Please follow us at:

www.testoHVAC.blogspot.com for the latest news, guides, and videos for your testing needs.

The answer is simple even though the combustion process is often not adjustable, the combustion process is dynamic, many factors of the installation and operation can and do affect the combustion process. With out verifying the combustion process is safe, stable and within the manufacturers or industry standard guidelines you are putting yourself and your customers at risk. Simply, what you cannot see can hurt you so the only way to know what you don’t know is to measure.

Furnace Basics you may have never considered.

Natural Draft
Natural draft or atmospheric appliances are relatively simple and as safe as modern appliances. In the case of an atmospheric burner, the combustion air is drawn in by buoyancy of the heated flue gasses and mixes with the fuel gas as it enters the combustion chamber. The fuel/air mixture burned in the combustion chamber quickly releases its heat (primarily by radiation) to the heat transfer surfaces surrounding it, and the hot flue gas escape through a draft hood into the flue.  The role of the draft hood is to prevent excessive draft or a back draft in the venting system from affecting the combustion process and also to provide dilution air to assure flue gasses do not condense in the chimney.  Because flue gasses have natural buoyancy, it is not necessary for the appliance to be connected directly to the flue to create flow through the heat exchanger. By design, as warm less dense air rises through the heat exchanger and moves toward the vent connector, fresh air will be required to replace it. Flue gas movement through the heat exchanger precisely controls the introduction of secondary air.  So long as draft is provided at the vent connector, (which means the flue pipe must be cemented in at the chimney) the low-pressure zone created in the draft hood will direct all of the flue gasses and the proper amount of dilution air into the vent pipe and chimney.  If a negative pressure is created in the appliance combustion/venting zone that is greater than the draft provided at the vent connector, spillage will occur.  The flue gasses will always move to the area of lowest pressure.  This will occur whether a draft hood or barometric damper is installed; hence the requirement of spill switches on both to improve safety. 

Induced Draft/Forces Draft
The modern gas furnace is as susceptible to causing or having combustion problems as older atmospheric equipment, if not directly, often through guilt by association. Several factors including a more powerful circulation blower, tightly controlled excess air requirements, complicated venting requirements, combination venting issues and low mass complicated heat exchangers have all had an impact on performance and safety of these appliances. Remember, a new or correctly operating appliance can easily affect the operation of the combined vented hot water tank, or create depressurization within the home back drafting atmospheric appliances, fireplaces, wood stoves or pulling auto exhaust from an attached garage.  

Many of the safeties incorporated into modern gas furnaces are required due to more complex heat exchanger and venting designs. Efficiency comes at a cost. Heat transfer has been increased by convection and residence time of the flue gasses in the heat exchanger.  Because of these complex designs, the flue gasses no longer naturally flow through but are rather drawn through with a draft assist fan (draft induced). The function of in inducer on a mid efficiency furnace is to create a negative pressure on the heat exchanger, The pressure on the discharge side of the fan will also remain negative when connected to a properly designed venting system, as the inducer should never overcome the negative pressure created by the chimney draft. On condensing models, the induced draft fan performs two functions, creating a negative pressure on the heat exchanger and a positive pressure on the exhaust system to expel the byproducts of combustion.   Pressure switches, roll out switches and fusible links all provide protection from a loss of draft or flow through the heat exchanger and ultimately reduce the risk of fire. Remember, a flame will travel toward its oxygen source. If combustion air is not entering the heat exchanger with the fuel, the flame will travel outside the combustion chamber in search of oxygen. Rollout switches and fusible links are designed to stop fuel flow by way of the gas valve should such an event occur. Draft induced appliances pose danger whether they are direct vented, or combined vented with an atmospheric appliance like a hot water tank. Even vented in PVC via sealed combustion, an improperly operating or vented appliance can pose a grave danger. Remember each appliance is part of a complete engineered system, which also functions as an independent system often referred to as the building envelope.  In short, there is no gas or oil appliance; atmospheric, induced, ventless or other that does not require combustion testing to assure safe and reliable operation.

