How to Flush & Descale a Bosch Tankless or Combi

Descaling is the most important Bosch maintenance task for Greentherm tankless and Greenstar combi. Scale insulates the heat exchanger, reduces output, raises gas bills, and eventually triggers Code A9 (overheat).

Tools needed

• Submersible pump kit ($40-60 on Amazon)
• Two 5-gallon buckets
• 4 gallons of white vinegar OR 1 gallon of commercial descaler (Hercules Sizzle, CLR PRO; Bosch-branded available)
• Two ¾" garden hoses
• Crescent wrench

Prep

1. Turn off gas at the unit's shutoff valve
2. Close cold-water and hot-water isolation valves
3. Open a hot tap to release residual pressure
4. Allow 10 minutes for the unit to cool

Connect the pump

1. Attach one hose to the cold-side isolation valve service port
2. Drop hose into bucket with descaler
3. Attach the second hose to the hot-side service port; drop into same bucket (closed loop)
4. Place submersible pump in bucket; connect to cold-side hose

Run the descale

1. Open both service ports; verify main isolation valves are closed
2. Plug in pump
3. Circulate 45-60 minutes
4. Fluid will discolor and may foam — scale dissolving

Rinse

1. Unplug pump; close cold-side service port; disconnect cold hose
2. Open cold-water main valve briefly to flush fresh water through and out the hot-side hose into a clean bucket — 5-10 gallons until clear
3. Close hot-side service port; disconnect hot hose
4. Open both main valves

Restart

1. Open a hot tap; run until flow is steady (no air)
2. Turn gas back on
3. Power-cycle the unit if any error code appears
4. Verify normal operation

Greenstar combi specifics

For Greenstar combi: descale ONLY the DHW heat exchanger (the same procedure as Greentherm). The hydronic-loop side has separate maintenance — air purge, dirt separator clean, pressure check. Don't run vinegar through the hydronic loop.

Vinegar vs commercial descaler

Vinegar is cheaper but slower. Commercial descaler (Hercules Sizzle, CLR PRO, Bosch-branded) is faster, stronger, food-safe. For routine annual: vinegar fine. For first-time descale 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 with dated photos for warranty defense. The 15-year Greentherm heat exchanger warranty is the longest in the residential gas tankless category — but it's defensible only with documented annual descaling.