Q&A for How to Maintain a Tankless Water Heater

Your heat exchanger has cracked and needs to be replaced. You can buy just that part, or replace the whole tankless heater. Replacing just the part is less expensive (for parts), but I replaced the whole unit because the water had leaked onto the electronics, and who knows when that would cause another failure.

Yes, as it's important to properly maintain a tankless water heater. The heating elements get extremely hot and bake calcium and other deposits on the heating units quickly. Filtering even clean municipal water should be done. Neglecting to maintain a tankless heater results in a very short life span. They're a very sensitive piece of machinery and require you to maintain them. Install a filter before the heater and clean them twice a year for a long lifespan. It is required. You're cleaning baked on junk off the heating element more so than dirt or debris.

No, the used vinegar has dissolved solids in it and is no longer "virgin." Discard the used vinegar and always use fresh vinegar for flushing.

No. You can switch them out when cooking, but the cider vinegar has added coloring which is not good for your heater. It may leave stains, while white vinegar will not.

Check the temperature fuses (thermostats) on each heating unit. Check for continuity through them using a multimeter.

Yes. Since your tankless water heater is most likely the source of all your drinking water and bathing water, using chemical cleaning solutions may be extremely harmful to your health. Use 2.5 gallons (9.46 liters) of undiluted white vinegar to clean your tankless water heater at all times instead of chemical solutions.

There is a minimum water flow rate required at your hot water devices (faucets, dishwasher, etc.) to turn the heater on. For some reason (such as water-saving faucets, plugged faucet filters, restriction(s) in the lines or lines too small to the faucets, plugged or partially-closed faucet isolation valves, etc.), the flow rate at these taps is probably too low to turn the heater on. Get a one-gallon pail and fill it from one of these taps, timing how long it takes to fill it, and then compare that to the heater's low-flow spec.

Yes, but it’s not needed for that long. Try one hour, then another hour with fresh vinegar the next day.

There could be many problems. First, how is water heated? What type of device is used? There can be something like safety problem (temperature could be so high, so it needs some kind of stop and "restart"). You'll have to call the super.

Yes. It will penetrate the metal. It is a chemical you do not want in your bath water, either.

Usually annually to stay within warranty. If you have hard water, you may need to do it bi-annually. Check with the manufacturer to be certain.

It should be about 30-45 minutes, but this depends on your method of cleaning (e.g., a larger pump will flush faster). The accompanying manual should tell you how long to flush.