Steam boiler: flame goes on, off, back on

This^^^. Some of the best in the business.

On the spill switch and window, though. It's trying to tell you something. Listen to it.

Sounds like you might not have enough combustion air in the room. Did they install a fan in a can or is there an existing combustion ducts with dampers on an outside wall? Opening up a window is an indication of lack of combustion air and creating a negative in the room.

But that doesn't jive with the flame going out and restarting. Unless its causing the flame to fall off the flame sensor. You need a better pic. wide view so we can see the set up and if you do a video take one of the sight glass. wondering what low water cut off they are using.

A couple of questions:

In the first video, when the boiler shutdown, how long had it been running?

In the first video, the boiler restarted almost immediately. Were you able to reset the spill switch that quickly?? It does not appear possible.

If the spill switch was at fault, it is highly unlikely that you could reset it that quickly. Its temperature would not have fallen sufficiently to achieve a reset.

with the window closed something in the system that is shutting it off.
until that’s found keep the window open.

The spill switch was not the culprit in the video as you agree.

However, based upon your description, it had gone out earlier due to the spill switch.

This would imply that you have TWO separate issues. I find this extremely doubtful and suggest that some data is lacking.

It would be beneficial if you could get an independent temperature measurement near the spill switch when you believe it tripped. I would not dismiss the possibility of a spill switch that is not properly calibrated.

A lot going on here. First of all, it must be properly installed. That means for the venting, it must be a listed liner, vent or chimney properly installed. The horizontal offset from the boiler to the chimney is stated in the code as is derating for elbows, which cause flow restriction. Each joint must have a minimum of 3 equidistantly spaced screws per joint. At this size, I believe the connector should be either 22 ga. galv. or 24 ga. ss. You have not provided any combustion analysis readings so everything is going to continue to be guesswork. A "spill switch" should be a manual reset. A flame rollout switch is typically a single use fusible link that would require replacement at each trip. On, off, on cycles are typically a problem with flame rectification. You can conduct simple systematic troubleshooting to rule that out: is the incoming power proper polarity 120vac RMS(108-132 vac),, peak voltage 153-185 vac, 60 cycle (<2volts neutral to ground) with less than 5% voltage drop, no bootleg grounds and low impedance conductors(< 0.0048 Ohms/ ft 14 ga./ 0.003 Ohms/ft. 12 ga./ <1 Ohms to protect people; < 0.25 Ohms to protect equipment. Must have surge suppression by code. Coming off the 24vac transformer L1 to Transformer Hot or TH terminal should be additive voltage for correct phasing. You should have a world class ground from flame rod to chassis. Where the burners are part of the ground, check for their contact and no corrosion. Need min. 4x the mass of the flame rod, which should be spotless and scrubbed every time you touch it or annually at min.

Measure your gas pressures: static inlet with everything else in the building off; just the boiler, full load everything firing concurrently; measure manifold pressure. If your inlet drops below min. or flutters, correct the gas piping. Check any MP regulators and venting; If the inlet gas is sufficient and stable, assess the manifold pressure- is it sufficient (determined by combustion analysis) and stable? Are all orifices correct, clear, properly engaged into the burner venturis? Is there anything near the boiler, such as objects or stray air currents that could adversely affect the free flow of air into the boiler cabinet and burner compartment? Air turbulence can disrupt the secondary air at an igniter causing loss of rectification on an otherwise perfectly firing appliance and controls.

Take your micromanometer and read the following: Note wind speed and direction. Do not test if > 10 mph. CAZ to outdoors via a basement window (Stack Effect). Try front vs. back sides; CAZ to first floor; CAZ to any adjacent space separated from the CAZ that could create a separate pressure zone. Now, starting at the front door, determine the Neutral Pressure Plane using a smoke puffer. Repeat at backside. If windy, try for upwind (windward) and downwind (leeward). If > 10 Pascals, stop test and wait for calmer wind. Locate the NPP. Now, survey the upstairs for air leaks. Check every window to ensure it is sealed. Same for attic stairs or hatch. Must be gasketed or seams taped. Any ceiling penetrations must be tight. All 'can' luminaires (ceiling lights) must be ICAT- Insulation Contact Air-Tight rated) if extend into attic. If any upper level leaks, repeat Stack Effect tests and note change. If the building continues to be very drafty, consider contacting an energy rater to guide weatherization with a blower door test. They can perform a Worst Case Depressurization Test if you feel overwhelmed. Otherwise, this may reveal some endogenous influence causing venting failure where there is nothing 'wrong' with the equipment or venting.

Measure your stack draft pressure at standby. Is there a flow reversal at standby? Do you have a cold backdraft? Compare your stack draft vs. your building Stack Effect. Who wins that tug of war? You want the NPP as low in the building as possible with the minimum Stack Effect.

Set up your DMM in series with the sensor to measure flame rectification in microamperes. Set up a video camera on the screen. Start recording 10 seconds, make a call for heat and record through a firing cycle. Have a second video on the burners recording flame patterns and your combustion analyzer running the whole time. Repeat at: call for heat-1 min; 5 min. 10 min.; 15 min.; open the switch; 5 min. post-purge. Lay the data over a timeline. Your problem should jump out at you.
Flame rectification requires a stable flame to conduct the signal and avoid dropout. A stable flame is a happy flame.

HTH,

Bob

BTW, if you need a chimney pro in North Joisey, contact Ryan & Sons in Landing, NJ or Bridgewater Chimney and tell them I sent you. Two of the best instructors in the country. Tell them I taught them and don't let kids hear their response.