Differences: Furnace vs. Boiler.

Gas Furnace:

A gas furnace, or forced-air heating system, reacts when the air temperature drops below your programmed setting on the thermostat. The igniter lights a burner inside the combustion chamber. The heat created is then pushed into the heat exchanger, where the air is then heated. The newly heated air moves through the ductwork and into the rooms of the house. The combustion gases used to create the heat are vented out through a flue in the roof or wall.

Boiler:

Unlike a furnace, a boiler doesn’t use forced air to provide heat, and doesn’t require ducts. Instead, it uses heated water in a tank which a pump then sends through a network of pipes in your home to the various radiators or baseboard heaters. These end points (radiators or baseboard heaters), move the heat into your rooms by using radiant heat, which is when you raise the temperature of an object so it warms the area around it.