Flushing the heat exchanger is the most important Navien maintenance task. Scale insulates the heat exchanger, reduces output, increases gas use, and eventually triggers Code 016 (overheat). Plan 90 minutes once a year.
Tools needed
- Submersible pump kit (½" or ¾" outlet) — $40-60 on Amazon
- Two 5-gallon buckets
- 4 gallons of white vinegar OR 1 gallon of commercial descaler (Navien-branded; CLR PRO and Hercules Sizzle also work)
- Two ¾" garden hoses
- Crescent wrench
Prep — NPE tankless
- Turn off gas at the unit's shutoff valve
- Turn off the cold-water inlet isolation valve (blue T-handle on the service kit)
- Open a hot tap inside the house to release pressure
- Allow 10 minutes to cool
Prep — combi boiler (NCB, NFC)
Combi flush is the same procedure for the DHW heat exchanger. The hydronic-loop side does NOT get flushed with vinegar — that's a separate chemical cleaning process if needed. For routine maintenance, only the DHW side gets descaled.
- Set combi to DHW-only mode (disable heating temporarily)
- Turn off gas at the unit shutoff valve
- Turn off cold-water inlet isolation valve
- Open hot tap to release pressure
Connect the pump
- Attach one hose to inlet isolation valve service port (typically ½" threaded)
- Drop the hose into the bucket containing descaler
- Attach the second hose to outlet isolation valve service port
- Drop into the same bucket (closed loop)
- Drop the submersible pump into the bucket and connect it to the inlet hose
Run the descale
- Open both service ports (small lever perpendicular to pipe = open)
- Close the inlet and outlet main valves (handles parallel to pipe = closed)
- Plug in the pump
- Circulate vinegar/descaler for 45-60 minutes. Fluid will discolor and may foam — scale dissolving
Rinse
- Unplug pump
- Close inlet service port
- Disconnect inlet hose
- Open inlet main valve briefly to flush fresh water through and out the outlet hose into a second bucket (5-10 gallons)
- Close outlet service port, disconnect outlet hose
- Open both main valves
Restart
- Open a hot tap and run until water is steady (no air bubbles)
- Turn gas back on
- Power-cycle the unit if any error code appears
- Restore combi to normal heating mode
- Verify operation
Vinegar vs commercial descaler
Vinegar is cheap and slow; commercial descaler (Navien-branded, CLR PRO, Hercules Sizzle) is faster, stronger, food-safe. For routine annual: vinegar fine. For first-time on a scaled unit: commercial.
How often
Hard water (>11 gpg) — annually. Soft water (<7 gpg) — every 2 years. With a softener — every 3 years.
Bottom line
Document the descale (date + photo of bucket water) for warranty defense. See full maintenance checklist. For combi boilers, the hydronic side has separate (less-frequent) cleaning requirements.