There is certainly more to the story which we don't yet know. I have been involved in blowing up houses on purpose with both natural gas and propane with full instrumentation packages and using stochiometric mixtures. It is very HARD to get enough natural gas into a house to create an explosion of this level with the reduced pressure side the residential appliances operate on. Having investigated a large number of building explosions, I know it will take some time to sort it out. The furnace...speculation at this point.