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@hankwylerjr, thanks for the update. Looks like you did everything you could to get the heater going. I just wonder how that soot started to accumulate… (Oops, this is a gas heater, they burn clean and don't make soot, according to the natural gas folks) carbon started to accumulate? Did you look at the boiler room for signs that you are getting enough combustion air? An open basement with sufficient cubic footage and reasonable amount of infiltration is all most gas boilers in basements need. But there are specific minimums that often get overlooked when someone refinishes a basement with insulation, paneling, and a drop ceiling. (or just seals up the home to make it more efficient) That open floor plan basement now becomes a comfortable living space with a heating appliance tucked into a confined space that can not provide enough combustion air resulting in SOOT on the HX.

Other reasons that carbon can start to form on a HX are:

1. The dryer vent from a natural gas clothes dryer is too close to the air intake of the burners.
2. The dryer vent from any type of clothes dryer is allowing dust and lint to block combustion air openings on the burners.
3. Wildlife in combustion venting system, air intake or exhaust
4. Combustion fan (if equipped) is not performing to specifications. Motor running slow or debris on the fan blades.
5. someone placed a box or other items too close to the combustion air intake of the burner.

There are too many things to list here. The bottom line is that carbon starts to accumulate when there is insufficient air for combustion, as the carbon builds up over time (days, weeks or months) and another carbon atom attaches to it, eventually the build up becomes part of the restriction. Even if the original cause of the insufficient combustion air is removed, once the blockage is there in the HX, it becomes self sustaining and self perpetuating, until it eventually goes from plenty of room for combustion to completely blocked in a matter of minutes.

Whenever I see soot, I look for the reason. The reason is there somewhere.  It’s our job to find it.

@EdTheHeaterMan thx for the detailed explanation, yes I am a plumber, a licensed and insured plumbing contractor, semi retired Any who, I don’t routinely work on these boilers. This morning I returned to the call. I took apart the vent and all the connections and top of the jacket.,I used my soot saw and brushes and throughly cleaned the exchanger. I had 2 inches of soot at the bottom, no pictures today the cellar was a complete disaster and carbon was everywhere. Used the respirator and was done in an hour. Job ran under 3 hours total I reassembled the unit and after changing out the old lux pro thermostat that was also bad I fired the boiler up. Although the flame was yellow due to the particles in the room it stayed on and she started getting heat within 40 minutes. I also cleaned out the vent and ran my Milwaukee drain camera to the roof it was clear but I recommend to install a ss liner asap due to the breakdown of the mortar. Had my brother in law the master HVAC technician bring his combustion analyzer over he ran the tests and he advised me that all have passed. Checked for co and natural gas leaks as I do every job all passed. Stayed another 30 minutes to ensure proper operation and passed handed the customer a modest invoice for our time she was happy now to install a new chimney liner in our parts in several grand but that’s not our job