How to Flush a Tankless Water Heater (and How Often)

Repipe Solutions is a licensed Houston plumbing company, and knowing how to flush a tankless water heater is the single most useful thing a tankless owner can learn. A flush (also called descaling) dissolves the mineral scale that builds up inside the heat exchanger. This guide covers why it matters in Houston, how often to do it, the steps, brand differences, cost, and when a flush will not fix the problem.

Why a Tankless Water Heater Flush Matters in Houston

A tankless unit heats water as it passes through a narrow heat exchanger. Every gallon leaves a little calcium and magnesium behind. Navien compares it to the lime that builds up in a coffee maker, and says those deposits can erode and clog the walls of the heat exchanger, reducing efficiency, reducing flow, and potentially causing premature failure.

Houston water carries enough of those minerals to matter. The City of Houston’s 2025 Water Quality Report lists total hardness in the main system at 43.5 to 229 ppm, with an average of 135.5 ppm. The USGS classifies 121 to 180 mg/L as hard and anything over 180 as very hard, so the city’s average falls in the hard range. The system blends lake water and groundwater, so readings vary across town. If your home is served by a MUD or another utility, its own annual water quality report will show your number.

How Often to Flush a Tankless Water Heater

Rinnai, Navien, and Rheem all recommend at least once a year. Navien adds that the interval gets shorter as three things go up: the mineral content of your water, the temperature the unit is set to, and how much hot water your household uses.

Do not wait for symptoms. By the time the water cycles hot and cold, scale has been building for a while.

What You Need Before You Start

• Isolation valves (service valves) on the unit. These are the valves with small hose ports on the hot and cold lines under the heater. Rheem’s flush kit instructions assume they are already installed and say to consult a plumbing professional if they are not. Without them, you cannot flush the unit.
• A small submersible or utility pump, two hoses, and a 5-gallon bucket. These are sold together as a tankless water heater flush kit. HomeAdvisor prices a kit at $100 to $120.
• Undiluted white vinegar or an approved descaler. Rinnai calls for 4 gallons of vinegar. Rheem’s kit calls for 2 gallons. See the next section before choosing.
• Gloves, eye protection, and towels. Water in the lines may still be hot, and the service ports can spray when opened.
• 60 to 90 minutes. Rheem says the process can take an hour or more.

Flushing a Tankless Water Heater With Vinegar or Descaler?

This is where the manufacturers disagree, so follow the one whose name is on your unit.

• Rinnai says to use undiluted white vinegar and to never use chemical solutions, because the water passing through the heater is the water you drink and bathe in.
• Rheem’s flush kit instructions say to use virgin, food grade white vinegar and “do not use any other flushing solution.”
• Navien allows either household white vinegar or an approved tankless descaling solution, and says to follow all safety requirements and directions for any chemical.

Vinegar is cheap and safe but slow on heavy scale. A second round of fresh vinegar helps on badly scaled units, as Family Handyman describes. Never dilute it.

How to Flush a Tankless Water Heater, Step by Step

These steps follow the published Rinnai and Rheem procedures. Check your own manual first, since power steps differ by model.

1. Shut off the energy source. For a gas unit, turn off the gas at the supply valve and unplug the heater. For an electric unit, switch off the breaker. The gas must stay off for the whole flush.
2. Close the hot and cold isolation valves. This keeps city water out of the heater during the flush.
3. Relieve pressure. Remove the caps on the service ports slowly. Rinnai warns there may be pressure behind them, so confirm the hot valve is fully closed first. Keep the small rubber seals inside the caps.
4. Connect the hoses. Run one hose from the pump outlet to the cold side service port. Run the second hose from the hot side service port back into the bucket.
5. Add the vinegar. Pour undiluted white vinegar into the bucket and make sure the pump and the return hose end are submerged.
6. Open the service ports and start the pump. The vinegar now loops from the bucket, through the heat exchanger, and back. Let it circulate for 45 to 60 minutes. Navien sets a minimum of 45 minutes.
7. Drain and dump. Turn off the pump, let the vinegar drain back into the bucket, and discard it.
8. Rinse with clean water. Route the hot side hose to a drain or a second bucket, close the cold service port, and open only the cold isolation valve so fresh water runs through the heater. Rinnai and Rheem call for about 5 minutes. Navien calls for at least 10. When in doubt, run it longer.
9. Clean the inlet filter. Close the cold valve, remove the screen filter at the cold water inlet, rinse out the debris, and reinstall it.
10. Restore service. Close the service ports, remove the hoses, and replace the caps snugly without over-tightening. Open both isolation valves, then open a hot tap until the sputtering stops. Rinnai says air can take 2 or 3 minutes to clear.
11. Turn the unit back on. Restore gas and power, run a hot tap, and confirm steady temperature. Check the valves for drips.

