Why are registers closed on shut down burners?

• A. It prevents false readings on O2 analyzer
• B. It reduces fuel consumption
• C. It speeds up shutdown
• D. It protects flame sensors

Answer: (A)

During shutdown, keeping the air registers closed is all about keeping the exhaust gas composition measured by the O2 analyzer accurate. If the registers stay open, outside air can mix with the stack gas as it winds down. That dilutes or alters the oxygen content seen by the analyzer, making it look like the furnace is still burning with a different air-fuel balance than it actually is. This can trigger unnecessary actions or confuse the control system when the burner is supposed to be off. By closing the registers, you prevent that air ingress, so the O2 reading reflects the true exhaust from the shutdown process and won’t cause misleading readings. The other options aren’t addressing the measurement accuracy, so they’re less relevant in this context.