What is the weather in that area like?
Is there are furnace in the basement as well?
Natural Gas? Heating oil? Boiler? Propane tank not properly stored maybe?
The limited info I see makes me think that either chemicals too close the heat, or a gas explosion. It would take more than a few jugs of powder, and cleaning solvent to cause that big of an explosion.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.