Burnham ES2 - ES27 Spill Switch

Have you looked at / inspected the actual flue passages in the boiler? If they are fouled or partially plugged, that can be an issue. To be clear, the blocked vent or spill switch and the roll out switch are often confused, does your roll out switch have a manual reset? is it located near the burner of near the chimney vent connector pipe/draft hood area?

4.68 wc gas pressure, is very very different than 3.5. Definitely possible that there was flame impingement leading to carbonization and partial blockages in the flue passageways

OK there is more info…. The wood shop in the basement can be a clue:

1. Any airborne particles of wood dust (the small stuff from sanding can stay airborne for a long time) the get caught up in the combustion process will cause the flame to burn rich and may deposit Carbon particles (The gas company does not call it soot) to stick to the walls of the boiler and a significant build up mau cause improper combustion gas flow thru the boiler. That is usuallt the roll out problem but can trip the spill switch under certain conditions.
2. Some shops have large exhaust fans to eliminate saw dust to the outdoors. If that vent is still connected and is operated for some reason, then you can get the downdraft in the chimney that would cause a spill switch to trip.
3. Also check for other exhaust fans throughout the home, bathroom exhaust, kitchen exhaust and the like can create a negative pressure inside the home. The air inlet for that negative pressure zone inside the home can be your chimney of all other openings are closed (windows, doors, etc).

@jdigangi said: "Its been on my list to remove the jacket and check out the actual passages between sections of the boiler." 

Don't pull everything apart unless you need to: You can use a light and a mirror to inspect the boiler sections from the burner area. With some burners removed, shine a light up from the floor and use the mirror to look up thru the cast iron sections for blockages. You can also put a light inside the flue collector at the top and use the mirror to look up to see any blockage.

Blowing out the sections is the worst possible idea. If there is actually soot, you will be scrubbing walls, cleaning or chucking clothing, linens, furniture. Gas boilers can certainly have soot or more precisely carbon deposits. Same thing for our purposes. You would be wise to remove the burners and take pictures from underneath. Pictures of in between the sections. Pretty easy to do. You might even be able to do that with the printers still in place.

Who's doing the combustion test? We don’t want more than 50 ppm CO up the flue. Especially on an atmospheric. And draft needs to be tested. Isn't that why you're here, draft? You don't know until you know.

You validate draft with a calibrated combustion analyzer.