My HVAC/gas heater made an explosion of some sort yesterday after it was running for a couple hours. When I opened up the panel there wasn't a whole lot of damage, mostly some burnt wires that needed to be replaced.

Id like to know what may have caused this fireball before I invest in any parts if anyone can tell me. I've already replaced the wires and tried to start the unit up again but it won't start unless I manually press down the contactors, Im also not sure if I need to replace the contactor or not since it looks kind of charred. I figured Id ask just in case there's a way to reset it or something. It seems like it may be some sort of safety feature that won't let me start it.

Also the unit is about 20 years old and there was a good bit of debris in the furnace area. Im not sure if that was the cause but I figure it might help in the diagnosis.

Id really like to fix this myself, I'm confident that I can do it as long as I have an idea of what parts I need to test or replace.

I don't know about the contactor, I'm not a furnace expert, but I do know that mine would not carry the flame to all the rails in the burner. The ones that didn't light dumped raw gas into the chamber until it reached the ones that did light. It made quite the bang when the gas lit. it was an older unit, and the burners had a lot of corrosion on them. I had to replace the furnace.