The same percentage applies to warm air lost in that same HVAC duct when the gas heating equipment is running at full blast.
I am honestly an unashamed fan of the Three Stooges. The plots of their short films often have them working at jobs where they easily mess things up resulting in their “brand” of comedic craziness! Aside from slapstick humor, the movies offer a look into what life was similar to in the 1940s… One short movie titled “An Ache in Every Stake” features “The Boys” delivering blocks of ice as was a normal thing during that time. In a single scene, “Curly” carries a large block of ice up a terribly long staircase only to learn it has melted to approximately the size of a small ice cube as he gets to the top. The funny image of that ice shrinking reminds me of how inefficient the regular Heating, Ventilation and A/C air duct is in the average house. Any AC’s efficiency is measured using the Seasonal Energy Efficiency Ratio (SEER), a ratio of the cooling output to the energy consumed. Unluckily, that cool output is further decreased as the air goes through the HVAC duct plus into your preferred areas to relax. According to different estimates, as much as 40% of the cool air produced is lost inside the ductwork system. The same percentage applies to warm air lost in that same HVAC duct when the gas heating equipment is running at full blast. There is honestly no way to eliminate heating or cooling losses in your air duct but there are ways to minimize it. Having great insulation is an obvious necessity to keep the conditioned air that you pay for inside the air duct. Another easy step to take if you notice your energy bills rising is to have a Heating, Ventilation and A/C equipment professional have a look for any leaks plus gaps in the air duct. Of course, keeping the air duct perfectly clean plus free of contaminants plus dirt will also help get more usable warm or cool air to its final endpoint.