This is a very common problem in areas that have hard water. With any type of check valve buildup inside the piece will cause it to Fowell up.
Part# 30024442A
Cost $6.95
You can get this piece in the NPE parts service kit.
It's very important to check this valve when you do your yearly or Quartey service on your customers unites.
It's also important to work the interior/exterior valve.
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Mike

PLEASE USE A LICENSED PLUMBER TO INSTALL AND SERVICE YOU NAVIEN UNIT. FOLLOW ALL STATE AND NATIONAL PLUMBING AND GAS CODES AND TO FILE A PERMIT WITH YOUE LOCAL BUILDING DEPARTMENT.