very disappointed - tried to use for garage heater thermostat

Have a gas garage heater and was looking for something basic to replace my old thermostat that quit working. This thing takes forever to accurately read the current temperature, at least 30 minutes for it to be right. In the pic attached I had set the thermostat to 70, and after awhile the garage got to be nearly 80 degrees and the thermostat still read about 50. I went and got my fireplace thermostat and brought it out to the garage for comparison - after 15 minutes the fireplace thermostat read 70 (correct temp), but this thermostat still said 56. This is a complete piece of junk... And most likely a fire hazard if this thing was allowed to continue to run. Extremely disappointed.

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