Published: August 13, 2015 - by Ray Wohlfarth

Ray Wohlfarth Pic

There is a disagreement in our industry as to when is the ideal time to perform a combustion analysis on boilers.

Some believe that it is best performed in the fall prior to the heating season. Others believe that it should be performed in the middle of winter when there is a substantial heating load on the building. There is also another group that does not believe in instruments at all and will simply eyeball the flame to perfect combustion. We will discuss those at a later time when we talk about psychics, mediums, alternate universes, and aliens.   So, when is the best time to perform the combustion analysis? Like almost every question asked in this industry, the answer is “It Depends.” Consider the advantages and disadvantages of each so that you may decide which is better for you.

Combustion efficiency of a boiler is a measurement of how much heat is transferred from the flame into the heating medium, like water or steam, and how much is lost up the stack.  Traditional boilers are usually between 70-85% efficient. Condensing boilers can be as high as 99% efficient.  Combustion efficiency testing will show what the ratio of air to fuel is during the burn cycle. For instance, a boiler that burns natural gas will require about ten cubic feet of air for every cubic foot of gas for perfect combustion efficiency.  In #2 fuel oil burners, the burner requires about 14 lbs of air for every pound of fuel oil for perfect efficiency.   Extra air or “Excess air” is added to the flame to assure safe operation and to protect the boiler from sooting. Sooting occurs if there is insufficient air or incomplete combustion for the burner.  Oxygen content for the combustion air can vary greatly by combustion air temperature and/or barometric conditions.  

When adjusting the fuel to air ratio, it is common practice to operate the burner for fifteen minutes to allow the flame and draft to stabilize.  An On Off burner will require about 10-15 minutes to adjust the combustion efficiency after the initial fifteen minutes.  A Low High Low Burner will require about 20-30 minutes to adjust. A fully modulating burner will take about an hour to properly adjust. If you add the fifteen minutes that it takes for the flame to stabilize, the combustion analysis and adjustment will take between 30 minutes for the On Off burner and 75 minutes for a modulating burner.

Let us consider a hypothetical building that is roughly 50,000 square feet. At design temperature, our building will lose 1,500,000 Btu per hour or 25,000 Btu per minute. Our heating plant will consist of two hydronic boilers with fully modulating burners. Each boiler will be 80% efficient and sized to provide 75% of the heating requirements of the building at design temperature.  Each boiler will have a rated input of 1,400,000 Btuh and an output of 1,120,000 Btuh.  The table below shows the consumption of fuel per minute per boiler at the most common firing rates.

Firing Rate

Btuh Input per Minute

Output per Minute @ 80% efficient

100% or High Fire









33% or Low Fire




The following is a demonstration of a pre-season combustion analysis and adjustment. We visit the jobsite on a fall day when the outside temperature is about 500F. At 500F outside temperature, our building will lose 346,154 Btu per hour or about 5,769 Btu per minute. This is about 23% of our design heat loss. The building hydronic loop temperature will be about 1300F at this outside temperature and will contain about 1,863,499 Btu. We start our first boiler and allow it to operate at high fire for fifteen minutes to stabilize the flame.  We then begin our adjustment of the fuel to air. Let us assume that it takes us about fifteen minutes to adjust the fuel to air ratio at each of the firing rate positions; low fire, 50% fire, 75% fire, and high fire.  At the end of the hour, we would have added the following Btu into the building.

15 Minutes @ High Fire to stabilize the flame      280,000 Btu

15 Minutes @ Low Fire                                          92,400 Btu

15 Minutes @ 50% Fire                                         140,000 Btu

15 Minutes @ 75% Fire                                          210,000 Btu

15 Minutes @ High Fire                                          280,000 Btu

TOTAL                                                                   1,002,400 Btu

At the completion of our adjustment of boiler #1, our building hydronic heating loop has gone from 1,863,499 Btu’s with an average temperature of 130F to a loop that contains 2,865,899 Btu with an average temperature of about 200F.  If we are lucky, we would have completed the adjustment before the boiler shut off on the operating or limit control.  Our next challenge is that we still have to adjust boiler #2. It will take 2.89 hours for the building to lose enough heat to lower the loop temperature to the original starting point.  What will we do for almost 3 hours? Perhaps play the latest version of Angry Birds.

Now, consider what happens when you perform the combustion analysis in the middle of winter. We will perform the same tasks as above but the only difference is the outside temperature.  If we perform our adjustments when the outside temperature is at 12F, the building will lose 1,269,231 Btuh or 21,154 Btu’s per minute. It will take about 47 minutes for the building to lose the amount of heat that we have added to the loop. This is about a third of the time that it took at  50F outside temperature. It makes us more efficient.  

I like to perform the combustion analysis and adjustment in the winter when the building is under a heavy load. In the pre-season inspection, I perform a cursory check to make sure that the fuel to air adjustment is close. 

Want to learn more? Check out my books, Lessons Learned in a Boiler RoomLessons Learned: Connecting New Boilers to Old PipesLessons Learned: Servicing Boilers, and Lessons Learned: Brewing with Steam.