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From the Q&A

With TJ Byers

This Old House Thermostat


My house has an open stairwell between the main floor and the basement, where I have an office and my electronics workshop. When the weather is neither too hot nor too cold, the circulating fan on the heat pump stays off for long periods. The cold air gravitates down and the hot air goes up, creating an uncomfortable condition in both places. Eventually, the thermostat detects this and the fan starts. To correct the problem, I built 555 timer to cycle the fan. This helped a lot, but ran the fan at the wrong time in the summer.

I then built a differential temperature device using two thermistors, an op-amp, and a relay. Unfortunately, my design was not sensitive enough — the temperature spread was too great. I kept changing feedback resistors, but was unable to get it right. I would like to build a controller that I can comfortably adjust to a 1 to 4°F differential.

C. P. Furney, Jr.
Loudon, TN


Maybe your problem is that thermistors are non-linear. That is, the resistance doesn’t stay in step with the temperature. Let’s replace the thermistors with the venerable LM34 temperature sensor (shop around for best price; it can vary widely). The LM34 is a precision Fahrenheit temperature sensor with a guaranteed 10 mV/°F linear output (80 mV at 80°F, 100 mV at 100°F).

Now, if you place two LM34 sensors on the differential inputs of an op-amp (any garden variety will work), you automatically get a voltage that reflects the difference in temperature between the upstairs and downstairs.

This voltage is first multiplied by 10 and then fed to a comparator with an adjustable temperature differential (SET) up to 10°F. This should give you enough range between upstairs and downstairs so that the rooms don’t become uncomfortable.

The 1M resistor provides a slight amount of hysteresis to prevent the fan from hunting — constantly going on and off. If you find the fan hunting, lower the value of the 1M resistor. To clarify: The SET determines the temperature difference between the basement and upper house. The 1M (feedback) resistor determines the temperature difference between fan on and fan off — a dead band that prevents hunting.