Check the Chimneys Before the Blower Door Test
I always tell clients not to have a fire going in their woodstoves or fireplaces prior to my blower door testing. I also request that they remove all ashes from these areas to prevent ash dust from being pulled into their home.
I have now learned to do one more safety check prior to running the blower door test. A client had followed my recommendations precisely and told me that he had not had a fire in the wood stove for over 24 hours. It seemed that I was good to go with the blower door testing.
After about a minute of running the blower door, smoke began to back draft out of the junction of the woodstove flue pipe and the chimney. I shut down the blower door fan and then opened the clean out of the chimney used for the stove to see what was going on. With a mirror I could see a large glowing mass that was below the flue pipe entrance but above the clean out door.
The client told me that he hadn't cleaned the chimney all the way to the bottom for quite some time. Apparently, there was a plug of creosote that was about 3 feet deep that extended from below the flue pipe to just above the clean out door. He could not see it when he was cleaning his chimney. Inside this plug was a glowing ember which ignited the creosote when all of the air was rushing down the chimney. His house was pretty tight so the slight gap around the flue pipe became the entry point for the blower door air. This resulted in a chimney fire. Once it became just a small fire we cleaned it out. Fortunately, his chimney was well lined and no damage occurred to the home. Even luckier, the wood stove was in his basement so we could isolate the smoky area and use a fan to ventilate the basement prior to running the door again.
Some lessons are very exciting to learn!