Any idea what would cause this? In some random spots underneath a tray, the mat is very warm to touch. If it were much warmer, it would almost start to burn me. I unplugged it because I didn't want it to burn the house down.
Some of the heat mats I've had got very hot, but I don't think any could have started a fire. All were Hydrofarm brand. I have measured soil in seed trays above them as high as 105, which is way too high. You only need them until the first seedling in the tray sprouts. Since I got T5 flourescents to grow seeds I have stopped using them all together. The lights heat up my seedling mix to 85-90, and higher with a humidity dome. If you are going to use them, get a thermostat.
I noticed mine was doing the same thing after being fine earlier in the year. Turned out that it was sitting on a shelf with a florescent light on the underside. The area above the ballast was hot and so were a few other spots. When I moved it to another area it was evenly heated but hotter than earlier in the year. I think that was because mine typically raises the temperature of soil about 10-15 degrees above that of the room air. Early in the season my home was several degrees cooler most of the time so the soil never got hot, just warm. Does any of this sound like it could be the cause of your temp swings?
I'm thinking it might be time to look into a mat with temp control for next year.
Thanks for the replies. naturegirl, not sure. It's on a workbench in the Garage. There are no lights underneath that I'm aware of. The only thing that is the same as you described is the fact that it's warmer now.
Probably I should get a temperature controller.
An easier solution might be to use a heat diffuser, like an old towel or a thin piece of metal, something to act as a buffer between hot spots.
The best example of a heat diffuser I can give is when I make candy on my gas stove, the ring of fire coming from the burner creates a hot spot in the pot, and I use my cast iron skillet under the pot to make sure the heating surface is evenly heated. Anyone who has made candy treats knows hot spots in a simple syrup will make a person go mad.