Waterbed Heater - Fridge - Ranco Dual Stage

I have an upright fermentation fridge setup in my garage. I am using a Ranco dual stage controller to switch on/off between the fridge and a waterbed heater that is inside.

The problem I am experiencing is uneven heating caused by the waterbed heater which I have placed in the bottom of the fridge about six inches below the rack that the carboys are on. The bottoms of the carboys get much warmer than the rest. I tape the Ranco probe to the middle of a carboy and set the temp at 66°F.

I have also placed a sheet of cardboard between the heater and the carboys as a direct heat shield. That helped a little, but not a whole lot. I have noticed that on a couple recent cold nights, fermentation gets explosive despite the Ranco's reading of 66°F. The bottoms of the carboys are getting warmer than the middle where the probe is taped.

Does anyone else use a waterbed heater in a fridge? If so, any advise or lessons learned about placement or setup of the heater to minimize uneven heating?

one more thing, I was contemplating placing a small fan in the fridge also on the heating circuit of the ranco. whenever the waterbed heater runs, the fan would too. i think this would help distribute the heat more evenly.

any thoughts?

Yes, a small fan blowing directly at the heater is how I would handle that. In fact, running a fan on low full time wouldn't be such a bad idea either because it would keep temps even everywhere.

Not sure how much room you have to work with but I'd try putting a shallow rubbermaid filled with water directly on top of the heater. Tape the probe to the side of the rubbermaid container or one of the fermenters.

the temperature of fermenting beer will be greater than that of the water in the container due to the heat output of the yeast doing their thing.

thanks for the responses. i am going to put a fan in there this evening.

UPDATE:

6 weeks ago I installed a fan that runs nonstop in the fermentation fridge. Works like a dreram. Now the carboy temps are uniform and the waterbed heater never gets as hot as it used to.

Now I think that a fan is a necessary piece of equipment for this type of setup.