Tooling up for Home Performance: Part 2 of 3: Combustion Safety Testing

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This article is the second in a three part series where we discuss the types of diagnostic equipment necessary for Home Performance work.

Last month we covered the 5 national standards governing much of the home performance work in the US ( as well as an overview of pressure measurements. (NOTE: an astute reader found some missing info and we have since updated that table.)

This time we will dig into the products used in combustion testing.

As we all know, combustion is the process of converting the chemical energy trapped in fossil fuel safely and efficiently into usable heat energy. Let’s tighten our focus to the two adverbs: safely and efficiently.

The goal of efficient combustion is to extract the heat energy trapped in the fuel. The age-old indicator of reaching of achieving this peak is attaining the maximum CO2 (Carbon Dioxide) available in the fuel.

Without getting too “chemical” on you, each different fuel (actually each different source of fuel) contains a different amount of hydrogen and carbon molecules – hence the term hydrocarbon fuel, which will produce different levels of maximum CO2.

For example the testo handbook on combustion states: (

Fuel MAX CO2
Methane 11.9%
Propane 13.8%
Fuel Oil 15.4%
Coal (soft)  18.4%
Coal (hard)  19.1%

Basically the more carbon, the more energy there is present in the fuel.

Interestingly, none of the standards we reviewed actually call out for combustion efficiency testing. I suspect this will change in the next couple of years, as a new performance standard will be released in the USA. It is called AHRI Standard 1260P (P for proposed): Performance Rating of Flue Gas Combustion Analyzers.
A group of combustion analyzer manufacturers have been hard at work for the last few years creating this standard as members of the AirConditioning. Heating and Refrigeration Institute (AHRI). This standard will allow for consistent end-user results from combustion analyzers that align with the standard. Consistent test results are a goal of Home Performance testing, such that all stakeholders can clearly identify conformance without doubt.

Equipment sales data shows natural gas is the current “fuel of choice” in homes across the US. Yet, that is not the only reason Gas Leakage tests are spelled out in the major standards. You see, all fuels must convert to a gaseous form before combustion.

In the simplest sense, Fuel Oil (a liquid fuel) must be atomized or heated to the point when the gases emitted can combust. Even wood does not burn directly. Temperatures of 500-600 degrees Fahrenheit cause the process of pyrolysis to release combustible (eg. ready to burn) gaseous vapors from wood creating those relaxing flames that dance before your eyes.

Gaseous fuels such as natural gas, (which is primarily methane) and propane are already in a combustible state just waiting for the right fuel to air ratio and a source of ignition to burst into flame; something a log of wood or chunk of coal will not do.

Hence the need for combustible gas leak detectors, to be sure that these fuel gas levels do not get out of hand and cause a hazard. Additionally, gaseous fuels can leak and move in the air to hidden locations to concentrate, something wood or coal cannot do.

Of course, not all gas leak detectors are created equally. Some are rated intrinsically safe, which means that they have been designed and tested not to make things go “BOOM” when the air and fuel ratio are ripe for combustion.

Other gas leak detectors actually give you a concentration reading and alarm, on top of the typical “beep beep”, more here, less there, Geiger-counter type detection mode. The new BPI 1200 standard actually calls for intrinsically safe gas leak detectors that can alarm/alert when concentrations are growing unsafe (eg at 10% of the Lower Explosive Limit (LEL) of gas concentration).

Carbon Monoxide testing also comes in two flavors: Ambient and Flue gas. Ambient CO detection is fairly straight forward, with feature rich, high quality meters having evolved to reasonable price points due to a growing market and commercial competition in response to Federal Regulations promulgated by the Occupational Health and Safety Administration.

In fact, recent rulings have residential attics and crawlspaces being considered as “confined spaces” that need to stay in OSHA compliance. (Perhaps the topic of another HP blog post!)

In the home performance industry, Carbon Monoxide (CO) is most usually produced as a result of a poorly installed, serviced, tuned or vented space or water-heating appliances. Yet, you should never let your guard down, as CO may come from other sources.

Flue gas CO meters are more advanced than ambient CO meters in that the hot, wet, dirty and cross-contaminant laden flue gas must be “prepped” before it is delivered to the CO sensor – which is a close cousin to the ambient CO sensor.

A flue gas CO analyzer requires a heat resistant probe to get into the center of the stack and a pump to move the flue gas sample through a particulate filter, a water trap (flue gas is typically 100% rH) and then (in the best case) a Nitric Oxide Filter (NOx filter) before being analyzed by the sensor for CO concentration.

Presently, the BPI 1200 standard is the only one that specifies a NOx filter must be used to conduct the CO test. You see, Nitric Oxide gas, present in pretty much all residential appliances in tens of PPMs, cross interferes with a CO sensor by a factor of 2 to 1, unless it is filtered out.

EG. If the flue gas contains 90 PPM CO and 60 PPM NOx, an analyzer without a NOx filter will show a CO reading of 120 PPM CO, possibly causing a technician to make a wrong “pass-fail“ decision on the appliance.

Beyond this, the BPI 1200 standard has aligned with the appliance manufacturers by calling out that CO-AirFree tests must be conducted. Appliances are design to meet national standard levels of CO-AirFree (ANSI Z21.1) and it is best to evaluate them in a similar manner in the field.

Coming up in our last segment we will discuss Optional/ Advanced testing equipment used in Home Performance.

Posted In: Building Performance, Residential Buildings

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