fan cycles on/off every 20 seconds

[tjslomba13]

alright... i have a gas forced air furnace, the burners will ignite, heat up, fan kicks on... 20 seconds later, fan kicks off.... 20 seconds later, fan kicks on... so on and so forth. all the while the burners stay ignited. i have changed the fan limit switch, forgot to pull pin to drop to low voltage, blew the spark box, changed that, blew the gas valve, changed that.... finally got all new parts put in, turn it on, and it does the same thing... only other thing i notice is that two burners flame are inconsistent, and sometimes roll out under neath... could this cause this ** i don't want to spend anymore money!!! please any help is greatly appreciated.... btw it will bel in the teens for the next week... please help

[Freon]

To help get posts readable, do not use dashes, slashes or question marks... they get replaced by an *. If, after you submit your post, you see an *, edit your post please so it's readable.

Make and model number of the furnace please. Can you describe the fan limit switch you replaced... make and model number.

In older furnaces the fan limit switch was a bimetal device that would rotate a temperature calibrated dial where you could set the fan on and fan off temperatures. Sometimes the airflow from the house would cool the heat exchanger enough to make the fan turn off waiting for the heat exchanger to warm. The goal was to not circulate cold air.

I don't know if this is the type of fan control you have. I mention it as one possible explanation. Many newer furnaces simple have a delay before starting the fan and then it's on constantly during the heating cycle.