Will the hole drilled into the furnace door cause problems? Gases etc. escaping?

Question

I recently had a furnace service. A 3 blink code came up. The flame sensor was dirty,and a curved white pipe outside on the roof had snow inside it. The tech drilled a hole in the furnace door so you could see the red "code" light.

As he had taken the module out before(to replace the fan, and never put it back properly). So you had to take the door off to see the light on the module. I placed sheet metal tape over the hole as l felt uncomfortable with the "repair". There is a Co2 detector 12 ft away which didn't go on before l covered the hole(or now).
Should l go for a more permanent repair to cover the hole drilled, or leave it? Should l call the tech back to repair the hole? I could use a magnet also. As l understand it isn't "code".
Another positive: the oil-canning stopped. The furnace seems to run slightly longer after the thermostat "clicks", but the loud metal bang is gone now.

isherwood · Accepted Answer

There's no problem. In a healthy furnace, combustion gases are contained inside various components housed in the upper cabinet.
If you think through the airflow path of your HVAC system, there's normally a vacuum on the lower chamber of the furnace cabinet because it's part of the return duct--it contains the fan. That means that air is drawn into that space, joins all the other incoming return air, and passes through the furnace's heat exchanger and out through the main exit trunk. A tiny hole just creates a miniscule return path from the furnace room.
I've done exactly this with one of my furnaces after upgrading the circuit board to one with an indicator, so that I could see the indicator from outside. I applied clear packaging tape over the 3/4" hole I made just for the heck of it. I put two pieces back to back to eliminate the exposed adhesive, then I framed that with tape on the inside of the panel. It held well. Obviously you don't want to use anything that could get drawn into the fan cage.

Will the hole drilled into the furnace door cause problems? Gases etc. escaping?

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