The sheriff to whom the scire facias is given for service shall add to the writ, as parties defendant, all persons, other than those named therein, who may be found in possession of the property described, or any part thereof, and in case no one is found in possession by the sheriff, he shall post a true copy of the writ on the most public part of said property; and he shall add to the said writ the names of any persons, not already named therein, whom he may ascertain to have an interest in the property described, or any part thereof, which writ shall then be further served as follows:
Where the said writ, or the brief notice of the contents thereof, have been advertised as aforesaid, the same shall have the same effect as if the writ had been personally served; and all those named therein, or added thereto, as to whom publication has been made, shall file their affidavit of defense, as required by the said writ, within fifteen days after the date of the last weekly advertisement of the said writ; and all those named therein or added thereto, who have been served as in case of a summons, shall file their affidavit of defense, as required by said writ, within fifteen days after such service. Service of any such writ may be made at any time within three months from the date on which it was issued, but it shall be served and returned at the earliest date possible, and the plaintiff may require its return at any time, whether or not it be actually served.
53 P.S. § 7186