Combustion Analysis Quick Reference

Combustion Analysis Quick Reference
Natural Draft 70-80%Fan Assisted 80%High Efficiency 90%+
Draft Test
Location of spillage test sampleAt the edge of the draft diverterN/AN/AIf too lowIf too high
Location of draft test sampleIn the flue AFTER the draft diverter.N/AN/A
Draft pressure-.02 to -.04″ WC-.02 to -.04″ WC.+.02 to .08″ WCCheck for draft blockage and inadequate combustion air. Check for depressurization of Combustion Air Zone. If 90% appliance check for disconnected vent pipe.If 90% appliance, check for blocked vent pipe.
Combustion Efficiency and Safety
Location of combustion test sampleAt the outlet of each heat exchanger tube BEFORE the draft diverter.In the flue, at least 2X the flue pipe diameter away from the unit.In the flue, at least 2X the flue pipe diameter away from the unit.If too lowIf too high
O25-10%7-9%4-9%Incomplete combustion. Check for draft blockage and inadequate combustion air. Possible overfired- check manifold pressure and orifice size per OEM.Inefficient combustion. Possibly underfired- check manifold pressure and orifice size and adjust per OEM. Investigate heat exchanger for cracks. A change in O2 when blower runs indicates heat exchanger crack leak.
CO<100 PPM<100 PPM<100 PPMVerify sample location is correct and reading flue gases.Incomplete combustion, poses health risk. If over 400 PPM, red tag appliance. Check draft and inadequate combustion air. Possibly overfired- check manifold pressure and orifice size per manufacturer. Check for burner misalignment and impingement. Investigate heat exchanger for cracks, warps, blockage.
CO26.5-8.0%6.5-8.0%6.5-8.5%Inefficient combustion. Possibly overfired- check manifold pressure and orifice size and adjust per OEM.Incomplete combustion. Possibly underfired- check for draft blockage and inadequate combustion air.
Efficiency75-84%78-82%89-98.5%
Stack Temp.325-500 deg F.325-450 deg F.80-120 deg. FExcessive airflow. Cold return air temperature. Too much draft. Condensation may form and corrode metal flue pipe and heat exchanger.Inadequate air flow. Possibly overfired- check manifold pressure and orifice size per OEM. Check for draft blockage and inadequate combustion air. Heat exchanger may overheat, warp or burn through.

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