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If your gas- or oil-burning furnace cycles on and off too frequently or there are major swings in room temperature before the furnace goes on, the thermostat may just need a simple adjustment. To adjust the thermostat:
1) Remove the thermostat’s cover. For a mechanical thermostat with a mercury switch (a small vial filled with mercury), first use a small level to make sure the thermostat is mounted level on the wall. If it isn’t level, it won’t measure temperatures properly.2) On many thermostats, you’ll see a small lever that moves along a calibrated scale (not the heat temperature lever) and may be marked “longer.” This is the heat-anticipator adjustment.
3) Adjust the heat-anticipator lever one calibration mark closer to the “longer” setting if the furnace goes off and on too frequently. If the furnace allows room temperature to drop too low or rise too high before the furnace goes on or off, move the lever one mark away.
4) Wait several hours for the thermostat to stabilize at this new setting.
5) Repeat the adjustment if necessary.
6) If making these adjustments doesn’t solve the problem, consider replacing your thermostat.
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