Yesterday had a guy call me up said he has a 10 year old Goodman 90% furnace that is not working. His stat has no display or nothing told him if it is power stealing then he is losing 24V somewhere. Get there today and turned on switch furnace fired off no problem. THEN the blower kicks in and one of the cell starting blowing the flame backwards causing major flame roll out in that area even rolling out of the burner box and no roll out switch tripped. Bottom line was Heat exchanger had huge hole around one of the rings. I red tagged furnaced wrote it up as danger waiting to happen had customer sign gave him his copy and before I left he told me you know once you walk out the door I am going to turn it back on. Told him do what he wants but if something should happen you just signed where I red tagged and I am no longer responsible. Why would someone put not only his but his family lives in danger like that? Makes no sense. And no he has no CO detectors in house told him it is now a law here to have them like smoke detectors. I posted this here so ALL can see especially visitors to this site. If furnace is red tagged their is a reason.