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Hello well that is not awful but borderline bad. The next step is to take some readings since you probably don't have a guage ( magnehelic) we will have to take the long way. First, look at you outdoor vent termination and look closely for any signs of soot. Next we will have to take the draft motor out and look into it to see if any of the fins are broken. Now you have to look close on this because everything inside the motor housing is black. When you take that motor out you will see a hole going into the heat exchanger this is the secondary coil it has a series of small tubes. Look in there as well for any debris or soot.
I might be a little ahead of my self here because i know this furnace like the back of my hand. So if you can take the burners out and blow air in them if you can to clean anything that could be in there. Next step is we will need to check the gas pressure and the vent pressure and we will need a gauge for this. Now getting the burners out is a little tricky but you don't have to take the gas valve off of the manifold just the 4 screws that hold it in place. If your good you can get the burners out. One last thing since this must be an older unit to have hot surface ignition the burners are most likely small Robert shaw individual burners instead of a solid manifold this may require you to take the burner box out. if it has the Robert shaw burners there is a little field fix for them if we get this far. Can you provide me the model/ serial number? Take the top door off and it will be on a tag to the side let me know Richard.
Hello, since you can see the burner i am going to assume it is an RGEB or EA ( older version) This unit used aluminumized heat exchanger cells. These were very prone to cracking. If your able to remove the burners and with a light look in side and up you may see them. A crack will cause the issue you have. It is possible it is just rust and debris however with this furnace and the age i would strongly recommend you or someone look. Now they did have a program for this furnace because they no longer make the heat cells. I looked it up but it expired in 08. If you look at the serial number there will be a F followed but 4 numbers. F1088 example, made 10th week of 1988. Many of these came with 20 year warranty so if it is less than 20 years you could qualify for something.
Getting back to your issue. Clean it , inspect the heat cells i can send you a picture of what they look like, then try again. Now this was the second generation (RGEB) of 90 plus furnaces so they had limited safeties built into them. Often the pressure switch would not take the furnace out or removing the door would make it run under a not so good condition. Now the heat exchanger is in a negative pressure so the chances of CO escaping is very small but don't take any chances here. Let me know