Technical insight ·
How Full-Premix Metal-Fiber Burners Keep NOx Below 30 mg/m³
Many cities now set strict NOx limits for gas-fired equipment. This article explains how OTTMEL’s low-NOx condensing water heaters keep NOx emissions below 30 mg/m³ and what that means for specifiers.
Where NOx comes from
Thermal NOx forms when nitrogen and oxygen in the combustion air react at high flame temperatures. The larger the high-temperature zone and the longer the flue gas stays in it, the more NOx is produced.
How the premix metal-fiber burner limits NOx
- Full premix. Gas and air are mixed in a controlled ratio before they reach the burner, giving even combustion without local hot spots.
- Metal-fiber surface. The flame spreads evenly across the metal-fiber layer on the burner surface. This lowers the flame height and shrinks the high-temperature zone in the combustion chamber.
- Short residence time. Flue gas leaves the high-temperature zone quickly, so less of the nitrogen and oxygen has time to react and form NOx.
OTTMEL’s premix models are specified with NOx below 30 mg/m³. The stainless steel condensing boiler table lists NOx ≤ 25 mg/m³ (class 6).
Efficiency comes from condensing
In the PRO series, flue gas passes through a three-pass heat exchanger. According to OTTMEL’s product literature, the first pass exceeds 1,000 °C and the flue gas leaves at around 45 °C. Recovering the latent heat of the water vapor in the flue gas is what allows thermal efficiency ratings of up to 107%. A built-in neutralizer reduces the acidity of the condensate before it is discharged.
Points to check when specifying
- The NOx limit and test method required by your local authority
- Gas type and supply pressure (published models are rated for natural gas)
- Site altitude: premix combustion adjusts the air-fuel ratio, and OTTMEL literature states suitability up to 3,048 m
- Flue arrangement: balanced flue (sealed intake and exhaust) or forced exhaust, and the equivalent flue length
- Condensate drainage at the installation location
