Best Of

Combustion is most likely out of tune and needs to be adjusted using a combustion analyzer. It could also be several other things like CSST piping, low gas pressure, undersized regulator, plugged intake, etc but this is a pretty common symptom of a boiler that was not properly commissioned. Being a Mascot that's cheap online, would it be safe to assume that this was not professionally installed and commissioned?

I would definitely check the vent pipe and air intake. Tuning it with a digital combustion analyzer is crucial. These things are not good to go out of the box. Professional installation and maintenance are needed for reliable operation.

Yes, its scary. Which is why there are qualified people in the trade to make sure your extremities stay attached to the rest of you. A few years is 3,4? Has it ever been serviced? Has it ever met a combustion analyzer?

Is that 1/4" OD aluminum tubing shown on the left side of the picture a vent line or does it connect to other tubing and go to other devices? That gas valve looks like what we used to call a Gas actuated device. A better picture may help. Pull the camera back some to get a better view. If that gas valve is just a standard on/off gas valve I would replace it as @EdTheHeaterMan suggested but if it is the other type, you need a good service tech to keep you from blowing your self up. Before I say more, I would need more information as to how it is controlled and what type valve it is. And, don't try to repair a gas valve yourself.