How a Thermostat Is Like a Teeter-Totter

By Hal Alles

I went to a three-room country grade school with playground equipment that today would be mistaken for weapons of mass destruction. Our parents never complained because it was still safer than the farm equipment we operated when we weren't in school. One of our favorites was a big teeter-totter made from a 14-foot piece of 2x12 pivoted about 3 feet off the ground. It was great fun to bounce a playmate off their seat for the 6-foot fall to the ground. Sometimes a third playmate would join in by standing at the pivot point, shifting their weight to make the launch off the seat faster and higher. Not much happened in the middle, but the ends bounced up and down like crazy.

This is like what happens with the temperature control in most homes. The one central thermostat controls the temperature at the thermostat. With luck, it is located near the central air return where the temperature is the average of the whole house. The thermostat is at the pivot point of a temperature teeter-totter. When running, the HVAC equipment produces a fixed amount of conditioned air that is distributed in fixed proportions to each of the rooms by the duct system. If the HVAC contractor did a good job of design and installation, this airflow to each room will be about right for the average loads in each room. However, the actual loads are seldom equal to the average since the loads vary dramatically due to sunshine, weather, people, lights, electrical equipment and appliances, etc. These can change the need for conditioning by more than a factor of 3. The mismatch between loads and the amount of conditioned air delivered causes the room-to-room temperatures to vary about 10 degrees in single-story homes and 15 to 20 degrees in multiple-story homes.

This is most apparent during sunny days when air conditioning is needed. The heat load on the sunny side of the house is typically three times the load on the shady side. The rooms on the sunny side warm up, so the return air warms up. But this air is mixed with the return air from the shady side. The thermostat senses some increase in temperature and runs the A/C longer and more often. However, cool air goes to both the sunny side and the shady side, so both the sunny and shady side rooms are cooled until the average return temperature at the thermostat is the set temperature. But the sunny side will never reach this set temperature because the return air from the needlessly cooled shady side lowers the average. The rooms are at the end of a temperature teeter-totter. Hot rooms are balanced by cold rooms. The central thermostat is like the playmate at the pivot point - it is a smooth ride while the rooms are out of control. Too bad we don't spend more time living by the thermostat. Fixed airflow and a central thermostat is a weapon of mass destruction to the comfort in the rooms. A good countermeasure is a well-designed and installed zoning system.

April 2007 Builder Architect Edition Issue

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