This is the second post in a 2-part series on the physics of moving air through ducts. If you missed it, click here for The Best Velocity for Moving air Through Ducts, Part 1.
The first thing to know about the velocity of air moving through ducts is that the slower you get the air moving, the better it is for air flow. That was the main point of my last article. It asked the question, is low velocity bad for air flow in ducts? And the answer was that, in terms of air flow, you really can’t get the air moving through the ducts too slowly.
But that’s not the end of the story. If it were, you’d always try to get the lowest velocity possible by using the biggest ducts that fit the space without blowing the budget. There’s another relevant fact, however, and ignoring it can lead to trouble.
In moving air through a duct system, we want good airflow, but remember that the object isn’t just to move air throughout the house. It’s to move heated air in winter and cooled air in summer. When that conditioned air is moving through the ducts, the second law of thermodynamics comes into play because we have a temperature difference between the inside and outside of the ducts.
The second law of thermodynamics says that when you have objects at different temperatures, heat flows from the warmer to the cooler object. In winter, the warm air in our ducts can lose heat to the surroundings. In summer, the cool air gains heat from the surroundings.
And the amount of heat that flows between the duct and its surroundings depends on three things:
The equation that ties these things together is:
Q here is the…