There’s an old HVAC joke where an engineer is troubleshooting a heating system over the phone with a homeowner and asks the question, “Are you comfortable right now?”, to which the homeowner replies, “I don’t know, let me check my thermostat.” We perceive that the little white box on the wall has magical powers to manage your heating bill, and ensure your comfort. In reality, a thermostat measures the air temperature of the room it’s located in, and closes a switch that turns your HVAC system on and off.
So what air temperature is comfortable to you? Before you answer that, I have to admit, it’s a trick question. Most of the time, we design an HVAC system to provide an indoor air temperature of 70 degrees on the coldest day of the year. 70 degrees is what most people would agree is the ideal indoor air temperature. But 70 degrees can also be very uncomfortable, say if you’re standing in the freezer isle at the grocery store. It’s true, the air temperature in the freezer isle is actually 70 degrees, don’t believe me, take a thermometer with you next time. But it’s so cold in the freezer isle, how can it be 70 degrees? It is because the freezers are excellerating the heat loss from your body by stealing heat. The same thing happens when you sit next to a window on a cold day. The window is colder than your body and because heat moves toward colder objects, that window starts stealing your heat.
True comfort happens when you combine indoor air temperature (what your thermostat says) and mean radiant temperature (what your skin feels). We call this Operative Temperature. Other factors that play into the Operative Temperature are air motion (draft) and humidity. Your Operative Temperature also varies based on what you’re doing, and what you’re wearing. For example, if you go on a jog your home may feel hot when it may feel cold cold if you’re sitting on the couch watching a movie.
We can’t control Operative Temperature because we can’t measure how objects in a room like windows or cold floors effect the human body. We have that thermostat. But, we can try and keep the radiant losses to a minimum. One method is to keep objects in a room from stealing your body’s heat. How do we do this? By making sure they don’t get cold, simple as that.
