Combustion Efficiency Tune-Ups, Parts I & II

No appliance, once installed in the field, can be guaranteed to operate safely without proper testing and setup.

What does maximizing equipment efficiency accomplish?
Before answering this question we might want to ask, what does maximizing equipment efficiency actually mean? It’s a fact that all equipment is designed and tested to meet one set of testing standards, conditions, and approvals. Unfortunately, this will be the only time equipment will ever be under these conditions or standards. In the field, every installation is different. Maximizing equipment efficiency means setting the equipment up on the job to meet the exact conditions under which it will now operate, using a digital combustion analyzer.
Maximizing efficiency means you’re also testing, evaluating and maximizing the safety of equipment operation. No appliance, once installed in the field, can be guaranteed to operate safely without proper testing and setup. The safer we make equipment the more likely it will operate at its best efficiency.
Maximizing efficiency reduces wear and tear on equipment. Rusting, condensation, cracked heat exchangers, high limit lock-outs, and more, are all caused by poor mechanical operation and efficiency.

What Affects Efficiency?

To understand why we need to perform precision combustion efficiency tune-ups in the field, let’s look at what affects efficiency.
1. Equipment size. Equipment should always be sized properly by doing load calculations but no matter what we do, equipment will be operating in an oversized mode 90% of the time. In Southern climates most heating equipment has been 100% to 200% oversized for years because of the necessity of larger blowers for cooling. Now comes the contradiction.
In 1987 an ASHRAE report stated that oversizing equipment 100% had little affect on actual efficiency. The only difference was number of cycles and temperature variations. But then some of this can be minimized by controlling the equipment a little differently. The key here is that equipment is designed to operate at it rating and nothing less.
2. Load and Ambient Conditions. Running a furnace on a 90 degree day versus a 20 degree day will give us different results. Equipment runs hotter under no load and appears to be less efficient but we just have to compensate for these things. It might even be cycling on limit, leading us to wrong conclusions.
Whenever we have multiple pieces of equipment sharing a single flue they can operate differently when firing alone versus firing together. If the same equipment is sharing the same gas line, (usually the case) all must be running to verify adequate gas supply. Both these conditions must always be considered when evaluating system operation.
Running a water heater with and without the air handler or furnace blower affects efficiency & safety and needs to be checked when they share the same space. Checking all equipment with additional ambient conditions altered is also critical for safe and efficient operation. This could include all exhausting devices in the building running.
3. Control Settings. A gas pressure regulator should be a settable control to optimize equipment performance after installation in its one of a kind environment. But this can only be done with proper instrumentation and training. Fan controls or settings, operating limits and differentials and energy management controls are all affective devices in improving equipment efficiency and are not things to be ignored.
4. Fuel. Gas, oil, propane, and other fuels are like the “refrigerant” of the heating equipment. Whether we have too little or too much fuel affects efficiency substantially. It’s been proven in the field that firing equipment less than it is rated at or mechanically capable increases the amount of energy wasted. Fuel produces the size of the flame. The size of the flame determines how hot the heat exchanger gets. The hotter the heat exchanger, the more heat that is transferred. The mass of a heat exchanger doesn’t change during operation and only one firing rate heats it most efficiently.
5. Combustion air. This is always a confusing subject. We put two grilles in a wall or two pipes and think we have combustion air when all they really have are a couple of holes in a wall. In 2000 an ASHRAE report concluded that supplying combustion air by mechanical means was the only way to deal with most environmental conditions or was the only true “functional combustion air”. But no matter what means is used to supply combustion air to a space, we also must verify it is getting to the appliance for safe and efficient operation.
6. Venting.Whether you have a brick or tile chimney, B-vent or liner or a pvc flue, they all affect how an appliance will perform. Natural draft chimneys can have constantly changing drafts that can have a distinct affect on the fuel air mixture of the burner. The length and number of elbows in a PVC flue affect the operation of high efficiency equipment operates. The GAMA venting tables specifically state they were developed for steady state conditions only. Until equipment is attached to its chimney, there is no way to preset equipment for proper operation in the field.
7. Ducting or piping. CFM and GPM determine how many BTUs are delivered to the air or water and then the space. In forced air heating systems, the size of the duct system and the amount of duct leakage are keys to efficient operation. It’s rare that the actual piping on a boiler system is wrong; or, at least, they seem to be more forgiving. As far as leaks on a boiler piping system, efficiency might be the least of your problems.
Changing airflow or GPM to attain a certain Delta T is not necessarily the proper way to set up any equipment if one wishes to maximize the efficiency of that equipment.
All of the preceding items affect efficiency and obviously are used to improve efficiency.

