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Gas furnaces can have many different types of controls. Often on units this age there is a fan limit switch. It will turn the fan on when temperature is reached but will also shut the burner off if the temperature gets too high in the furnace. Others have a timer that brings the fan on after a set period of time.
The sail switch that Bsiple is talking about may be on a forced draft furnace and is used to prove there is indeed forced draft before it will allow the burner to ignite.
One thing to look for is a change in the flame when the blower comes on. This could indicate a bad heat exchanger which is reason to condemn the furnace and get a new one.
You can check this by starting the furnace and watching the flame as the fan starts. If the flame changes at all there is a problem.
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Kevin
'79 Coupe
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