You circulate a descaling solution through the heat exchanger for about forty-five minutes, using the isolation valves fitted either side of the unit. Shut off the gas or power, close both isolation valves, connect hoses to the service ports, and pump white vinegar or an approved descaler round in a loop from a bucket. Rinse with clean water, clean the inlet screen, put it back in service. About an hour, most of it waiting.
Full sequence below, plus the two things that most often stop people before they start.
What do I need before I start?
- Two washing machine hoses
- A five-gallon bucket
- A small submersible pump
- About four gallons of white vinegar, or a descaler the manufacturer approves
- Isolation valves with service ports either side of the unit — without these it cannot be done at all
What are the steps?
- Shut off the gas at the unit, or the power for an electric unit. Do not skip this.
- Close both isolation valves, hot and cold, so the heater is cut off from the house.
- Connect a hose to each service port. One from the cold port to the pump in the bucket, one from the hot port back into the bucket.
- Fill the bucket with about four gallons of white vinegar or approved descaler.
- Open both service port valves and run the pump. Circulate for around forty-five minutes to an hour.
- Stop the pump and dump the vinegar. Refill the bucket with clean water and circulate for another five to ten minutes to rinse.
- Close the service ports, disconnect the hoses.
- Clean the cold water inlet screen filter. This is the step people skip and it is often where the actual restriction is.
- Reopen the isolation valves, run a hot tap to purge the air, then restore gas or power.
Scale is what kills a heat exchanger. The exchanger is the expensive part.
Why does it matter more on a tankless than on a tank heater?
Because a tankless heat exchanger is compact and it scales up faster than a tank does, and the exchanger is the part you cannot afford to lose. A tank heater collects sediment on the bottom and grumbles about it for years. A tankless unit runs water through narrow passages at high heat, which is exactly the condition scale likes.
That is also the argument for treating the water going into it. If you are putting in a tankless system, dealing with scale at the source is a reasonable thing to do at the same time. See water filtration and treatment and tankless water heaters.
What if my unit has no service valves?
Then it cannot be flushed, and fitting a set is the first job.
Isolation valves with service ports either side of the unit are what make annual maintenance possible at all. Plenty of tankless units get installed without them to save a little on the day, and the owner finds out years later when the first service is due. It is the same principle we apply on water treatment installs — isolation valves both sides and a bypass loop, so the thing can be serviced later without cutting pipe.
Use white vinegar or a manufacturer-approved descaler and nothing else. Household drain chemicals and strong acids will damage a heat exchanger and will void the warranty, which turns an hour of maintenance into a replacement unit. This is not a place to improvise.
How often should it be done?
Once a year is the usual recommendation, and more often in hard water. Check your own manufacturer's requirement, because on some units documented annual maintenance is a condition of the warranty — and a warranty claim is exactly the moment somebody asks for that documentation.
When should I stop and call somebody?
If there are no service valves. If the unit is somewhere awkward. If it is throwing a fault code — a flush is maintenance, not a repair, and it will not clear a fault. Or if you would rather it was documented properly for the warranty.
And if the unit is still under warranty and something has actually failed, the manufacturer decides what gets replaced, not us. On a Mont Belvieu Rinnai the fault turned out to be the thermal fuse. Rinnai handle warranty work through their own service department, so we got on a video call with their technician, went through the unit with them, they made the call and shipped the part, and we fitted it. You do not sit on hold trying to describe a fault you cannot see.
Worth knowing before you buy one, too: parts availability matters more than brand. We sell and install State, Navien and Noritz, and for those we diagnose the unit ourselves, call for the part, and it is usually in the Houston area. On that Rinnai, parts took two days to arrive and the customer was without hot water for three days.
Common questions
How do you flush a tankless water heater?
You circulate a descaling solution through the heat exchanger for about forty-five minutes using the isolation valves fitted either side of the unit. Shut off the gas or power, close both isolation valves, connect hoses to the service ports, and pump white vinegar or a manufacturer-approved descaler round in a loop from a bucket. Then rinse with clean water, clean the inlet screen, and put it back in service.
How often should a tankless water heater be flushed?
Once a year is the usual recommendation, and more often in hard water. Check what your manufacturer says, because on some units documented annual maintenance is a condition of the warranty. Scale is the thing that kills a tankless heat exchanger, and the exchanger is the expensive part.
What do I need to do it myself?
Two washing machine hoses, a five-gallon bucket, a small submersible pump, and about four gallons of white vinegar or an approved descaler. The whole job is roughly an hour, most of which is waiting while the pump circulates.
What if my unit has no service valves?
Then it cannot be flushed without fitting them, and that is the first job. Isolation valves with service ports either side of the unit are what make annual maintenance possible at all. If yours were not installed with them, having a set fitted is worth doing before anything else.
Can I use something stronger than vinegar?
Use white vinegar or a descaler the manufacturer approves, and nothing else. Household drain chemicals and strong acids will damage the heat exchanger and will void your warranty, which turns a maintenance job into a replacement.
Should I just have it serviced instead?
If the unit has no service valves, if it is in an awkward spot, if it is throwing a fault code, or if you would rather it was documented for the warranty, have it done. If your tankless is still under warranty and something has actually failed, the manufacturer decides what gets replaced - we get on the video call with their technician, get the part approved and shipped, and fit it.
Last updated · written by a licensed master plumber, MPL 38224