Yearly Tankless Descaling Checklist: The 8 Steps to a Clean Heat Exchanger

Before you start: three safety warnings

Quick refresher on the unit's vent path and combustion-air clearances before you crack any service port — combustion safety is covered in our FAQ on tankless venting requirements.

The 8-step yearly descaling checklist

Work through the steps in order. The 45–60 minute circulation in step 5 is the actual descaling window — everything else is setup, flush, and restart.

1. Turn off the unit (power/gas) and shut the cold-water isolation valve. Electric: breaker OFF at the panel. Gas: gas cock OFF at the unit, then breaker OFF. Then close the cold-water isolation valve on the cold-water service port.
2. Close the hot-water isolation valve. This isolates the heat exchanger so opening the service ports does not drain hot water back through the home's hot-water lines.
3. Connect a submersible utility pump and 5-gallon bucket to the two service ports. Most residential tankless models have port-labeled caps marked "C" (cold) and "H" (hot), or use an aftermarket service-valve kit. Attach the manufacturer-supplied hoses (or the service-valve hoses) from each port to the pump inlet and a return line back to the bucket, following the arrow direction on the pump.
4. Fill the bucket with 2–3 gallons of food-grade white vinegar (or a citric-acid solution if the manufacturer specifies one for your model — check the owner's manual).
5. Run the pump and circulate the vinegar through the heat exchanger for 45–60 minutes. The vinegar dissolves the accumulated calcium and limescale inside the heat exchanger. Do not run the unit during this window — the pump alone does the circulating.
6. Drain the vinegar back into the bucket, then refill with clean water and circulate for 5–10 minutes to flush any residual vinegar and dissolved scale out of the heat exchanger.
7. Disconnect the pump and hoses, restore the hot- and cold-water isolation valves to the open position, and confirm no leaks at the service ports. Leave the service-port caps off for a minute while you pressurize so any leak at the hose bib is visible — then hand-tighten the caps back on.
8. Restore power/gas, restart the unit, and run a hot tap for 1–2 minutes to bleed any trapped air out of the lines and confirm the unit is firing normally. Check the controller for an error code — if you see anything other than a clean firing confirmation, stop and call a service pro.

When to flush more often than yearly

Annual is the minimum that protects the warranty on most residential tankless units. The right schedule for your home depends on feed-water hardness:

Hard water (>7 grains per gallon). Union County sits in the 5–8 gpg band — on the low end of "hard." If your home pulls from a private well and tests above 7 gpg, drop to a 9-month cadence and consider an inline scale-preventer cartridge on the cold-water feed. Charlotte-Mecklenburg Utilities and Union County Public Works customers are typically fine on the annual cadence.

You see visible scale at the aerators or showerheads. If white crusty buildup shows up at fixtures within months of cleaning, your feed water is on the high end of hard — schedule every 9 months.

The unit fires error codes for "scale buildup" or "flow restriction." That is the controller telling you it detected scale. Do not ignore it — flush immediately and increase the cadence.

When to call a pro instead

Descaling is homeowner-friendly, but some signs mean a licensed service tech should handle it:

The unit shows an error code after descaling. Recirculate clean water through the pump for another 5 minutes, restart, and if the error persists, stop and book a service call. Likely culprits are air trapped in the heat exchanger, a sensor that needs re-calibration, or scale that has migrated into a spot the pump cannot reach.

You have no hot water after restart. Could be a stuck flow sensor, a tripped high-limit switch, or a closed isolation valve that did not fully reopen. A service tech will isolate the cause in a single visit instead of you chasing it.

Recurring scale buildup even after a clean flush. Usually means the inline scale-preventer cartridge is exhausted or was undersized for your feed water. A pro can swap in the right cartridge and verify the descaling is sticking.

Any leak at the service ports — even a small drip — once they are re-capped. Service-port caps wear out and start to weep; a tech can rebuild them in 15 minutes and prevent a future water-heater-room flood.

And — before any work on the unit — confirm your home's venting and combustion-air clearances still meet code. We cover the full venting breakdown in our FAQ on tankless venting requirements.