(a) Average heating value for all gas distributed in each municipality or part thereof on a volumetric basis shall have a continuous:
(1) monthly average heating value of not less than 1,000 British thermal units per cubic foot or that amount specified by the company in its tariff schedule;
(2) three-day average heating value of not less than 980 British thermal units per cubic foot or 98 percent of that amount specified by the company in its tariff schedule.
(b) Where therm billing is employed, subdivision (a) of this section does not apply.