My thermostat will not turn my furnace on when I put the temperature gauge under the current house temp. But the furnace will turn on if I go downstairs and tap the pipe leading to the furnace. Any suggestions on what I can do to fix this. It causes the house to get very cold when I'm not home because it won't start up when the temp drops below the level set on the thermostat until the pipe is tapped. Thanks in advance for whatever help you can give.