Can Anybody advise on Takagi TK-3 problem with moisture condensate and entering into the water heater appliance and dampening the igniter thus turning off the water heater supply. Moisture on the bottom of the unit and dripping down into the unit from the vent.

I have installed all venting stainless steel pipes made by Tagaki. The vertical drain Tee model number TK-KPDRV4 is not available for sale because it has been discontinued by Tagaki since product does not comply with the "Safe Drinking Water Act" and maybe other reasons that Takagi doesn't want to disclose. Some plumbers tell me it is a design flawed that Takagi doesn't want to admit. Even if I want to replace the old vertical tee, the new Vertical drain Tee manufactured by Nova Vent (Z-Vent), is not compatible with the old venting style. Meaning the T-Vent style and Z-Vent parts are not compatible and should not be used together. Can anybody explain why this product is discontinued or if this could be the cause of the problem? Any advice given to stop this moisture from dripping into the unit is appreciated.

Could the exchanger be leaking into the venting system?

it could not be a leak from the heat exchanger because if the leak was coming from the heat exchanger it would have been nonstop but not intermittent. I don't see a consistent flow of water leak from the heat exchanger every time we turn the hot water. The condensation is not always consistent sometimes there is and sometimes there is none.