Rinnai, Navien, Rheem warranty terms commonly require documented descaling. Skip it and a warranty claim for a clogged heat exchanger is on you. Write the descale date on a piece of tape stuck to the unit cover.
Gear you need
- Submersible pump + hose kit ($50-$90) — Eccotemp EZ-Flush, Calci-Clean, or generic. One-time buy, lasts forever. Don't try to skip this.
- 5-gallon bucket (clean, no detergent residue).
- Two 3/4" washing-machine hoses — usually come with the pump kit.
- 4 gallons white vinegar (5% acetic acid, grocery store) OR CLR Pro Calcium Lime Rust Remover (1 gal concentrate diluted per label). Vinegar = non-toxic, gentler, 60 min. CLR = stronger, 45 min, more careful disposal.
The 7-step procedure
- Power off + gas off. Shut off the unit switch or breaker. Shut off the gas valve. Wait 15 minutes for the heat exchanger to cool.
- Shut off water + isolate. Close the hot-side and cold-side isolation (service) valves — handles labeled HOT/COLD. Open the small drain ports on each service valve over a bucket to relieve pressure.
- Connect hoses. One hose: from the cold-side drain port to the submersible pump (inside the bucket). Second hose: from the hot-side drain port back into the bucket. Now you have a closed loop.
- Fill with descaler. Pour 4 gallons of vinegar (or CLR per label) into the bucket. Open both service-valve drain ports.
- Run the pump 45-60 min. Descaler circulates through the heat exchanger and back to the bucket. You'll see scale chunks accumulate in the bucket — that's the goal. On hard water, the bucket will look like the bottom of a coffee pot.
- Flush with fresh water — the half-step nobody skips. Turn off the pump. Dump the descaler bucket. Disconnect the pump return hose. Open the cold-side isolation valve briefly to flush fresh water through the heat exchanger and out the hot-side hose into the bucket. Run for 5 minutes. Vinegar/CLR residue left in the exchanger eats brass fittings and feeds biofilm — flush thoroughly.
- Reconnect + restore. Close service-valve drain ports. Disconnect hoses. Open both isolation valves. Restore gas, then power. Run a hot-water tap for 30 seconds to confirm flow + temperature.
Also do this while you're in there — clean the inlet filter screen
Cold-side has a small filter screen behind a cap near the cold isolation valve. Unscrew the cap, pull the screen, soak in vinegar 5 minutes, rinse, reinstall. Scale and pipe debris accumulate here and restrict flow — most "low hot-water pressure" complaints I take on tankless units trace to this $0 fix.
Vinegar vs CLR — which one
| Factor | White Vinegar | CLR Pro |
|---|---|---|
| Strength | Mild (5% acetic acid) | Strong |
| Time required | 60 min | 45 min |
| Safe for stainless heat exchangers | Yes | Yes (per CLR Pro spec sheet) |
| Safe for copper heat exchangers | Yes | Yes (but flush thoroughly) |
| Disposal | Drain (it's literally vinegar) | Per label / household chemical disposal |
| Cost (annual descale) | $12 (4 gal grocery store) | $25 (1 gal concentrate) |
| Best for | Soft water, mild scale | Hard water (15+ GPG), thick scale |
Brand-specific notes
- Rinnai (RX, RU, RXP series): isolation valves with built-in drain ports are standard on units sold post-2018. If your install is older, you may need to add isolation-valve kits ($80) before you can descale.
- Navien (NPE, NPN): built-in maintenance kits standard. Same procedure. Some models display a maintenance reminder on the panel after 1,500 hours of runtime.
- Rheem (RTGH): isolation valve kit usually included with new installs. Check for hot/cold labels — Rheem's are sometimes reversed compared to Rinnai conventions.
- Bosch (Greentherm), A.O. Smith, Stiebel Eltron Mini: same general procedure, but verify your specific service-valve port location in the manual before connecting.
FAQ
Vinegar or CLR?
Both work. Vinegar is non-toxic, gentler, 60 min. CLR stronger, 45 min, more disposal care. Hard water (15+ GPG): CLR. Soft water: vinegar.
How often do I descale?
Annual minimum. Every 6 months on hard water (15+ GPG). Warranty terms often require it.
Can I descale without a pump kit?
Not effectively. Static fill doesn't circulate. Pump kit is $50-$90 one-time.
Will descaling fix low hot-water pressure?
Sometimes. Also clean the inlet filter screen (cold side, under cap near isolation valve) — most common cause of low flow.
Related guides
- Best tankless water heaters 2026 — what to buy when descaling is too late
- Tankless vs tank water heater — the comparison that decides the next replacement
- Best smart water softeners 2026 — fix the upstream cause of scale
Editorial standards: Procedure verified against Rinnai, Navien, Rheem service manuals. Reviewed by Al, Building Doctor — IUOE Local 39 Stationary Engineer.