Using Thermo Cube for greenhouse heater

In a previous post I mentioned purchasing a Thermo cube to control the heater for the greenhouse.  Up until recently it has not been cold enough to adequately test the unit, but that changed a few days ago.

Our weather has changed dramatically, with several inches of snow and temperatures well below freezing, both during the day and at night.  The plants were brought back indoors until the weather moderates, so with the greenhouse empty I decided to test the cube.

The Thermo cube was a dismal failure when it came to controlling the heater, while it seemed to work, somewhat, with a small LED light.  My thinking was that the resistance to the electrical current draw of the 1500 watt heater was causing a heat build up in the cube circumventing the thermostat.  When I spoke with one of their representatives, I was told that that was not possible, as the thermostat was bi-metal construction.  

We all know that when an electric appliance is working, the cord and outlet, even the circuit breaker, get warm.  How do they expect the heat caused by resistance within the cube not to affect the bi-metal thermostat?

The above photo shows the cube with just the LED lights which draw a watt, or less.  The temperature is in the thirties and the cube has the light turned on.  As seen in the photo there are two very accurate thermometers in proximity to the cube and circuit.

When the heater is plugged in, the grey cord on the left, the cube turns off in a minute or so, as you can see by the lights.  The test has been repeated several times over the last 24 hours with the results being the same.  Additionally, I have tested two different cubes with identical results.

If you have purchased a Thermo cube and intend to use it to control  greenhouse heating, beware, test it thoroughly before you trust  it.  These tests were conducted with four different heaters: 600 watts, 900 watts, 1300 watts, and 1500 watts.  The published rating on the device is 1800 watts, so all of the heaters were within the device's specifications.

This morning I will be returning the cubes for a refund and will go back to a simple mechanical timer and oil filled radiator for heating the greenhouse.  These have served me very well for a number of years now, so it just proves: Don't mess with success.