Question: what is the difference between low nox and ultra low nox?
Answer: Low NOx vs Ultra-Low NOx The difference between Low NOx and Ultra-Low NOx is determined by local EPA standards and based on the type of heating equipment. For example, It's not uncommon to see an Ultra-Low NOx water heater that emits less than 10 nanograms of NO2 per joule of heat output (10 ng/J).