i was in a rental at one time.
Moved in during the summer, then fall came, and BOOM!!!!
Same as you describe, but with a natural gas forced air furnace.
Flames were shooting out of the front of the unit and caused me great concern.
Turns out there were three baffles in the combustion chamber mounted off the main gas line, that the gas was pushed out of into the chamber.
They were mounted perpindicular to the front of the furnace.
There was a pilot light on this model, and it was positioned in front, directly over the middle baffle.
When/if the unit functioned perfectly, the gas coming out of the center baffle would ignite immediately and spread to the other two baffles on either side of it.
It was not functioning perfectly to say the least.
After viewing the explosion process, we noticed that the flame did not start at the front of the center baffle closest to the pilot, but above it from the gas emitted from the other two baffles.
basically it would enter the chamber, swirl around a bit, filling it, until it hit the pilot light, then BOOM!!!!!!
Took the baffles out and some previous renter had plugged the middle baffle orifice with a small wad of paper, preventing any gas from entering the baffle.
No idea why.
After clearing the blockage, it worked fine.