I am thinking about mounting a 1000W 120V wall heater on an exterior wall, in my bathroom. Assuming I have room to do so, would it be possible to put rigid insulation behind the heater's mounting box? Could this be a fire hazard?
Yes, it will be a fire hazard. The smoke when it burns is toxic, the reason fire-fighters first started using oxygen tanks. IRC requires it be covered for fire safety in an attic or crawlspace under most conditions....
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!