Article published in Home Energy Magazine
Issue Dated: May/June 1995
Written by: Rob deKieffer

Heating contractors, inspectors, and energy auditors all have different approaches to inspecting combustion appliances. Combustion problems come in various sizes and shapes, and individual tests may not by themselves prove if the house is actually safe. Those of us who work in buildings must be able to understand the signs, and know what and when to test, in order to ensure that a small oversight does not result in a long-term health problem for our client.

Knowing some simple facts about combustion will make analysis easier. Most common fuels have carbon (C) and hydrogen (H) locked up and ready to heat. Add a consistent ignition source and some oxygen and we have combustion. Breaking apart the carbon-hydrogen bond produces heat and releases carbon and hydrogen to find a new bond. On a good day, this will produce water (H₂0), carbon dioxide (CO₂) and a bunch of hot air. This means that the carbon had to find some oxygen. To do this, the combustion products must stay hot, and oxygen has to be available. If there is insufficient oxygen, the carbon has no choice but to remain carbon or become carbon monoxide (CO).

Sometimes the rules just do not make sense. For instance, houses with gas ranges dump all of their combustion products directly into the kitchen, so why all of the concern about these other appliances? In Colorado, an average of more than six people a year are killed and 50 people accidently poisoned by carbon monoxide produced from furnaces, gas appliances, and kerosene heaters. These numbers represent only the cases that are properly diagnosed and reported to the state.

The problems with combustion appliances are not simply with the devices themselves, but how they work within the building The safety of the units depends on their installation, operation, and maintenance. Other concerns such as competing air sources, house tightness, and effects of remodeling all can be important to the overall operation of the system.

In developing a program that will check for the safety of combustion appliances, three questions need to be answered:

• Who are you trying to keep safe?
• What are you trying to keep them safe from?
• What is the appropriate level of testing?

Once these seemingly simple questions are answered and the goals of the safety program are defined, standards and procedures can be established to ensure the program will provide the desired level of protection.

The first and most immediate concern should be the safety of the staff going into buildings and conducting tests. A good safety check involves gathering information from the client, visually inspecting the building, and running specific physical tests. Of these, the visual identification of potential safety problems is the most important.

The immediate problems can be defined as those that pose imminent danger. Two groups of concerns are air quality and fire hazard.

Carbon monoxide detection systems have advanced significantly in recent years. Digital equipment can be used to provide an indication of elevated ambient levels of CO. Sensors should be turned on and calibrated in a noncontaminated environment, typically outside the building. When preparing to test all of the combustion devices, an ambient reading should be taken and be close to zero. If the initial readings are measurable--0-9 parts per million (ppm) depending on the meter--the source needs to be identified. In no case should exposure exceed 35 ppm.

Gas leaks pose a potential immediate threat of fire or explosion. Leaks can be detected by smell, gas leak detection, or fluid and electronic gas leak detectors. A combination of techniques can be used to identify, and ensure the repair of, any leaks prior to additional testing.

Flames unexpectedly coming out the front of the appliance ("roll-out") indicates serious combustion problems. Visual indications of roll-out will be seen on the appliance body, in the form of black or rusted areas in front of the burners, burnt wires, and carbon deposits. In these cases, a client may mention that they occasionally hear a "boom" when the furnace turns on.

We don't want our staff breathing combustion products for extended periods of time. Combustion products can enter into the living area of the house through disconnected venting systems or systems which are not venting properly. Vents should be visually checked for integrity. All draft diverters should be tested with smoke to ensure that all of the combustion products are leaving through the vent. Testing the draft of the appliance will assist in determining the adequacy of the venting system, but spillage is an immediate hazard.

There are additional factors that affect the testing for these immediate hazards. Our intent is to evaluate the house for combustion safety under conditions that are most conducive to creating a problem. To do this we need to have the house set up in that condition. This means turning on air handling equipment that may move air to the outside (dryers, bath fans, kitchen fans) and closing the doors and windows.

A protocol must be established and followed. Basic combustion tests should be performed on a consistent time schedule. Observation of the appliance's cycle provides indications of operational components. The observation time should be at least five minutes after ignition. This allows the device time to establish a draft and reflects how it will operate on a consistent basis. The protocol also needs to address different configurations of appliances. If a furnace and water heater are connected to a common vent, both appliances should be tested independently and together.

Client Safety

The second level of concern is for the short- and long-term safety of your client. Visual inspection of the house and appliances is extremely important in this analysis.

Are there any signs of carbon monoxide being created? Is there any carbon in the burner area, flue or vent? How are the flames burning? Are there any visible signs of a problem, such as flames burning erratically, no flames visible on part of the burner, weak flames, or white tips on the flames? Regardless of the visual inspection, a test must be performed to verify that there is no CO in the combustion gases. The sample should be taken from each flue (exhaust port), before additional dilution air is added to the gases. In a furnace with four burners, at least four tests should be taken. In a water heater there should be a test taken on both sides of the internal baffle. On a stove, a test should be taken in the oven vent and above each burner. Sealed combustion units can and should be tested as well.

