Coffee with Caleffi: Combustion in Hydronic Systems

Jody Samuell, manager of engineer education, leads training sessions on boiler efficiency, design
Coffee with Caleffi: Combustion in Hydronic Systems

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For any boiler, the overall efficiency is driven by the burner. In last month’s Coffee with Caleffi presentation, Jody Samuell, manager of engineer education for Caleffi  Hydronic Solutions, examined the combustion process and how it affects hydronic system operation.

This month, Samuell discusses “Designing for Condensing Boiler Performance.” The webinar runs from noon to 1 p.m. CT on Thursday, Oct. 22. There is no cost to participate, although registration is required.

Topics include:

  • AFUE and combustion efficiency: What do they mean and how close is it to delivered performance?
  • Squeezing out the water: The basics of dew point.
  • Can baseboard and condensing work together?
  • Outdoor reset: A must to achieve maximum performance.
  • Balancing your system for efficiency.
  • What system Delta T are you designing for?

The following is a follow-up question and answer session from last month’s webinar: “Combustion: Theory and Concepts for Hydronic Systems.”

Q: After commissioning a boiler, what can happen to change excess air, O2 and CO2 levels?

A: 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.

Q: Can you briefly summarize excess air and dew point relationship?

A: 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 degrees 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 gases 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 degrees F.

Q: How does relative humidity (RH) affect excess air?

A: Because RH is a minor portion of air, it has a minor effect on the excess air reading. For example, at 50 degrees F and 50 percent RH, the water content in the air is 4 grams per kilogram of air. If the RH increases to 100 percent relative humidity, the increase is an additional 4 grams of air per kilogram.

Q: How is it possible for Viesmann to claim 100 Btu capacity at 5,000 feet?

A: There is a coding change that is made by the installer at 5,000 feet. That changes the operation of the blower.

Q: Due to the density of air as it combusts, a 35 degrees F change in primary air will change the O2 in the exhaust about 1 percent 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?

A: 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.

Q: If N2 just passes through the combustion process, why do manufacturers advertise their boiler/burners as being "Low NOX?"

A: 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.

Q: Please discuss which combustion parameters need to be adjusted for high altitude (low density) applications (both air and fuel [gas] sides).

A: 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.

Q: 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?

A: 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.

Q: 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)?

A: Generally when we think of incomplete combustion we think of insufficient air causing the CO formation. If I maintain 17 to 30 percent excess air, then I will minimize the formation of CO. Due to a number of factors, CO will be present in the flue gases even when I have excess air. Too much excess air or recirculation of combustion gases in the combustion chamber can drive up CO even though there is sufficient oxygen to support clean combustion.

Q: Wet kit tells you excess air?

A: 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.

Q: What can air tell us with regard to combustion issues? e.g. Boiler fires in high flames and out in low fire, etc.

A: 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 six-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.

Q: When installing a new oil-fired boiler and reusing an existing stainless steel chimney, is it necessary to clean the chimney itself? How important is it?

A: 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.

Last month’s webinar, “Combustion: Theory and Concepts for Hydronic Systems,” can be viewed below:


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