Section R307-208-3 - Visible Emission Standard(1) Visible emissions for all outdoor wood boilers shall be limited to a shade or density no darker than 20% opacity as measured by EPA Method 9, except for the following: (a) An initial fifteen minute start-up period; and(b) A period of fifteen minutes in any three-hour period in which emissions may exceed the 20% opacity limitation for refueling.Utah Admin. Code R307-208-3