I have a gas furnace that is 4 years old. For about 3 weeks now it has been blowing out cold air. Occasionally it will blow out the heat its suppose to, but not for hours. I looked inside the furnace and see the flame is still lit and don't understand why it won't kick heat on. I have thermostat set at

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70 degrees in house woke up this morning it was 57 degrees in the house. Help.. we are freezing.