Our field experience has shown that problems with most units that create CO in excess of 25 ppm in the flue can be corrected. Most field standards are higher than this (less than 100-200 ppm). One of the key components of this step is to determine why CO is being created, since CO is a symptom of something being wrong with the building or unit.

Draft and Venting

The draft of the appliance measures the power of the venting system to exhaust. The measurement of the draft is coupled with a visual inspection of the venting system to determine the probability that all of the combustion gases are getting out of the structure. If the draft is measured in cold weather, it can provide an indication of the ability of the appliance to exhaust in warmer weather (if the draft is weak in cold weather, it will be weaker in hot weather). The standard we use was developed from both technical analysis and field testing .

The source of the combustion air for the appliance must be adequate and not come from a prohibited location. (Having a combustion appliance draw air from, and be connected to, a bedroom is not a good idea.) The first test for adequacy of the combustion air has already been completed: no CO, no spillage, and sufficient draft. This test is to identify problems you might have if you change the building tightness. Having an open or not sufficiently connected return air system is a primary concern. Open returns provide a significant depressurization source in the immediate vicinity of the unit.

Standards range from simply ensuring that the appliances work under the worst case conditions, to installing combustion air that meets current building code.

Figure 1. Schematic of a combustion furnace.	Figure 2. Air paths in a combustion furnace.

Most appliances are equipped with safety devices to prohibit them from overheating and from allowing fuel into the device if there is no ignition source. You should check the limits if you are doing anything that might affect the heat transfer. This can include cleaning blowers, repairing ducts, insulating boiler pipes, and so on. The gas supply to a forced air furnace should be turned off before the plenum temperature reaches 275deg.F. If this occurs during the normal operation of the unit, it can be indicative of an overheating problem and should be corrected. Checking the pilot safety and 100% closure of the main burner valve can also be done as a precautionary procedure.

Examining appliances for a breach in the heat exchanger is potentially significant, but of lesser importance than the previous tests. Checking for cracks is done by examining the flames for interference when the blower is operating and by direct inspection of the heat exchanger.

There are a number of other code and safety issues that programs can address. These are primarily addressed due to a specific program bias or documented regional safety problems. These include inspecting and repairing gas lines due to galvanized pipe, copper pipe or soldered flex lines, testing fuses, replacing venting due to improper materials or insufficient clearance, gas pressure, and other items that do not meet current building code.

Those who work in programs that provide any significant service inside a building should be knowledgeable of the unanticipated consequences of their actions. Changing a furnace filter, changing a client's use of the thermostat, fixing a bath fan, or hanging a door, can all potentially effect the operation of combustion devices.

The long-term liability will sit with those agencies that ignore the facts. The facts show that the air inside buildings is connected to a myriad of systems. These systems use the air in different ways. If you want to avoid liability problems with building failure or combustion problems, you need to test the building and the appliances. Testing must identify problems that exist before any work is done. Tests must also be done on completion of work to ensure that the systems that were in place were not adversely affected.

Other combustion issues have a potential impact on liability such as unvented combustion appliances and kerosene heaters. At Sun Power we will not work on the building unless we can eliminate those types of combustion appliances.

Setting Your Standards

For every program there needs to be a written set of standards. These standards provide the basis for any work done and give management a basis to evaluate the coverage and liability exposure. An example of such a standard is: No furnace or water heater may have over 100 ppm carbon monoxide in the flue. This establishes the quality threshold which needs to be maintained.

Written policies and procedures must be in place to establish how these standards are to be met--or, more importantly, what is not wanted. As in the previous example, the policies will establish how much money to spend to correct the problem, what to do if the problem cannot be fixed, whom to contact, and so forth. The procedures will provide the guidance on what to do first.

A quality assurance program will help staff follow the policies and procedures in pursuit of the quality standard. Effective communication and training rely on having a quality assurance program which can change and have direct input into improving the staff's abilities. In the case of CO production, we want to make sure that we have found the true cause of the problem and that we have corrected the CO problem as inexpensively as possible. We do not necessarily need to inspect the unit to determine if the problem was properly diagnosed. Systems should be in place to determine if staff understand the process and a training component should be available to improve abilities.

Focused evaluation makes sure that the program intended to be delivered is really in place. Many programs adopt standards that do not relate to their building stock, climate, or staff's ability. A good evaluation will show the strengths and weaknesses in the system.

Evaluation also needs to be used to examine technical issues such as: do we need to simulate a fireplace draft when testing the other appliances? Should we test the ability of the house and appliances to backdraft the fireplace? How do the results of the testing vary if the furnace is hot?