This is my first year with a harman hydroflex 60 boiler piped I to my baseboard heat and direct power vent. Has worked well for the first 2 tons of pellets this year and have been running the feed rate at 4. Had cold temps this week and boiler wasn't keeping up so I increased the feed rate to 6 and I notice a black soot outside at the exhaust vent and the ash in the stoves black rather than the grey / white it has been. Stove didn't seem to keep up any better. Also did a cleaning of the stove and noticed the twirly things (technical term) in the heat exchanger tubes were also black. It doesn't seem like this stove adjusts the air flow when the pellet feed rate is adjusted, true? Do people adjust feed rates when it is colder outside or just leave them as-is for a particular fuel? Is the soot a result of too much fuel and not enough air? Thanks.