Electric Floor Heat

[Bill Hudson]

I plowed my drive today. When I got done, the Cub still had half a tank of amps.

I haven't laughed this hard in a loooooong time.

Bill

[Barnyard]

I plowed my drive today. When I got done, the Cub still had half a tank of amps.

Jim must be the guy behind that electric Cub we read about.

[Jim Becker]

There is another possibility to operate all three . . .

I had to mull that over a bit. Essentially the current thermostat and 2 large elements get priority and the second thermostat only gets power when the large elements are turned off. I can think of a couple ways to do that. Some would require changing the breaker to a GFI to avoid replicating the non-protected situation I started with.

I'm going to stick with my earlier plan to leave element 3 disconnected. I will keep this option in mind in case we decide to start using element 3 later on.

[John *.?-!.* cub owner]

One thing to remember, hot elements normally have a higher ohm reading than cold ones, how much difference depends on the design and material.

[Jim Becker]

I switched to the two large elements and ran it quite a while last night. After running it several hours today, I measured the two together at 15.6 ohms. Who knows if the last digit means anything on my meter, but it is about 1/2 ohm higher than they were cold. The temperature was probably up 10 to 15 degrees from cold. Yes temperature makes a difference, but heating wires buried in mortar won't have a temperature swing like a light bulb. I did all my initial observations and calculations based on cold elements and a full 240 volts on the lime. That way I was generally looking at worst case.

It has been running with the two larger elements and no contactor for long enough now that I think it is OK with that configuration. I am going to permanently wire it up that way. I just need to run a new wire from the thermostat to the junction with the elements (couple feet away). Then change out the contactor enclosure for a box with a splice to run power to the thermostat.

[Bus Driver]

Those elements typically will not exceed 110 degrees and the flooring material is wicking away heat from the elements all the time. The empirical evidence indicates that the contactor coil has some slight leakage (fault) to the non-current carrying metal parts of the contactor. No doubt the contactor was mounted in a metal enclosure that was grounded, as it is required to be. The slight current to ground actuated the GFCI.
Have you noticed how sharply the horizontal heat migration stops at the edge of the heating elements? Two inches away the tile will be cold.

[Jim Becker]

The metal box for the contactor was grounded through the ground wire in the power cable. One of my early tests was to remove the ground from the box and put a milliamp meter between the box and the ground wire. The meter never showed anything (at least while I watched). With the ground disconnected, the GFI went longer without tripping but I don't know if that was meaningful or just a random variation. Leakage between the control coil and one of the main poles could trip the GFI without any leakage to ground too.

Based on the ohm readings (implied element length) and the area covered, it appears that the cable was set at a pretty close spacing. I haven't been able to use heat to trace the actual path (might be able to with am IR thermometer). But it is easy to tell the difference near the edges where they didn't run the cable.