Navien NFB-301C User's Information Manual page 17

To flush the Heat Exchanger:
A
System
Supply
Circulation Pump
Flushing Kit
1. Disconnect the power cord to turn off the boiler.
2. Close the "C" and "D" valves on the supply and return water
lines.
3. Connect one tube to the "A" valve and place the free end in the
bucket.
4. Connect one of the tubes to the circulation pump outlet and
the return water line at the "B" valve.
5. Connect one tube to the circulation pump inlet and place the
free end in the bucket.
Navien
Manifold
System
B
Pump in
C
Manifold
System
D
System Return
6. Pour the cleaning solution into the bucket.
7. Open both "A" and "B" valves.
8. Turn on the circulation pump and allow the solution to
circulate through the boiler. Refer to the manufacturer's
literature for information about the circulation time.
9. Rinse the cleaning solution from the boiler:
a. Remove the free end of the drain tube from the bucket and
place it in the condensate drain or laundry tub (wherever
the boiler drains).
b. Close the "B" valve and open the "D" valve. Do not open the
"D" valve yet.
c. Allow water to flow through the boiler for 5 minutes.
d. Close the "A" valve and open the "C" valve.
10. Disconnect all tubes and valves.
11. Reconnect the power cord to turn on the boiler.
Maintaining the Boiler 17