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Capable package, great price 
Good capability for the price, but here are some considerations:
- Can only display in degrees C.
- Maximum temperature is 110 C both for setpoint and alarm.
- Minimum temp. is -39 C.
- Total sensor length is about 12 inches, but could be extended.
- Can be configured for heating (relay contacts open on reaching temp.) or cooling (contacts close).
- Relay contacts Normally Open when unit is unpowered.
- Relay case shows contact rating of 20 Amp. at either 14 VDC or 125 VAC, although I would use
it to operate a heavier duty mechanical or Solid State Relay for continuous currents that high.
- Comes with a single page of instructions that are easy to follow.
Device Looks Good, A Little Tough to Program
I bought 8 of the THM-4s to replace defective thermostats in electric space heaters. The THM-4 needs 12VDC, but they don't tell you what voltage range it can use. Powered by 12.0 VDC it draws 30mA when the relay is not engaged and 80 mA when relay pulls in.
When being programmed, the device will switch back to "idle" mode if you stop pressing the buttons for more than a few seconds. Best to have a list of the parameters and values you want to program before you start.
Press and release SET button to program the temperature set point (press + and - buttons to change value).
Press and hold SET for 5 seconds to access parameters P0 through P6. After "P0" appears in the display, use + and - buttons to select the parameter you want. Then press SET again to access the parameter value. Use + and - buttons to get the value you want. Press SET again to go back to the list of parameters to select another.
The display shows "LLL" if the temperature sensor is unplugged.
I have not used the THM-4 enough to know how the relay will stand up to a 13amp/120VAC heater load switching maybe several times an hour 24/7. I am prepared to use 40A solid-state relay if the on-board relay can not take it.
Great thermostat
This is a great thermostat, it works very well. You can not only set the temperature but also the hysteresis and whether you want it to control a heater or a cooler/AC. The instructions that come with it explain all the settings very well. You will need an external 12VDC power supply. The only two caveats I found with this board is it only reads in centigrade (if anyone figures out how to make it read in normal degrees, please post it here) and when the relay engages or disengages there is some kind of transient that makes the temperature reading jump by approx 0.2 degrees, but it quickly settles back down after a second or two.
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