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I can't get the draft differential between above the fire and the flue to be less than .400. I've disassembled the boiler and cleaned it. I've also checked smoke and CO2 which are both fine. What else can I do to get the differential down? I believe the differential should be much tighter and it's currently getting too much draft, increasing flue temperature to around 600 degrees F.
Current readings: -.400 (flue), -.001 (over fire)
Boiler: Weil Mclain WGO-2
Flue diameter: 8"