https://www.fcwa.org then click on Customer Service, then Rules & Regulations, then MAINTENANCE OF SERVICE CONNECTION and final read section F. Your house must be constructed prior to December 4, 1972.
F. Pressure Reducing Valves
Customers that experience greater water pressure than they desire may elect to install a pressure reducing valve within their plumbing system. All such work must be in accordance with applicable Fairfax County Plumbing Codes. The purchase, installation, and maintenance of pressure reducing valves will be the responsibility of the Customer except that under the following circumstances the Authority will provide a pressure reducing valve to be installed and maintained at the Customer's expense:
1. If a residence was constructed prior to December 4, 1972, the date on which the Fairfax County Plumbing Code was amended to require installation of pressure reducing valves, and water pressure to that residence exceeds 80 psi; or
2. The Authority has made changes to its distribution system which increased the water pressure after a residence was constructed and the pressure exceeds 80 psi.