A furnace including a combustion chamber for burning fuel can have increased fuel burning efficiency, increased heating efficiency, and decreased harmful emissions of combustion byproducts. A combustion air delivery system delivers primary and secondary combustion air to the combustion chamber. Primary and secondary combustion air may be delivered at amounts that increase burning efficiency. An amount of secondary combustion air can be controlled by a valve system. A heat transfer device efficiently transfers heat from products of combustion for heating an enclosed space.

An air-conditioning apparatus for air conditioning indoor environments has a containment structure having a casing delimiting a first compartment and a second compartment. A pellet stove configured to operate in heating mode and provided with a first duct for movement of combustion air in input to the pellet stove is housed inside the first compartment. A monoblock portable heat pump configured to operate in heating and/or cooling mode and provided with a second duct for movement of heated air in output from the monoblock portable heat pump is housed inside the second compartment.