In mod-con boilers, the flue temperature should be equal to or higher than the return water temperature. You can’t heat the water in the initial heat exchanger if the heat exchanger is the same temperature or cooler. Low flue temperature provides higher efficiency readings, but unfortunately, they are misleading.
Tankless water heaters will operate with similar O₂ and CO readings. The difference here is that tankless water heaters heat water that is much cooler than the water in boilers.
Guidelines for Performance
NCI protocols are guidelines for equipment maximum performance and safety. These were created with years of testing equipment under actual installed operation. Usually, the only instructions that come with equipment are for adjusting gas pressure. That provides little information on acceptable operating parameters.
On the other hand, before electronic combustion analyzers, combustion testing was tedious, time-consuming, and not very informative.
By consistently verifying combustion air and venting and focusing on the core measured values — O₂, CO, and temperature — technicians can ensure equipment operates safely and efficiently. Even as newer technologies emerge, maintaining CO below 100 ppm and achieving the lowest stable O₂ levels remain key indicators of proper performance.
Built on decades of real‑world testing, NCI’s protocols help uncover issues that would otherwise go unnoticed and provide the framework for optimizing equipment in any environment. To deliver safer, more reliable results, make these measurement‑driven practices a standard part of every combustion test and adjustment.
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