How Can I Tell If Pests Are in My Heating System?

You usually detect a heating-system pest problem by its evidence, not by seeing the animal — mice are nocturnal and insects emerge from hidden voids. Check for these, and if you find several, treat it as time to call for an inspection.

• Scratching or scurrying in walls, ceilings, or near ductwork, especially at night

  Means: Likely rodents active in voids near warm equipment.

• Dark droppings pointed at both ends, about 1/8 to 1/4 inch long, near vents, in cabinets, or along walls

  Means: Active rodent presence, per UNH Extension sign guidance.

• Gnaw marks on wiring, duct insulation, or baseboards

  Means: Rodent gnawing near the system — have wiring inspected before continuing to use the equipment.

• Flickering lights, unexplained breaker trips, or intermittent no-heat

  Means: Possible chewed control or energized wiring — stop and call a licensed professional.

• A musty or ammonia odor, or a decomposition smell, near vents

  Means: Urine and dropping buildup, or a dead animal in a duct or wall cavity.

• Sluggish flies or lady beetles at windows on warm winter days

  Means: Overwintering insects emerging from wall voids — a heat_reveals pattern, not a heating-system fault.

• Nesting debris — shredded insulation, twigs — at vent or flue openings or in the blower cabinet

  Means: Nesting inside ducts or equipment.

• Reduced airflow from registers, or a combustion appliance that won't draft

  Means: Possible duct or flue blockage — treat a flue blockage as a carbon monoxide concern.