How to Evaluate the Equipment Efficiency
Over and over we hear you need a combustion efficiency analyzer when in fact it is just a combustion analyzer. Because all efficiency calculations are based on a single set of possible characteristics or parameters, the likelihood of them giving us accurate calculated data is about nil. Keeping it simple, a combustion analyzer, or a combustion efficiency calculation chart rarely comes close to evaluating true delivered performance of combustion appliances. But they do give us a means of knowing where we are starting and how much change did we make. To measure true performance or efficiency it is necessary to measure the actual heat transferred to the air, water or steam of our system versus the amount of fuel input. This is called thermal efficiency.
We have formulas to calculate thermal efficiency of furnaces and boilers (in this case water not steam).

The formula for furnaces is:
CFM x Delta T X 1.08 = BTU Output
The formula for boilers is:
GPM x 8.33# X Delta T x 60 minutes = BTU Output (8.33 X 60 = 500 or GPM X 500 X Delta T = Output)
The only way these formulas can be used correctly is if CFM and/or GPM are measured. Simply plugging in numbers to these formulas to calculate CFM or GPM provides undependable results.
In the field, CFM or GPM can be approximated by measuring Static Pressure or Head Pressure and then looking at the blower or pump performance charts. As long as we use the same methods and protocols each and every time our results will be reasonably accurate.

Ex. 100,000 BTU input 80% induced draft furnace - G.P. 3.5”
Static Pressure: .7”w.c. – Blower Speed – Medium Hi
Blower Chart: 1100 cfm @ .7”
Plenum T: 120 degrees
Return T: 70 degrees
Thermal Eff: 1100 X 50 X 1.08 = 59,400 BTU or 59.4% efficiency
100,000 BTU input 90% condensing furnace – G.P 3.5”
Static Pressure: .5”w.c. – Blower Speed – Medium
Blower Chart: 1000 cfm @ 7”
Plenum T: 120 degrees
Return T: 70 degrees
Thermal Eff: 1000 X 50 X 1.08 = 54,000 BTU or 54% efficiency
This is the method to evaluate equipment performance. We can determine the efficiency of the whole forced air system by taking temperature measurements at the supply registers and return grilles and the using the estimated CFM.
Measuring the head pressure on a circulating pump might be difficult but most boilers required 1 GPM per 10,000 BTUs output. Plugging this into the Thermal Efficiency formula we find that a 20 degree rise is needed for proper operation.

Ex. 100,000 BTU input boiler, 80,000 BTU output
Pump GPM : 8gpm
Return T : 140 degrees
Supply T : 150 degrees
Thermal Eff: 8gpm X 500 X 10 = 40,000 BTU or 40% efficiency
If this is the way to evaluate true efficiency, why do we need a combustion analyzer in the first place?

Why does all equipment have to be tuned with a combustion analyzer?

