"What is the draft in your system? What is the altitude of your system? Where (in relation to exhaust) do you measure the draft? the exhaust temp?"
Easy answers first, then a little to mess it up. I have a Condor probe thermometer inserted in the black stove pipe about 12" vertical above the flue collar exit from the boiler (Tarm). I never have measured draft because of my belief that temperature and draft are directly related (and I did not have nor wanted to buy a draft gauge). Since I started with much higher temp, I know my draft is good, as I had to reduce the draft by partially closing the forced draft fan damper to bring my temperature down. This is the only air in point in my boiler, so the vacuum action of a good draft also is pulling air through this damper.
We are located about 1000' above sea level. The boiler has 4-5' of black stove pipe exiting vertical from a 90 degree flue elbow diretly from the flue collar. The stove pipe connects through a celing fitting to 18' of 6" stainless, double wall, insulated chimney pipe, with a standard rain cap at the top. About 12' of the stainless chimney is in an unheated, well-ventilated attic and the remainder is above the roof. The top of the chimney is 2-3 feet above the roof peak. Outside air temp this winter has been a low of -26, with low's typically 5-15.
Measuring flue temp or draft readings for the typical boiler owner will be guides to proper boiler operation and not scientific set points, as there are many variables from burn to burn and even during a single burn. Examples mentioned earlier and others include accuracy of temp or draft gauge, type of wood, mixture of wood types, moisture content, size of splits (surface area), outside air temp, outside wind speed and direction, clean/dirty heat hx tubes, presence of turbulators in hx tubes, load mix valve on boiler vs direct return from heating system, type of chimney (steel, clay, masonry, etc.), . . . .
For me the goal was the reasonable minimum flue temp with the temp probe I had which also resulted in the heat I needed (within boiler capacity) and no smoke, no creosote, no condensation, practically no ash, and no problems. Based on info provided by others, I knew my flue temp should not be less than 300 (assuming an accurate probe; others have indicated material differences in readings between different probes, even of the same brand), and each 100 degree increase in flue temp translates approximately to 2.5% reduction in efficiency. My goal is not maximum efficiency all the time, but reasonable efficiency all the time in consideration of all the variables. As mentioned previously, with burning dry pine slab wood this is achieved with a probe temp of 450-500 during most of the burn, with a peak of just under 600, and end of burn down to 300-400. I found that if I pushed that peak temp much lower, while the early part of the burn was good, I did not seem to achieve the same good burn durning the latter part. So I compromised, knowing that 600 is above the most efficient temp but not dangerous, and the entire burn was reasonably efficient. I want to be able to fire my boiler and walk away without any adjustment during the burn.
Another solution for some may be to add turbulators to the heat hx tubes. This will be particularly true if excessive flue temp is a problem which cannot be solved by draft adjustment, which was my case, as turbulators will result in more heat transfer, reduced flue temp, reduced draft, and also increase in efficiency. They also should add to efficiency in most other cases, so long as a good burn may be achieved and the desired flue temp/draft maintained, and draft adjustment likely will be necesarry if turbulators are added.
