Q88. If a combustion unit with a heat input capacity less than 250 mmBtu per hour burns both natural gas and a liquid fuel, and the quantities are known can Tier 1 be used for the natural gas calculations and Tier 2 for the liquid or must Tier 2 be used for both?

A88. The tiering requirements are applied to each fuel in a combustion unit, not to the combustion unit as a whole.  Therefore, in this example, different tier calculation methods can be used for the natural gas and the liquid fuel combusted in the same combustion unit. Also note that Tier 1 could be used for the liquid fuel if it provided less than 10 percent of the annual heat input for the unit, provided Tier 4 is not required and you do not routinely perform fuel sampling and analysis for the fuel high heat value (HHV) or routinely receive the results of HHV sampling and analysis from the fuel supplier at the minimum frequency specified in §98.34(a) (see §98.33(b)(1)(iv) and (viii)).