Ventilation Air Test
No combustion test should be completed until the venting system has been inspected, and safe removal of flue gasses can be assured. Building pressure testing can be a complicated science. Everything from stack effect to temperature, to prevailing wind can affect the pressures within a building. At the end of the day, you can have all the pressure measurements you want, but when the rubber meets the road, what’s really important is does the venting system really operate properly under even the worst conditions that could occur with the home.

The Ventilation Air Test, or Worst Case Depressurization Test should be performed every year on all atmospheric and induced draft appliances after an ambient CO test is performed. Additionally ambient CO should be monitored while performing the ventilation air test. Remember, CO can come from ANY fossil fuel burning appliance, so use caution while testing. The procedure for the ventilation air test is outlined in the International National Fuel Gas Code. (ANSI Standard Z223.1)

This procedure should be performed prior to any attempt at modification of the appliance or of the installation.  This includes servicing, clean and checks, and/or mechanical changes. It should also be preformed after any new installation including that of a sealed combustion appliance.

If it is determined there is a condition that could result in unsafe operation, the appliance should be removed from service (shut off) and the owner advised of the unsafe condition.  If there is not sufficient air for combustion and/or ventilation, the homeowner and/or technician will be at risk by operating the appliance under worst-case conditions.

This test is performed to insure that the building into which you are going to install, or have installed, a fossil fuel appliance has enough ventilation/infiltration air to replace the air used in the combustion and venting process. As homeowners are constantly making changes to the home, (caulking windows, adding weather stripping) this test should be performed on an annual basis.
(For additional information performing the ventilation air test, please consult the International Fuel and Gas Code or the Testo Combustion Guide)

Understanding that each appliance requires combustion testing is the first step in providing a reasonable standard of care to your customers. Performing the ventilation air test is critical to assure safe operation. Additionally, testing generates revenue and hopefully profit. Thousands of systems operate either constantly or intermittently with combustion/ventilation air deficiencies. Finding and correcting these problems not only increase customer safety, but also provides an additional revenue stream and service that should be part of every companies service procedures.  

Sidebar
If you ever end up in a court of law, you will get a quick lesson in what standards are. Most tasks we perform in the HVAC industry can have a standard of care attached to it. A standard is nothing more than the usual and customary practices in the delivery of products or services within a particular business sector or generally accepted operating procedures, practices and requirements defined by national trade associations, and state and local government laws relevant to utility service.  When it comes down to it, with all equipment inspection there is an implied warranty. The service technician must be aware that it has been upheld in court that unless the consumer is notified in writing that there is real, or potential problem; the person performing the inspection has determined that the appliance is operating safely and correctly. If you work in this industry in the capacity of a service technician, or installer, you are considered to be an expert. Furthermore, if it can be proved the technician (expert) was negligent in his or her job, they could be liable for the consequences.

Being negligent is the failure to exercise the standard of care that a reasonably prudent person would have exercised in a similar situation in order to protect others against unreasonable risk of harm. Determination of the appropriate standard of care is an issue of law. In general, a “reasonable person” standard of care is applied. Whether a defendant has met that standard of care is an issue of fact to be determined by the jury. Because “standards” that are adopted by associations or other nongovernmental entities may represent a consensus regarding what a reasonable person in a particular industry would do, such standards may be helpful to the trier of fact in deciding whether the defendant has met the standard of care that is due in a particular situation. Hansen
v. Abrasive Engineering and Manufacturing, Inc, 856 P.2d 625, 628 (Sup. Ct. Or. 1993).

Homes are dynamic and need to be treated as so.  While finding and fixing thermal bypasses and air leaks may seem like the right thing to do, it can have deadly consequences if the combustion equipment in the home is not accounted for and tested prior to, during, and after repairs are made. The old saying one man’s junk is another man’s treasure could not ring more true considering that the thermal bypass or air leaks (junk) you’re sealing might be providing all or some of the air (treasure) required for safe combustion of the fossil fuel appliances. Do not assume that sealed combustion appliances are safe either, as building pressures can easily overcome even the best designed appliances under the right (wrong) conditions. 

