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If you have an older wall thermostat (non-digital), it is important to keep it clean inside. Over time, a thin layer of dust can accumulate on the metal bimetallic temperature-sensing strip inside. This dust insulates the bimetallic strip from the temperature changes in your house. This causes uncomfortable and wasteful large temperature swings, as much as four degrees in some situations.
It is easy to clean out the thermostat. First snap off the cover and you will see the temperature-sensing strip inside. It often has a mercury filled bulb attached to it. Brush it off VERY GENTLY with a small soft brush. You can use your vacuum cleaner, but hold the nozzle at least several inches from the thermostat. You will probably see a little slide lever inside with some numbers on it. Do not adjust it. It is a heat anticipator and only your furnace contractor should adjust it.