Until you begin to understand all the variables associated with ash cleaning intervals in a pellet boiler system, the question, “how often do you have to remove the ashes?” seems simple enough.
I decided to test my MESys 4000 boiler to point of failure by not removing ash until a “Problem! Low chimney draft” error indicated it was shut down due to the problems of ash build-up. Through a couple of test cycles, it became clear that around .8 tons (1600 pounds) of the pellets I’m using would create enough ash to fill the boiler enough to shut it down. By test, we know that those pellets contain 1% inorganic ash; so, my system can handle 16 pounds of ash before a cleaning out is required.
My boiler is purposefully undersized for my house, yet it has managed to carry the house’s heat and domestic hot water demands even into this week’s sub-zero weather. This morning at -12F in my backyard, boiler water temperature was at 177F when I checked it at 6:00 a.m. To provide this heat for the house, I am burning approximately 100 pounds of wood pellets per day. That means that I should plan to clean the ashes out of the boiler every two weeks during this weather in which the pellet burner is running at maximum output full time. (I clean it every other Sunday; it takes a half hour.)
The MESys 6000 boilers are using roughly 120 pounds of pellets per day when they are in full fire conditions. They will handle 40 pounds of ash or more between cleanings. That means that regular cleanings for those boilers should be scheduled approximately every 30 days if they’re operating under full fire conditions using pellets that generate 1% ash.
The last qualifier is important. Pellet ash content varies among manufacturers and even among mill runs. Pellets produced and tested to PFI (Pellet Fuels Institute) standards that are labeled “premium” will contain inorganic material to produce 1% ash, or less. It’s not uncommon for pellets to have 0.5% ash, which would double the cleaning intervals.
Some of our boilers are in the Arctic Northwest, and the pellets there are so clean that ashscrape cycles are set to the available maximum setting of a scrape every 144 pounds and, even then, the scraping is unnecessary, and ash content of the pellets is at, or below, 0.3%.