Skip to content NREL Buildings Research National Renewable Energy Laboratory (NREL)
Field Test Best Practices: A Resource for Practical Residential Building Science

Main menu

Combustion Analyzer

Combustion analysis generally refers to measuring the temperature of and concentration of various gasses in the combustion products from vented or unvented combustion appliances (furnaces, boilers, water heaters, space heaters, ranges, etc.). These measurements, along with knowledge of the basic fuel chemistry, allow:

  • Measurement of the "thermal efficiency" of the device, i.e. how much energy has been removed from the combustion products by heating air or water
  • Measurement of the combustion mixture (excess air ratio)—the amount of air available for combustion as compared to the minimum (stoichiometric) amount required for complete combustion
  • Measurement of carbon monoxide produced in combustion, important for evaluation of incomplete combustion and identification of hazards
  • Sometimes, measurement of other combustion products including NOx
  • Adjustment of fuel pressure and air/fuel ratio to achieve cleaner and more efficient combustion.

In general, combustion analysis is performed under "steady-state" conditions, after an appliance has reached normal operating temperature. Steady state can be gauged by the temperature of combustion products in the vent system—a temperature that changes less than about 10°F in a minute can be considered steady state.

Most combustion analyzers directly measure oxygen and carbon monoxide concentration, using an internal pump to sample combustion gasses through a wand. Carbon dioxide is generally a calculated value, based on the other measurements and assumed fuel chemistry. Some more sophisticated units also measure NO, NO2, and/or SO2.

Combustion analyzers using sampling pumps have moisture traps to collect condensation, and these traps must be cleared periodically.

Measurement of the steady-state efficiency of condensing-efficiency appliances with conventional analyzers is problematic. At temperatures above the condensation point, the energy removed in the heat exchanger is reflected in a proportionate decrease in combustion products temperature, but when condensation begins, the relationship of temperature to efficiency changes. Some modern efficiency analyzers incorporate a correction in their internal calculations when combustion products fall below the dew point, and some do not. Equipment specifications are not always clear on how this issue is handled, and it may be necessary to talk with a distributor or manufacturer's rep for clarification.

Most analyzers provide a measurement of "air free" carbon monoxide (CO), i.e. the concentration of carbon monoxide adjusted to conditions of no excess air. Removing the influence of excess air added after combustion allows more consistent characterization of incomplete combustion across various appliances. CO measurements should always be reported as either "as measured" or "air free."

Many analyzers also have the capability of measuring draft pressure. Draft pressure is an indicator of the performance of appliance venting systems, but is not required for basic thermal efficiency measurement. It can also be measured using a separate instrument.