Bill, from the Champion catalog the w16 is a standard plug while the w18Y is an extended tip that runs a bit hotter but not by a lot. The hotter part is just the tip of the plug not the whole engine. the heat of the fire in a fireplace doesn’t have much to do with how hot the match was that started ignition. The exception to this analogy is if the hot tip is enough to cause pre ignition (knock). The point of the hotter tip is if you are prone to spark plug fouling. If not, the standard plug would seem to be the choice.