72 Pa. Stat. § 5860.611
Service of the rule shall be made in the same manner as writs of scire facias are served in this Commonwealth. When service cannot be made in the county where the rule was granted, the sheriff of the county shall deputize the sheriff of any other county in this Commonwealth, where service can be made. If service of the rule cannot be made in this Commonwealth, then the rule shall be served on the person named in the rule by the sheriff, by sending him, by registered mail, return receipt requested, postage prepaid, at least fifteen (15) days before the return day of the rule, a true and attested copy thereof, addressed to such person's last known post office address. The sheriff shall attach to his return, the return receipts, and if the person named in the rule has refused to accept the registered mail or cannot be found at his last known address, shall attach evidence thereof. This shall constitute sufficient service under this act.
72 P.S. § 5860.611