Why does the fan come on and then shut off in less than a minute?

Hi Friend:No doubt you have left out something important that I would find useful. ( like electric heat or gas? ) lol,,,, Not to worry though. It sounds that in all likely hood weather heat or gas you have a defective heat delay seaquencer or you have a gas burner that is defective and letting the blower air come on and blow backward through the heat exchanger there fore cooling off your heat sequencer and causing it to shut down the fan.Please take my advice!!!!!! Seriously get someone on it that knows a thing or two to safely remedy this for you. The thermostat fan switch turned on to on while you hold a long barbeque cigarete lighter up inside the burner box housing while heat is off will quickly tell you if the heat exchanger is collapsing and blowing your flame out. It blow the flame on the lighter vigorasly and perhaps even blow it out. Try each burner port. ( just a little trick from times past ) Hope this helps; Jimiwane