In the absence of written instructions, a disputed bill or a payment arrangement, payments received by a public utility which are insufficient to pay a balance due both for prior service and for service billed during the current billing period shall first be applied to the balance due for prior service.
52 Pa. Code § 56.24
The provisions of this §56.24 amended under the Public Utility Code, 66 Pa.C.S. § § 501, 1301, 1401-1419, 1501 and 1509.