As noted in an earlier Halacha Yomis (with regard to electric stove tops) many heaters and air conditioners are controlled by digital systems. In a digital system, even if all one is doing is turning what appears to be a simple manual knob, in reality one may be programming a computer. Therefore, even if the heat is currently on, one may not raise the thermostat. However, if one has an old heating or air conditioning unit, and one is certain that the thermostat is fully mechanical, one may raise the setting on the heater while the fire (or burner) is on to make it warmer, and one may lower the temperature on such an air conditioner while the compressor is cooling to make it colder.
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