When it comes to home construction, there is no such thing as too tight, actually the tighter the home the better. Remember though if you build or seal tight you must ventilate right for safe combustion. Mechanical ventilation is preferred to natural infiltration for several reasons, but primarily because it is controlled rather than a variable.  Air is needed for combustion, dilution, and ventilation. A ventilation air test is needed after any repairs are made, and after mechanical ventilation is added. CO and stack O₂ should be monitored prior to, during and after repairs are made. The question is “What do we need to know about safety and testing for CO and the measurements and tools needed to assure the safety of all involved?”.

Jim White, senior researcher with the Canadian Mortgage Housing Corporation, has a straightforward summary of this point: “Both people and houses need continuous ventilation to stay healthy, but natural ventilation does not provide continuous ventilation. Therefore, we need good mechanical ventilation. Good ventilation means balanced ventilation, and balanced ventilation works best in tight houses. Therefore, we need tight houses with good balanced ventilation systems in them. Anything less is just not good enough.”

Tools Required for Testing

If you are considering or working in the building science, weatherization or HVAC industry, at minimum the following tools are required for your safety and to determine the safe operation of combustion appliances. While you may not be qualified to correct the combustion problems you may create, you must at minimum need to know if they exist. If you are not qualified or trained to make repairs to a heating system consult a qualified and trained HVAC contractor preferably who has been formally trained with dealing with issues of Carbon Monoxide (CO) and combustion testing.

Tools of the trade:

• Blower door to determine natural ventilation rates and aid in identifying thermal bypasses and points of air infiltration.
• Personal CO meter for your safety
• Combustion Analyzer (with CO and O₂) for measuring and documenting ambient CO levels CO AirFree (COAF) levels or appliances, COAF stack levels for furnaces and boilers, and draft.
• Draft Gauge (included in most combustion analyzers) to assure that minimum draft requirements are kept for all vented appliances.

CO (Carbon Monoxide)

All fossil fuel appliances including those burning natural gas, propane, fuel oil, bio-fuels kerosene, wood, wood pellets, or even corn can produce dangerous levels of  combustion byproducts if not set to burn properly.  During combustion, carbon in the fuel oxidizes through a series of reactions to form carbon dioxide (CO2). However, 100 percent conversion of carbon to CO₂ is rarely achieved under field conditions and some carbon only oxidizes to the intermediate step, carbon monoxide or CO.

Carbon monoxide is a pollutant that is readily absorbed in the body and can impair the oxygen-carrying capacity of the blood (hemoglobin). Impairment of the body’s hemoglobin results in less oxygen to the brain, heart, and tissues. Even short-term over exposure to carbon monoxide can be critical or fatal to people with heart and lung diseases, the young or the elderly. It may also cause headaches and dizziness and other significant medical problems in healthy people. Studies are now linking long term exposure to low levels of CO increased heart and other health problems especially among the young, elderly, and breathing compromised. 

Ambient CO Testing and Personal CO meters

Enough cannot be said about the need for personal safety and CO. The fact that most first responders don Self Contained Breathing Apparatus (SCBA) when levels exceed 35ppm CO should be enough to convince you that low levels of CO can quickly become high levels as you move toward the source. Personal CO detection with an audible alarm set at or below 35ppm should be worn at all times on your person, preferably one like the testo 317-3 which does not need to be zeroed in ambient air. Because CO is a colorless odorless gas it can quickly overcome anyone, often mimicking flu like symptoms prompting the victim to sit down for a minute rather than to get out of the contaminated area. If ambient CO levels exceed 35 ppm (as low as 9 ppm in some jurisdictions) during testing, shut down (isolate the fuel source) the appliance immediately. If the appliance is out of arm’s length and cannot be immediately be shut down, the best thing to do is to evacuate the premises then ventilate,  in that order. Shut off the fuel source outside the premises so you are not exposed to dangerous CO levels as you approach the appliance.  Typically CO only enters the home and or is produced because the flue gasses cannot exit, so depressurization and draft problems are the first things you need to investigate.

