It got down to about 14F last night and my heat stayed on all night. We had the thermostat set at 68. The thermostat would read 68 but the heat stayed on like it couldn't get the temp above 68 to cut it off. We live in a two story house with only one unit. If I dropped the thermostat to 65 the heat would shut off. Is this normal or should I have someone come check it out?