There is no way to eyeball a flame and determine if you have efficient fuel/air mixture or are operating safely. All equipment is designed to meet one set of operating conditions or parameters. But just how many of these will we encounter in the field?
A. The actual BTU value of a fuel in any given area is going to be different. Then according to GAMA in 2005 the btu value can fluctuate daily. Then there is the question of how much usable energy can we actually produce. Two exact cars with exact engines using the exact same gasoline get different mpg. The usable energy one engine is producing is greater than the other because of better combustion and this can only be accomplished with some type of analyzer.
B. How does altitude affect operation? Sea level is tough enough but when you start getting above 2000’ everything becomes unpredictable. Btu values of the fuel and air densities are not fixed commodities. Another wrench was thrown in the fire by a recent ASHRAE study that stated actual testing proved that equipment has been over-derated for altitudes between 2000’ to 6000’. This report would seem to imply that everything installed at higher altitudes was set up wrong and needs to be set up with a combustion analyzer.
C. Every piece of equipment has slight mechanical differences. What is drawn on paper is not necessarily the same piece of equipment that is assembled, rattled, bounced, tipped over, bent etc., that we get in the field. Each one has to be set up to its own ability and that can only be done with a combustion analyzer.
D. The venting system, flue or chimney has a direct bearing on how a piece of equipment will perform. Most look at the chimney as only removing the by-products of combustion, when if fact, it is also controlling the amount of combustion air that gets into the burner. The venting system also limits the firing rate of equipment. You can’t produce more flue gasses than the chimney can remove. A draft reading alone does not provide enough information to do our job correctly. A combustion analyzer is the only way to verify an appliance is venting 100%.
E. Combustion air — no matter how it is supplied— must be verified getting to the burner. Building pressures and leakage can determine if air is getting to the burners. It is quite difficult to visually observe how much air is getting into the burner. Equally hard is trying see whether the air is mixing with the fuel properly.
Combustion analysis is the only true indication of this action.
F. Duct systems and piping systems are unique and even if we have identical buildings with identical systems, they will not operate the same. Air flow and water flow are important when it comes to how many btus are actually transferred. Combustion analysis, by means of flue temperature, is the only way of knowing if these flows are maximizing heat transfer and efficiency
G. Factory settings can only be tested and adjusted for one set of possible field conditions. For safety reasons, most manufacturers take into account the worst conditions that might exist and design their equipment accordingly. This extra safety does sacrifice considerable efficiency when these conditions do not exist.
Clocking meters on gas equipment would be similar to weighing in the charge on A/C units and always using the exact same amount. Combustion analysis is the only method of establishing proper settings in the field.
H. A final consideration is the wear and tear on older equipment. A furnace, boiler, water heater etc., cannot not be mechanically the same after a few years of use. Parts expand and contract, orifices can become pitted, burners can change. Heat exchangers surfaces change on both the fire side and air and water side. Settings year to year have to show some slight changes. Knowing what changes have occurred and what changes we need to make are almost impossible to do without combustion analysis.

Combustion Diagnostic Operating Guidelines
A. Oxygen – O2 Guidelines/Vented Appliances
6% - 9% :This is the most often attainable setting in the field on most conventional atmospheric and induced draft equipment. The lower the O2 the hotter the flame and the longer the heat stays in the heat exchanger for maximum heat transfer.
Lower O2 readings are possible on a few pieces of equipment and these are even better.
Oxygen – O2 Guideline/ Unvented Appliances
Carbon Monoxide – CO Guidelines/Vented Appliances
0ppm – 99ppm: It is recommend to keep the CO readings below 100ppm for maximum safety in the field. Industry standards allow higher amounts but this would leave little margin for error. Reducing CO as low as possible does not necessarily make equipment safer and definitely not more efficienct.
Carbon Monoxide – CO Guidelines/Unvented Appliances
0ppm – 50ppm: In this case the lower the CO the better because it has little or no affect on efficiency.
C. Stack Temperature – Flue Temperature Guidelines
Stack temperature has different ranges for different efficiency ratings and whether we our heating air, water or making steam. Here the flue temperature is more of a differential than just a range.
Stack Temperature - 70% - 75% efficiency: Atmospheric
Air, Water, Steam temperature + 270 degrees minimum
Stack Temperature – 80%+ efficiency: Induced draft
Air, Water, Steam Temperature + 170 degree minimum
Stack Temperature – 90%+ efficiency: condensing
Equal to or greater than Air, Water, Steam Temperature
D. CFM – Airflow
100 CFM per 10,000 btu input 70% - 75% efficiency
130 CFM per 10,000 btu input 80%+ efficiency
150 CFM per 10,000 btu input 90%+ efficiency
Blower speeds are not perfect. The best way to choose the best blower speed is to check the flue temperature.
1 gpm per 10,000 btu output
GPM on newer equipment could vary from the old design standard and can also be verified by checking the flue temperature.

Conclusion: It's impossible to get an appliance to operate at its maximum efficiency and safety without performing a Precision Combustion Analysis. No other measurements can be used to define or adjust the best performance of equipment in the field.

Jim Davis is senior instructor for National Comfort Institute, Sheffield Lake, OH. He specializes in carbon monoxide and combustion, oversees NCI’s CO/combustion content development, and provides technical support in these areas.He can be reached at 800/633-7058, or through the NCI website,

TAGS: Service
Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.