COAF (Carbon Monoxide Air Free) Stack CO Testing

When testing in the stack, a combustion analyzer like the Testo 327 or 330 with a CO an O₂ sensor is recommended. CO and O2 levels should remain stable. CO levels should not exceed 400 ppm COAF at anytime during operation and typically appliances should operate below 50 ppm COAF.  The graph below (done with a Testo 330 equipped with EasyHeat PC software) shows how a typical combustion appliance should operate.

 Look closely at the blue line, the O₂ soon after light off stabilizes, and the COAF (ppm CO undiluted, the green line) operates close to 0 ppm during operation. The rising COAF at shutdown in this case is only seen due to the rising O₂ and its affect on the COAF calculation. COAF and O₂ both remain stable during operation of the burner and during the ventilation air testing process.  

In the graph right, a typical burner plagued by combustion air problems is shown. As can be seen, the undiluted CO (COAF) was stable at 0 ppm CO indicating what appears to be an excellent operating characteristic. The telltale issue to operational problems can be seen in the instability of the O2 during operation. As can be seen from the graph, when the O2 dropped below 8% in this case the COAF went from 0 to 3500 ppm COAF almost instantaneously. Due rapidly rising CO the technician quickly pulled the probe to avoid damage to the CO sensor through saturation so the actual levels might have been considerably higher.  This graph shows why a combustion analyzer and not a single gas CO meter is required for combustion testing as a rising CO or a dropping O2 both indicate potential CO problems.

Carbon monoxide is typically measured in parts per million (PPM); the number of CO molecules present in a million molecules of air.

CO is also measured and referenced as CO Air Free or ppm CO undiluted. This is a unit of measurement designed to compensate for the excess air to the burner and is only used to express CO levels in a flue gas sample as opposed to ambient air testing.

Because CO can be diluted by excess air, a COAF air free measurement is needed for an apple to apples comparison of equipment operation. A COAF measurement requires an O2 sensor and requires a two gas combustion analyzer for testing. Simply put, the COAF measurement is a standard for stack and appliance CO measurements. COAF readings should always be below 400 ppm and in reality COAF readings over 50 ppm indicate combustion problems. Typical industry standards dictate no burner be left in operation with levels over 100 ppm even though the ANSI standard is 400 PPM COAF. Levels below 50 ppm are easily achievable. 

In today’s equipment, high levels of carbon monoxide emissions primarily result from combustion dilution and ventilation air problems and or incomplete combustion due to poor burner maintenance or firing conditions. Examples would include an improper air-to-fuel ratio or possibly a misaligned burner.

No US standards exist for CO levels in indoor air.  The U.S. National Ambient Air Quality Standards for outdoor air are 9 ppm (40,000 micrograms per cubic meter) for 8 hours, and 35 ppm for 1 hour (time weighted). 

No CO (0 ppm) is the best level in the home.  This cannot always be achieved due to smokers in the home and/or appliances like stoves or unvented space heaters that produce small but acceptable levels of CO during operation. Even an operating electric ovens will produce levels of CO as the Carbon in the food residue is oxidized. When CO is present in the home, the source should be determined and corrective action taken if required.  The goal is to assure occupant safety and minimize the occupants’ exposure. 

The local authority having jurisdiction should be consulted when determining the maximum safe level allowed in the home before shutting down the appliance and or making it inoperable.  An appliance with rising CO or dropping O₂ should always be shut down no matter how low CO production is at the time of testing.  Rising CO problems are usually the result of improper venting and/or lack of combustion air, and a dropping O₂ in the stack is a tell tale sign that combustion problems are imminent when the oxygen levels are no longer sufficient to support proper combustion. 

