I just recently did some rearranging in my home. As part of the rearranging, I put in a dedicated 20 amp circuit for my laser printer that makes the lights flicker if they're on the same circuit. Since it goes into a bedroom, I used an AFCI (my jurisdiction is on the 2002 code still). Home Depot recently switched to selling combination AFCIs, and this is the first one I've used.

Last week, I was feeling a little under the weather and wanted it warmer in that room. I used a portable space heater, and figured I'd plug it into the printer circuit - I wasn't printing and figured having it on a dedicated circuit would be good. It kept tripping the circuit after a few minutes use, even with the printer unplugged.

I moved the heater to the 15 amp circuit that's shared with several rooms. No problems using it there. This circuit is on a pre-combination AFCI. No trips.

Should I be concerned about the new circuit or the heater? The printer works fine - no trips yet!

When you say "tripping the circuit" is it tripping the breaker or the arc fault protection of the breaker? Or does the breaker indicate what tripped it, an overload or an arc fault? Apparently if it doesn't trip a 15 amp breaker then it's not a heater problem.

I might expect a bi-metalic thermostat to trip an AFCI if the heater has been used a fair bit. Just because that contact could easily arc.

It definitely is the thermostat, which behaves like a bimetallic one. Thanks for setting my mind at ease!