Combustion: Theory and Concepts for Hydronic Systems
Combustion: Theory and Concepts for Hydronic Systems
September 17, 2015
with Jody Samuell, Manager of Engineer Education
Q&A Documentation
Question:
After commissioning a boiler, what can happen to change excess air, O2 and CO2 levels?
Answer:
The biggest impact is air temperature; temperature results in density changes which can result in the
amount of oxygen to vary. Other common impacts include changes in the caloric value of the fuel
which can result in a dirty fan, causing less air movement.
Question:
Can you briefly summarize excess air and dew point relationship?
Answer:
Dew point is related to the amount of moisture in the air, such as grams of H20 per kilogram of air. If
my moisture content is 95 grams per kilogram of air, the saturation point is 122°F. If I increase the
excess air, I will still have the same amount of H20 from the combustion process. But because the
volume of my exhaust gasses has increased, the amount of H20 per kilogram of air will decrease
resulting in the dew point lowering. For instance, because of increased excess air, if the H20 content is
lowered to 75 grams per kilogram of air, the new dew point is now 113°F.
Question:
How does relative humidity (RH) effect excess air?
Answer:
Because RH is a minor portion of air, it has a minor effect on the excess air reading. For example, at
50°F and 50% RH, the water content in the air is 4 grams per kilogram of air. If the RH increases to
100% relative humidity, the increase is an additional 4 grams of air per kilogram.
Question:
How is it possible for Viesmann to claim 100 BTU capacity at 5000 ft?
Answer:
There is a coding change that is made by the installer at 5000 ft. that changes the operation of the
blower.
Question:
Due to the density of air as it combusts, a 35°F change in primary air will change the O² in the exhaust
about 1% on a conventional power burner. What effect does it have on the 21st century direct fuel-air
coupled type burners (i.e. Dungs Whirlwind design) being used on modulating condensing boiler today?
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Combustion: Theory and Concepts for Hydronic Systems
Answer:
Same effect: it is a function of the air density. If I am moving a given volume of air (CFM), as the
density changes, the O2 reading will change.
Question:
If N2 just passes thru the combustion process, why do manufacturers advertise their boiler/burners as
being "Low NOX?"
Answer:
Do not confuse N2 with NOX. N2 is nitrogen. NOX is generic for mono-nitrogen oxides: the X is used
because it may be an NO or an NO2 molecule. Most of the N2 in the combustion air exists in the boiler
as it came in as an N2 molecule. In the presence of excess air and a hot flame, some of the N2
combines with the O2 of the excess air and forms NOX which is a greenhouse gas. The low NOX
burners are designed to reduce the NOX formation.
Question:
Please discuss which combustion parameters need to be adjusted for high altitude (low density)
applications (both air and fuel (gas) sides).
Answer:
This depends on the boiler manufacturer. It may be as simple as the output deration based on a
chart. It also may be either a coding change on the boiler control or it may involve an orifice change.
Because the process of changes and even the amount of deration is manufacturer dependent, I have
to default to the manufacturer of the boiler you are considering.
Question:
If nitrogen is a conductor of heat, how can a Lenox burner cool the flame? Wouldn't a colder flame
produce higher nitrogen dioxide levels?
Answer:
With NOX formation, it is all about the temperature needed to support the chemical reaction. I need
to exceed a threshold temperature to support the reaction. As temperature increases beyond the
threshold, the rate for formation increases. When I reduce the combustion temperature, then I can
reduce the rate of formation of NOX.
Question:
The formulas are addressing complete combustion which does not occur in our heating systems. What
about incomplete combustion resulting in the production of CO (carbon monoxide)?
Answer:
Generally when we think of incomplete combustion we think of insufficient air causing the CO
formation. If I maintain 17 to 30% excess air, then I will minimize the formation of CO. Due to a
number of factors, CO will be present in the flue gasses even when I have excess air. Too much excess
air or recirculation of combustion gasses in the combustion chamber can drive up CO even though
there is sufficient oxygen to support clean combustion.
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Combustion: Theory and Concepts for Hydronic Systems
Question:
Wet kit tells you excess air?
Answer:
Not directly. It will show you the CO2 or O2 reading which is directly related to excess air. In order to
convert the reading to excess air, a conversion chart or table is needed.
Question:
What can air tell us with regard to combustion issues? e.g.: Boiler fires in high flames and out in low fire,
etc.
Answer:
CO2 or O2 readings are just an indicator of a problem. Understanding what supports the combustion
process is as important. Is it a gas problem? Is the pressure too low or too high? Is the gas line
undersized? Is there water in my propane causing a regulator problem? Do I have the proper orifice
in my gas valve? Was the boiler converted over to propane? Do I have debris stuck in the screen in
the gas valve? Because of the age/size of the natural gas lines in the street, does the gas pressure
fluctuate over the course of the winter?
And then there is air. Am I cross-contaminating my make-up air? Is my make-up air properly sized?
Is there an insect nest in the combustion air line? Does the basement have sufficient volume to
support combustion and, if so, is there sufficient air infiltration to support combustion? Room
dependent air with tight houses bring in more questions. Does the problem only exist when the
exhaust hood for the 6-burner range is running?
Your combustion analyzer can indicate if there is a problem, but it is the understanding of boilers that
will steer you in the right direction. Sometimes it is a process of elimination that gets you to the
actual problem.
Question:
When installing a new oil-fired boiler and reusing an existing stainless chimney, is it necessary to clean
the chimney itself? How important is it?
Answer:
If I am dealing with a chimney, developing a proper draft is paramount. I cannot have any obstruction
that would restrict the flue gas movement. For the bare minimum, I would suggest an inspection of
the existing chimney is needed. Additionally, if the boiler being removed is caked with soot, a
chimney cleaning is probably in order. Another consideration as equally important is sizing;
particularly I don’t want it oversized. NFPA 31 (National Fire Protection Agency) has sizing tables near
the back based on efficiency and firing rates. If the new boiler is much more efficient than the old
boiler the current chimney may not properly draft with the new boiler.
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