Ambient CO Limits (Recommended)

0 ppm           Desirable level of CO in a home

1-9 ppm        Acceptable short term levels within a home.  If there are no smokers, investigation is recommended. These levels will be measured above outdoor ambient levels in most cases because the CO instrument has been zeroed in outdoor air.

10-35 ppm    Advise occupants, check for symptoms, (slight headache, tiredness, dizziness, and nausea or flu like symptoms.) check all unvented and vented appliances, including the furnace hot water tank and or boiler, check for other sources including attached garages or small engine operation

50 ppm         Maximum concentration for continuous exposure in any 8 hour period. (National Occupational standard)

36-99 ppm    Recommend fresh air, check for symptoms, ventilate the space, recommend medical attention

100+ PPM    Evacuate the home (including yourself!) and contact emergency medical services (911).  Do not attempt to ventilate the space.  Short-term exposure to these levels can cause permanent physical damage.

400 ppm       Frontal headaches within 1-2 hours life threatening in 3 hours

800 ppm       Nausea and convulsions, death within 2 hours

1600 ppm     Nausea within 20 minutes and death within one hour

12,800 ppm Death within 1-3 minutes

Maximum CO Levels in Equipment

Vented (Note CO reading must remain stable and are measured on an air-free basis)

• 400 ppm (CO-Air Free: CO-AF) Stack ANSI Z 21.1
• 100 ppm CO Stack recommended

ALWAYS FOLLOW REQUIREMENTS of AUTHORITY HAVING JURSIDICTION

Unvented appliances

• 30-50 ppm stable
• Less than 10 ppm recommended
• Ovens Less than 300 ppm (100-300 ppm) install CO detector if left in operation.

Draft

Appliances need to exhaust the byproducts of combustion outside the dwelling. This is accomplished through natural or forced draft. Air sealing a home can result in changes in draft characteristics that may be undesirable or dangerous.  Depressurization of the CAZ (Combustion Air Zone) and the actual appliance draft need to be verified to fall within acceptable limits for outdoor air temperature as outlined in BPI and RESNET standards and or within limits outlined in the Testo Combustion Applications Guide. Again, local jurisdiction may have authority. 

The moral of the story is: “If you find it and fix it, test it for combustion safety.” Finding problems is only the first part of fixing the problem. Don’t let your excitement for fixing air leaks get the best of you and hurt your clients. 

REFERNCES

BPI Standards: www.BPI.org

RESNET Standards: www. RENET.us

Testo Combustion Applications Guide: www.testo.us/appguide

Why don’t technicians make good measurements? The answers are plentiful and almost painful. Not enough time. Hard to get the same results twice. The equipment “works” without proper setup even if it doesn’t work right. No idea what to measure; primarily I find technicians have never been instructed how and why. Let’s face it; we have service technicians with 20 years of experience and those with one year of experience 20 times. Many keep repeating the same incorrect procedures and processes with substandard measurement instrumentation and techniques over and over again each time expecting a different or better result. One might just as easily chalk all this up to insanity.

The result; high rates of system failure, unnecessary warranty claims, poor system performance, loss of confidence by the consumer and technician, callbacks, and dissatisfied customers. A chain of events leading to a mess! According to a recent EPA Energy Star presentation a quoted study shows that up to 70% of residential A/C systems installed today have improper airflow, and additionally 74% of A/C systems have improper charge. While our industry is in need of repair, I am afraid there is no quick fix to provide the needed answers. Furthermore, neither will capture hoods, vane anemometers, hotwires or pitot tubes, digital gauges or combustion analyzers fix the problem, by themselves. Quite simply having the tools or knowing that you need them to perform the required work is only part of the solution. More importantly the technician must know how to use them, and how to evaluate the measured results. The key lies in technician and consumer education.

