PSA: Stop using blower door readings to diagnose dryer vent clogs...

I had a customer call me out to her house in Tacoma last Tuesday because her dryer was taking three cycles to dry a single load... she handed me a blower door test report saying the vent was fine, but I pulled 14 feet of lint out of it anyway. The pressure drop under negative house pressure is totally different from what the dryer sees under positive flow - has anyone else had to argue with a home inspector about this?

2 comments

2 Comments

victorcoleman1mo ago

The real problem is how these blower door tests get misinterpreted. Under negative pressure the vent system might show fine because air is being pulled through slowly, not pushed with hot humid air at high velocity. Did you check if the inspector actually understood the difference between static pressure readings and dynamic flow conditions? That 14 feet of lint proves the test failed you, not the vent. These home inspectors love their gadgets but hate learning the real physics behind them. What did she say when you showed her the actual blockage?

victorcoleman1mo ago

My neighbor Bob actually tried using one of those cheap home inspection smoke pencils to test his dryer vent last year, and the smoke barely moved. He thought it was fine until I stuck my shop vac on it and pulled out what looked like a small furry animal made of lint. The inspector I deal with now actually admits blower door testing only works right if you understand the difference between pulling air through a clog and pushing hot moist air through it at real operating speed. She finally agreed the 14 feet of lint was the real story after she saw me pull it out with a leaf blower attachment I rigged up.