Integrated & Retrofitted A Coils
One of the greatest advantages of a forced hot air system is that your existing ductwork can double as a central air conditioning distribution network. By adding an evaporator coil (commonly called an "A coil" due to its triangular shape) inside or on top of the furnace plenum, along with a matched outdoor condensing unit, you gain full central air conditioning without separate ductwork.
Integrated A Coil (New Installation)
When installing a new furnace, the most cost-effective approach is to plan for central air from the outset. An integrated coil cabinet is selected to match both the furnace's airflow capacity (measured in CFM) and the A/C system's tonnage. The coil sits directly on the furnace supply plenum. Refrigerant lines run from the coil to the outdoor condensing unit. A drain pan and condensate line are included to handle the moisture the coil removes from the air during cooling season.
Retrofitting an A Coil to an Existing Furnace
An existing furnace can often be retrofitted with an A coil, provided:
- The blower motor is capable of moving enough air at the higher static pressure introduced by the coil (typically 0.5–0.8 inches water column additional resistance).
- There is vertical space above or below the supply plenum to install the coil cabinet — typically 18–24 inches of clearance is needed.
- The existing ductwork can handle the airflow rates required for cooling (often slightly higher than heating-only systems).
- The furnace heat exchanger is in good condition — refrigerant leaks onto a hot heat exchanger create serious hazards.
Retrofit coils come in two configurations: upflow (air moves upward through the coil, used on upflow furnaces) and downflow/horizontal (for counterflow or horizontal furnaces). Your installer will select the correct orientation. A new refrigerant line set is run between the coil and the outdoor unit, and the system is charged with refrigerant by a licensed HVAC technician.
Dual-Fuel Systems
Some Maine homeowners opt for a dual-fuel system pairing an oil or propane furnace with a heat pump as the outdoor unit. The heat pump handles heating efficiently down to about 35–40°F, and the furnace takes over below that threshold — a smart strategy given Maine's climate, where temperatures often drop well below the heat pump's efficient operating range.