When the heating value of the gas distributed in the system is under the control of the utility, the following limits shall apply: Daily Btu. The average heating value on any one day may not exceed or fall below the total heating value (see COMAR 20.55.03.03B(2)) by more than 5 percent, except that the heating value may be increased by more than 5 percent during peak shaving or emergency operations in order to maintain the proper combustibility of the gas supplied to the customer. Monthly Average Btu. The monthly average heating value shall be not less than the total heating value filed in the utility's tariff.