Did the repair tech ck the heat exchanger integrity? A 40 yr old unit is likely cracked or rusted through. Here's a quick test, BUT NOT FOOL PROOF. With the main flame OFF, push in the white button on the fan/limit switch and the fan will start. Watch the pilot flame as the fan starts and if it begins to waver, the heat exchanger may be compromised, allowing carbon monoxide into the space! Pull the switch to stop the fan, now turn the tstat up and watch the main flame as the fan starts. Better yet, use a borescope to ck each bay.
Follow previous advice and replace it. Hang a unit heater with closed combustion that uses outside air for combustion. You will accomplish 2 major improvements, added floor space (which is golden) and removing the OPEN FLAME form the garage for safety. Plus, it may be more efficient.
Yes, I hear everyone out there saying a torch or sparks are an open flame. Would you light a torch or strike an arc if the space was full of flammable fumes? The furnace can't make that decision! Your life, your call! Except when your grandson is helping.
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