What is the proper type of furnace when ducts are in a concrete slab?

When ducts are embedded within a concrete slab, the appropriate type of furnace to use is a downflow system. This is due to the design of downflow furnaces, which direct heated air downwards into the ductwork. Because the ducts are located within the slab, they benefit from the natural heat rising from the furnace, allowing for effective heating of the space above.

A downflow furnace is specifically engineered to work efficiently in scenarios where the ducting is below the furnace. This setup promotes optimal airflow and heat distribution, ensuring that the heated air effectively rises through the living spaces. By directing the warm air downward, it minimizes heat loss and maximizes efficiency, which is critical in maintaining comfortable temperatures in environments with concrete slab ductwork.

In contrast, upflow systems, including highboy and lowboy configurations, are designed to push air upwards through the ducts, which is not suitable when the ducts are located beneath the furnace in a concrete slab scenario. Gravity-style systems typically rely on the natural rise of heated air, which may not provide sufficient airflow dynamics needed for spaces with slab-level ducting.