ambient CO above zero but when you see it take the time to INVESTIGATE. Flame Displacement One of the oldest ways to check for heat exchanger cracks is to simply observe the flame when the blower starts. Most furnaces will have a blower delay to get the furnace up to temperature before moving air over the exchanger. This procedure is as simple as watching the flame and observing if it the flame moves or changes when the blower starts. Because the burner and heat exchanger is isolated from the airstream there should be no change in the flame when the blower starts. If there is you need to begin looking for connection or leakage between the burner/heat exchanger and the air. won’t necessarily tell you if you have an exchanger leak but it will tell you if you have a high CO which can help you prevent a dangerous situation for your customers if there is an exchanger crack. Exchanger Isolation Pressure Test When you suspect the system may have an exchanger leak you can place a manometer probe in the exchanger and seal off the inlet/outlet of the exchanger as best you can (with the gas shut off obviously). Next, turn on the blower and see if there is a change in the Combustion Analysis Testing combustion
exchanger. Any change is an indication of a heat exchanger leak. Visual inspection In some cases, an old- fashioned visual inspection makes the most sense, either by removing the blower or the high limit (or both) and using a mirror or borescope to inspect. When you do find a cracked exchanger it should be quoted for repair and the furnace turned off. While finding cracked exchangers is worthwhile I would place it below or on par with other things like – • Testing for spillage due to depressurization in the combustion air zone • Insufficient Combustion Air • Blocked intake and/or exhaust • Recirculation of combustion products into intake vents or soffits • Gas pressure testing / clocking the meter for overfiring or underfiring • Testing heating airflow • Testing for CO in the occupied space • Visual inspection of vents, flues, and chimneys for proper installation as well as gaps and cracks • Visual inspection for return gaps pulling in air around the furnace When you do find exchanger cracks on newer units you need to also look for causes like low airflow, incorrect orifice size or over firing that could have caused the issue.