Furnaces will be putting in some overtime, as the first single-digit temperatures of the winter hit Missouri. Sandy Nock-Sheena with Artic Air Heating and Cooling in Gladstone says the most common reason for a heater not keeping up with the cold outside is regular maintenance to keep it efficient. She says her repair crews are busy anytime it’s cold, but especially when the cold snaps last longer. Nock-Sheena says a longer stretch of normal cold temperatures are harder on heating systems than just a few days of extreme cold.