OK, First let me say hi. Second Please don't roast me for dredging up this 2 year old post. I have been reading thru some of these older post for some entertainment as well as knowledge. I have the answer you are looking for here. It is actually very simple. The furnaces that won't work properly have flame sensors that resemble a short piece of stiff wire that is in the spark lit pilot or burner flame. This is the wire that sometimes needs to be cleaned when the furnace does not light properly. Most people mistakenly think these work like conventional thermocouples...They do not. They are a ground path for a milivolt current that passes thru the flame from the controller. The controller is looking to see if there is a ground path and therefore will keep the gas valve open. The carbon in the flame acts as the conductor. If there is no ground present the controller thinks that there is no flame and shuts down the burner. Since a generator is not connected to a ground (usually) it can not sense the ground. I can't tell you how many homeowner installed furnaces I have to fix because of no ground circut. I also have to fix many in the late part of fall after the handyman decided to rewire his basement over the summer. The local pool supply company also has me on their speed dial to fix pool heaters for the same issue. Hope this helps, Dan
