One of the most common issues in cold weather is frosted over fresh air intakes. You furnace requires air to combust the fuel in much the same way your car pulls air into the engine through the air filter. When the air intake is blocked or partially blocked the furnace will operate poorly. Not only will the furnace run poorly and produce less heat but it will also be far less efficient as a lot more fuel will end up going out the exhaust vent. If the blockage is severe the furnace may even shut down. It only takes a minute to check and clean the air intakes so have a look at yours today.