I reside in area 2A (Florida) with a liquid petroleum gas furnace which is 15 years old. The unit has always heated the 2,800 sf residence with no issues. Routine preventative maintenance has been performed over the years with the filters being replaced on a monthly basis. At the beginning of this heating season the furnace would cut-off prior to reaching the thermostat-requested temperature. My HVAC contractor has replaced a roll-out switch, control board, auxilary limiter switch, and the thermostat. The thermostat was replaced when the contractor detected a drop in voltage during the heating cycle. It was next recommended that I change to MERV 8 filters rather than MERV 11 and 12 filters.

The premature recycling condition continues in spite of all these changes.

The current recommendation is to increase the size of the return air flow. Presently the system has 2 14″ x 14″ returns downstairs and a single 12″ x 12″ return in a small upstair room. All ducts (supply and return) have been visually inspected with no blockage or kinks noted.

Since the system has worked for 15 years with no apparent air return issue, I question the need to increase the air return capacity. Any thoughts on the appropriate size of the air returns or other factors that could be causing this perplexing issue?