Tens of thousands of new and existing pieces of heating and cooling equipment are installed and serviced each year without ever knowing if they are operating at or delivering their designed and/or rated capacity. Operational measurements are made to verify temperature rise and drops, airflow, superheat and or subcooling, but often the total system performance is never quantified.  Equipment is sized for a minimum delivery, yet usually never tested after installation for actual performance verification. Measurements are made without an expected result, improper techniques are used to make measurements, and antiquated tools are used for critical measurements. It is like attempting to measure a distance in feet with your car’s odometer.

Consider, if you do not know what the airflow measurement should be what difference does it make what it is? For example, to select a register, we need to consider not only air volume, but also the face velocity and throw.  Technicians are not often privy to information regarding the register selection such as room air requirements, intended application or design. As can be seen from past experience, many times a proper heat loss and gain are not performed during the design phase, or when modifications are made to the ducting system, or when replacement equipment is selected in the field. I would care to bet register selection is more often based on cost, color and size. Measuring CFM delivered at the register is only a small part of the equation; face velocity and throw are just as integral parts of information. If the heat never gets off of the ceiling or the cooling never gets off of the floor, a correct quantity will not provide comfort do to stratification of the air in the room.

Technicians should not get in the habit of making estimations whenever a true measurement can be made. With the cost effective solutions in instrumentation that are available, technicians not only need to make an investment in technology, but also in its application and proper use. Again, making measurements without knowledge of the expected results is a valueless proposition. Measuring is the most critical part of all service and sales calls.  Before any system commissioning is complete; any evaluation of existing equipment is made, or during routine service, accurate measurements of operating conditions and system performance must be made.  When replacing existing equipment, a complete evaluation of the ducting system including verification of proper airflow at the registers is warranted.

HVAC performance testing setup and service is principally based upon accurate measurement of temperature, pressure, airflow, humidity, and O2 and CO when combustion safety is concerned. All of these measurements bear direct correlation to equipment operation performance and safety. Each segment and process requires test instruments and measurement probes specifically designed for the task at hand. Airflow for example can be measured with a hotwire, Pitot tube, or mini vane anemometer. Each probe has a specific application it is best suited for. No one is better than the other, just better for a specific measuring application.  Where a vane is an ideal tool for residential airflow measurement, it would not work as well as a Pitot tube in a small diameter duct with an exceptionally high velocity of air.  Refrigerant charge is more critical than ever for proper equipment capacity and efficiency. Digital gauges with accuracy of 0.5% full scale are now desirable. Manufacturers are not requiring a range for subcooling on TXV equipped systems; they are requiring a very specific result. Quantifying system performance requires not only high accuracy measurements but also high resolution as 1 difference in wet bulb equates to ¼ ton error in a cooling capacity calculation. Depressurization or pressurization of the combustion air zone or outlying rooms is also considered part of the total system performance evaluation as more powerful blowers can negatively impact these parameters. Pressure imbalances are measured in Pascal’s or 1/25 of an inch of water column.  

Digital instruments and new sensor technology is just better, Sensors convert the measured parameter directly, temperature can be measured by contact or non-contact sensing, no mechanical gearing drag, or large masses. Parallax and fluttering of a needle are a thing of the past.  Digital has resulted in greater precision, readability, and speed of response. A variety of probes for every application is available and displays and data management capabilities never conceivable with analog counterparts are available today. Today’s instrumentation is smaller, smarter, easier to use, more reliable, and supported with applications. Well worth the investment. 

Equipment malfunction is like a crime scene. Expert knowledge is required to determine how the equipment was operated, how it was installed, to decipher the original design specifications, or changes that took place in the environment after the installation was performed. The quandary however is why the criminal is many times investigating the crime with the same tools with which the crime was originally committed. If we don’t take the initiative now I am afraid it will be the equivalent of Nancy Drew competing with CSI. As the ANSI standard for QI/QC (Quality Installation/ Quality Contractor) moves forward and building scientists continue to cross the line into the realm of HVAC performance evaluation, it may be time you do some investigation on your own as to what the future will hold.  It’s time to look at what you do and how you do it, after all it’s your reputation. How is your equipment performing?  Don’t know? Just wait long enough and I am sure someone else will tell you.