to put in an anti-freeze system on your heater.

I think using the thermostat you have would work, but it would come on at whatever the minimum is. That's probably 55°F or so, which
means most likely, your heater is going to be running quite a lot when you're not there.

I did a similar thing with my aquahot. I used a thermocouple activated industrial heat tape controller. My father-in-law had
one from a jobsite he had worked on in the 70's. It's heavy duty, but it was free. I attached the thermocouple to the distribution manifold
copper pipe. I then wired it to a 28v relay which controls the electric element in my aquahot, and also wired it to activate
the heating circuit which turns on the circulation pump and the fan in the water bay. The thermostat turns on at about 35°F. I found last
week that my electic element can't keep up when temps drop to about 0°, but there's no reason I couldn't have it activate the diesel burner
instead.

I realize you are trying to use stuff you already have, but maybe you can find an affordable heat tape controller somewhere.

Another idea is to find a normally open snap-disc thermostat switch that closes at around 35° and opens around 40° and attach that to your
pipes and connect to a relay coil that controls the heater.

[mung]

Yes you can use a programmable thermostat to control something besides your AC.  I used one in an AC duct to turn on a duct booster fan so that it would only run when cold air was pumping through the system.  Worked like a charm. 

[eagle19952]

the thermostat controller from an old diesel fired torpedo heater will do what you want without relays if 110v is what you use for control voltage...

[zubzub]

Well sometimes the simple stuff is complicated...I think it was Len who suggested using a programmable thermostat.  I don't want minimum heat I want full off then on
  The proheat takes care of itself....I want to have it start at a specific time....but since all the programmable thermostat have a min on temp. As soon as the temp is below 55 the thermo turns on overriding the program. Same for an ac thermo. The cold would trigger an off signal.



I

[zubzub]

Well sometimes the simple stuff is complicated...I think it was Len who suggested using a programmable thermostat.  I don't want minimum heat I want full off then on
  The proheat takes care of itself....I want to have it start at a specific time....but since all the programmable thermostat have a min on temp. As soon as the temp is below 55 the thermo turns on overriding the program. Same for an ac thermo. The cold would trigger an off signal.



I

[Emcemv]

Zubzub

You can do this but these thermostats have a relay in them that is only rated for 24 volts and usually about 1 amp of current, so you would have to use the thermostat to control a 120v rated relay if you want to control heat.  So you would need a 24 v relay with contacts rated for 120v and the appropriate current for what you need to control.

[kyle4501]

FWIW
Line voltage programmable thermostats are out there that will switch 16 amps at 120 volts.

Honeywell # TH115-A-120S/U 
Can be set as low as 40F, ambient temp 32F