If your gas furnace has suddenly started blowing cold air instead of heat, you'll need to troubleshoot your furnace so you will know whether or not you will need to call an HVAC contractor. Here are the basic DIY steps in troubleshooting a gas furnace that has suddenly started blowing cold air. 

Are the thermostat settings correct? 

The first thing to troubleshoot is your thermostat to see if the settings are correct. Perhaps another household member flipped the thermostat from heating to cooling and this is why there is cold air blowing out of the vents. If the settings are incorrect, you'll obviously want to change them to the correct settings to see if that fixes the problem. 

Did you run out of gas? 

The next thing to check is to see if you ran out of gas for your furnace. If you have a tank for the gas, check the level of the tank to see if there is an adequate supply of gas for your furnace. If your gas is supplied by a gas line, be sure your gas wasn't shut off by the gas company for non-payment or for repairs on the gas line. Contact your gas supplier to find out if there is a problem with the gas supply. 

Is the pilot light still on? 

If the thermostat is correct and there is a supply of gas, check to see if the pilot light is still lit. The pilot light is what ignites the gas burners to produce the heat. If the pilot light isn't working to ignite the burners, the furnace would blow cold air. If the pilot light is out, try the reset button to relight it. If you prefer having a professional do this, call your HVAC technician. 

Are the filters dirty? 

The next thing to troubleshoot are the filters. If the filters are dirty, the furnace won't get an ample amount of air to heat, so the air that gets sent through the vents and into your home would be cool air. Check if the filters are dirty and replace them if they are. If you do find that the filters are dirty, get into the habit of replacing them on a regular, scheduled basis so you don't have this problem again in the future. 

If you've taken these troubleshooting steps and still have not found the cause of your gas furnace blowing cold air, contact an HVAC contractor to service your furnace. 

A company like American Services can provide more information.