Steve, the switch in the refer bay turns on the upper chimney fan automatically when the air temp inside the bay gets to a certain temperature. It works well in combination with the lower fan. I had installed something similar to this on my class C Tioga motorhome, and it helped a lot. The two fans I installed were solar powered, and so were on whenever the sun was out, whether or not it was hot. I can see the quantifiable difference on a hot day with and without the fans on. The switch in the bay is set at 105. I'm sure the temp inside the chimney gets hotter than that, but the low threshold in the bay, which when closed, and sun is hitting the door heats up quite quickly inside, and, therefore turns on both fans (the one venting the bay, and the one helping vent the chimney. It's what I would call a pro-active setup.
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