I'd been thinking about how I was going to switch these fans on and off and initially thought I'd just wire them through a light switch and force myself to remember to turn them on when the fireplace is running and off when it's shut off. Then I thought about automating the process and bought a thermal switch that would trigger the fans on when it reached 180 degrees. Mounting that device would be easy as it had a magnetic bracket attached to it, but any service or replacement of that device would be very difficult, if not impossible due to where it would have to be placed.

Instead, I grabbed this gizmo off Amazon. It's an inductance powered switch. You run the line side of a circuit through a hole in the device and when whatever that circuit is powering turns on, it closes a small relay, energizing some other device. The fans I bought are like 0.17A each so the 0.5A rating of the relay is more than enough to drive both fans. My final solution/installation of this turned out pretty slick.