Performed routine inspection on 3-year-old natural gas furnace that was experiencing minor noise and intermittent second stage heating issues. Diagnosed code 311 caused by inadequate aftermarket MERV 16 filter; replaced with customer's alternate filter which resolved the heating stage problem and recommended proper healthy climate MERV 16 filter for future use. Added accelerator to exhaust pipe, noted heat pump in good condition, and found HRV motors non-operational with neglected core and filters; disconnected HRV unit and will provide quote for replacement motors or complete HRV system.