After my heater fires I hear a loud bang a see flames shooting out the blower. The heater keeps working but happens every time it starts up. why?

Check out the pressure switch . and the snap disk .

Hi Loretta:
I'd contact the installer. This sounds as if there may be a slow leak. When the gas concentration is high enough it will ignite which would be the bang that you hear. The popping sound could be the flame at the leak source point re-igniting for a few times.
DEFINITELY CONTACT YOUR INSTALLER OR GAS INSPECTOR. THIS IS A SAFETY HAZARD.

When unit come on you need to watch the flames that shooting in all should blue. If there is any red flame kick out that would trip the flame role out switch. If the is the case you need a need heat exchanger.

flame sensor, air flow restriction or blower not producing enough pressure. clean the flame sensor to start. inspect venting, etc

I suggest you call for service. This "bang" could be a serious problem in the future - meaning the next time it tries to start.
Gary

This is backwards. The blower comes on first then after the air pressure is good in the exaust it will turn on the power to the ignitor and open the gas valve. If it is doing what your describing there is something wrong with the way it is wired. But many times people will mistake the noise of the blower comming on for the flame and the sound of the gas valve and the ignitor for the blower kicking in. This is probally the case. If you have water in the fire box this is a much bigger problem with your heat exchanger and it will do damage to other components. If this is the case it is time to replace the heater. All of this is symptommatic of bad water chemestry and you will need to address this or you will damage the new heater and chemecal damage is never covered by warrenty. Good luck.

Clean or replace the thermo couple.

It must detect a flame a few seconds after the main gas valve opens or it shuts it down. Since you are hearing the burners fire up the ignitor is working and all the other safety circuits are working to allow the furnace to fire up.

The thermocouple is a small silver tube that protrudes into the flame of the main burners, remove it and scrub with scotchbrite pad or anything mildly abrasive. You just want to remove carbon and corrosion build up, re-install and see if it fires up.

I have two Goodman furnaces and if you are to maintain them your self you should keep on hand a thermocouple and a new ceramic ignitor. They will inevitably fail on the coldest weekend of the year and the only place to get them will be from a service company with a emergency service call ...$180.00 VS $60.00 off ebay or internet sales.

Also if your power vent fan is making ANY noise it is on its last legs and buying on in advance online will save you about $80.00 over the loca ldealers price......been there done that

You will need to pull your tray from the heater to check for Wasp nests or other problems. You need to be careful with the wasps if you smoke them out be very careful not to burn any wires or components on the heater. If you have access to a co2 fire extinguisher the cold and lack of oxigen will hold them at bay for a few minutes. Back to the heater...
Your dead on about the gas build up. This comes from something blocking your pilot so that your burner flame isn't lighting on the burner that is connected to the pilot. If you have a wasp nest on the burner this will do be the problem. I want to think that when the heater fires this would fix that problem. What is more common is on the gas pipe that feeds the burners you have a gas orface for each burner. Small spiders will sometimes build webs that resemble lint inside the orfaces. You can clean these with a bent wire.Other than that anything blocking the burners will be evedent. I hope this helps, good luck.

Blower motor should not be keeping your heater from heating up. Something must be going on with your flame. Have you cleaned the heater recently? Is the igniter working? Is your burner ring in good condition? Maybe a sensor has gone bad? Trouble shooting at the rural energy website might be helpful.