According to furnace manufacturers, an annual inspection from a professional technician is recommended, during which the technician performs or lets homeowners know about necessary maintenance. Routine maintenance might involve checking for blocks and leaks, cleaning the unit and repairing damaged parts.
Outside of routine maintenance, or if homeowners skip annual maintenance, certain signs point to a need for furnace repair or replacement needs. If a heating unit runs constantly without impact to warmth in the house, or if a furnace doesn't power on or stay on, then the furnace unit is in need of maintenance. Maintenance may also be required if the furnace makes odd or loud noises or fails to pump a good flow of air. Water pooling under or inside an HVAC unit is a sign that professional assistance may be required, and any homeowner that notices carbon monoxide indicators blinking or sounding should call a professional immediately.
Regular annual maintenance may reduce the chance of some issues with a furnace. Since heaters are likely to run more in cold months, most homeowners schedule annual inspections in the early to mid fall so they have time to take care of any maintenance requirements before relying on the furnace full-time.