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I have a miller furnace model M1 w/ horizotal heat exchanger. The draft is 0. Complaint of smelly blue smoke coming out of chimney. I checked pump psi= 100. I increased it to 140 and smoke cleared up. I still had 0 draft. I tore down burner ,fan, and plennums to expose heat exchanger. It looked fine. I snorkeled out exhaust tubes rapping them as i vacuumed. No soot build up. Put everthing back together, Still 0 draft! I went up on roof, exhaust vent is good condition and intake air vent is in good shape. My pump psi is 140-- pump cut-off is good --cad cell resistance around 600-800,-- ignitor good. It fires up and runs good but can't get draft. Should I be concerned