You can tell when a furnace is bad or failing by watching for several key warning signs: the unit is over 15 to 20 years old, it requires frequent and increasingly expensive repairs, your monthly heating bills are spiking due to lost mechanical efficiency, it blows lukewarm air or cycles on and off rapidly (short-cycling), it emits loud banging, screeching, or rattling noises, or it produces a yellow burner flame instead of a crisp blue flame, which can indicate dangerous carbon monoxide leakage.
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