I have gas heat and a very old thermostat. My thermostat has a round dial and I set the top for the desired temperature; the bottom supposedly represents the temperature of the house. When I make the top number higher than the lower number, the heat comes up perfectly and stops coming up when the lower number matches the top number. The problem is when I turn the top number down. No matter how cold it gets the lower number will be at the same position as the top number and the heat will not come on. I have to manually turn the top number higher in order for the heat to come up.

Any idea what the problem could be?

Get yourself a digital thermostat their more accurate