In today’s HVAC systems, both the heating and the cooling functions are almost always delivered through the same duct and diffuser systems. These systems are normally designed with the cooling function in mind. That said, a diffuser system that works well in cooling often fails miserably in heating mode. The heat stays high in the room, and the floor is frequently cold. A large mass of hot air (diagram 1) stays high in the room, including the heat and ventilation air.

As a control system technician and engineer with more than 30 years of experience, I have been frequently frustrated by air distribution systems that lacked a solution that allowed them to work well in both heating and cooling. If the diffusers were directed downward, they would work in heating, but when that same system went into cooling, the occupants were terribly uncomfortable because of drags. Upon reviewing literature on heating from the ceiling, I discovered some references that suggested changing the direction of the diffusers twice yearly. That seems unworkable.