Our professional tankless descaling service uses the same core process and adds a full inspection.

Flushing a Rinnai or Navien Tankless Water Heater: Brand Notes

Sources: Rinnai flushing procedure, Rinnai Technical Bulletin 188, Navien, and the Rheem flush kit instructions.

Flushing a Rinnai tankless water heater: Rinnai’s SENSEI bulletin says the power must actually be unplugged, since the on/off button will not position the internal valves. Leave the inlet filter in place during the flush, and if pump flow drops noticeably, pause and clean the filter.

Flushing a Navien tankless water heater: Navien condensing units have a condensate trap, and in Houston homes it tends to clog sooner than Navien’s own schedule expects, so have it checked at every flush. On ComfortFlow models, the buffer tank and recirculation pump should be looked at during the flush as well. If you are still choosing a brand, our Navien vs Rinnai vs Rheem comparison covers the differences.

Tankless Water Heater Flush Cost

Angi notes that tankless units cost $75 to $100 more to flush than tank heaters because they are more complex. A kit costs about as much as one professional visit. What a DIY flush does not include is the burner, flame rod, venting, and condensate inspection, or a dated service record. Rheem itself lists flushing the heat exchanger and checking sensors and igniters among the tasks best left to a licensed professional.

When Flushing Won’t Fix It

A flush removes scale. It does not fix a failed part. If you have flushed the unit and still see any of the following, the problem is somewhere else:

• An ignition or flame loss code that returns after the flush (Navien E003 or E012, Rinnai 11 or 12, Rheem 11 or 13)
• A venting or air supply code, such as Rinnai code 10
• A condensate drain fault, such as Rheem code 29
• No hot water at all, even with gas and power on
• A drip at the unit or the valves that does not stop after the caps are tightened
• Water that still cycles hot and cold after a full vinegar circulation and rinse

Those usually point to a flame rod, flow sensor, gas supply, venting, or a heat exchanger already damaged by scale. Our tankless water heater repair service in Houston starts with the code history and diagnosis before any parts. For basic checks before you call, see why a tankless water heater stops making hot water. For leaks, see our water heater leaking guide. If the unit is old and the repair quote is large, our tankless installation cost guide shows what replacement involves.

Call a plumber instead of flushing it yourself if: your unit has no isolation valves, you smell gas, you are not comfortable shutting off the gas supply, the service port caps are seized, or the unit is under warranty and you want a dated service record on file.

Frequently Asked Questions

Can I use CLR or a store-bought descaler instead of vinegar?

Only if your manufacturer allows it. Navien permits approved tankless descaling solutions. Rinnai says never to use chemical solutions, and Rheem’s kit says to use vinegar only. Check your manual first.

How much vinegar does it take to flush a tankless water heater?

Rinnai calls for 4 gallons of undiluted white vinegar. Rheem’s flush kit calls for 2 gallons. Use enough to keep the pump and return hose submerged, and never water it down.

Why did the vinegar come out dark or cloudy?

That is dissolved scale. Family Handyman notes the vinegar darkens when there is significant buildup. If it is very dark, run a second round with fresh vinegar.

Can I save the vinegar for next time?

No. Rinnai’s technical bulletin says to discard used vinegar and not reuse it for future flushes.

My tankless unit has no service valves. Can it still be flushed?

Not until valves are added. Rheem’s instructions say to consult a plumbing professional if there are no service valves. We quote a valve kit upfront, and it makes every future flush simpler.

Does a water softener mean I can skip the flush?

It can stretch the interval, since Navien ties flushing frequency to the mineral content of the water. You should still clean the inlet filter and have the unit checked at least once a year.

Need Your Tankless Water Heater Flushed in Houston?

Repipe Solutions flushes and inspects Navien, Rinnai, and Rheem tankless units across Greater Houston, and leaves a written service report for your warranty records. Every price is quoted upfront, with no trip charge. See our tankless water heater maintenance service in Houston or contact Repipe Solutions to schedule a visit.

Featured photo: “Tankless water heaters” by Downtowngal via Openverse, licensed under CC BY-SA 4.0, converted to WebP and shared under the same license.