Pellet Boilers: What Are You Seeing For Max Flue Gas Temperature?

It's in the 40's here in central NH so the P4 that we heat our warehouse and offices with is ticking along nicely at about 40% output. The boiler is being used with 500 gallons of buffer so between the generous buffer volume and the modulation, the boiler is doing a nice job of matching the load and we are getting nice long, steady burn cycles of 6 to 8 hours between re-fires. Cool. Looking at the graphs the peak exhaust temp over the last 24 hours has been 260F. As it gets colder I would expect stack temps of more like 300 degrees F. I would not worry about cleaning until you see 400+ or get an error.

Temp rise from ambient air is what you want to look at. Mine is in my garage so it will see ambient air near freezing for much of the winter. As such I will see flue temp at 100% output in the 200-230 range.
Add another 40* to that for a nice warm basement and you can see you are falling about dead in the middle of that at 269.
When mine throttle down to 30% fire I'll see flue temp only 10-15* about water temp. Normally in the 160-175 range so flue gas is at 200 or below. You can get away with that because pellets are so dry. Most of the time between 3-7% Moisture content. At that level of moisture you can see flue gas dew points around 110-120* so even with the exhaust running <200* there is no danger of creosote formation in the heat exchanger or the flue.

I don't have an internal temperature probe and have not check the flue temp with a laser thermometer, but my Chimguard magnetic thermometer rarely will go over 200 even on a long burn. Just now the boiler is idling and the thermometer is reading about 150. The water temperature in the boiler and buffer tank is 185.

Are any of the pellet boilers near the point that an induced draft and low temp flue allows PVC flue horizontal to the outside where any combustion condensation can just drip to the ground, like a high efficiency NG condensing boiler?

The P4 is available in Europe with a condensing add-on module for their 8 - 25kW boilers, but not here yet, Jim. Interestingly, the driving factor is lower emissions, not so much higher efficiency. As you can imagine, the condensate does a pretty good job of collecting particulate. It is my understanding that the condensate is not as easy to deal with as that from a mod/con LP boiler. Will come here eventually, I suppose, but no one is really looking for ways to make pellet boilers MORE expensive in this country and our emissions regulations are lax enough that we don't need it yet. Of course the whole issue of venting a solid fuel boiler without any natural draft is another issue!

Of course the whole issue of venting a solid fuel boiler without any natural draft is another issue!

Bingo! We have a winner.

It would take a lot of convincing for me to install such a beast if it were even available here for this reason alone. I will gladly trade a few points of efficiency for nearly 100% safety.