83 Ill. Adm. Code 501.530
Heating Value
Section 501.530 Heating Value
Each utility shall exercise in good faith all reasonable
best efforts to maintain a consistent heating value for the natural gas it
provides to its customers. A utility shall take what actions it deems necessary
to avoid daily heating value fluctuations in excess of 5% for the gas it
provides to individual customers. A utility may allow the daily heating value
of gas delivered to its customer to fluctuate in excess of 5% when a failure to
take the gas would result in a supply shortage, or when, for the purpose of
meeting the requirements of a peak load or an emergency, a utility makes use of
a reserve or emergency supply, such as liquefied petroleum gas.