A Home with Only One Light Switch?!
By Hal Alles
Imagine a home with only one centrally located light switch so every light is either on or off. Anyone needing light in any room needs to turn on every light; anyone wanting the light off in any room needs to turn off every light. My guess is the lights would usually be on and family members would use blindfolds when they wanted personal darkness. I think most people would agree this is not sensible.
However, this is exactly how a thermostat controls a typical forced air heating and cooling system. One central thermostat turns on the heating or cooling when the temperature at the thermostat reaches the set point; then the furnace or air conditioner runs until the temperature at the thermostat changes a degree or two. The control is all on or all off in every room. When on, the airflow to each room is fixed, no matter what the temperature is or what temperature is wanted.
When conditioned air is delivered to a room, an equal amount of air must leave the room. The temperature of the air that leaves a room represents the temperature of that room. All of the air that leaves the rooms mixes on its way to the return, reaching an "average" temperature. A thermostat can only control the temperature where it measures the temperature. When the thermostat is located near the return, the thermostat controls the average temperature.
In a room, if the supply of conditioned air is different than the need for conditioned air, the temperature moves away from the average. Each room has a limited effect on the average temperature, so it has a limited effect on how much conditioned air it receives. In the summer, as rooms on the sunny side warm up and cause more cooling, rooms on the shady side must cool down to keep the average the same. Hot rooms don't get enough, cold rooms get too much.
Control using a single thermostat is fatally flawed. It is based on the false assumption that the need for conditioning varies the same way for all rooms. But that is obviously not true. For example, 1 square foot of west-facing window needs about 5 CFM of continuous cooling (or 2 CFM less heating) to compensate for solar radiation. So, a 4 x 5 window requires about 100 CFM (6" duct) more continuous cooling when the sun shines than when it does not. That becomes 200 CFM (8" duct) if the A/C needs to run only 50% of the time to maintain the average temperature. Everything makes heat. Each person needs about 12 CFM, and each 100 watts of light or equipment needs about 17 CFM.
Host a Sunday dinner for your family in a sunny dining room, and you may be serving a hot dessert &mdash even if it's ice cream. You will probably be sweating after a few hours working in your home office. The kids will be grumpy after playing in the bonus room over the garage. To be safe, keep your baby's crib near the thermostat.
There are many compelling reasons to have temperature control in every room. There is only one reason to have a single thermostat for an entire house &mdash it is cheaper to build.
July 2007 Builder Architect Edition Issue