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  • Writer's pictureTim Wiley

Tankless Water Heaters and their Maintenance

Updated: Jan 20, 2020

Tankless water heaters offer many advantages, but lack of maintenance is not one them.

Gas-fired tankless heater with front panel removed

Why Get A Tankless Water Heater?

Tankless water heaters have four main advantages:

  1. They generally last longer than conventional tank-style water heaters (20-30 years vs 10-15 years for tank-style)

  2. Higher efficiency:  They do not waste energy keeping water hot during idle times

  3. You will never “run out” of hot water (unless of course you lose your energy source)

  4. They take up considerably less space than conventional water heaters

However, the initial cost may be significantly more expensive.  And in the end, you may only break-even on the increased cost vs energy savings, if at all.  It is frankly a personal decision about what is important to you and what your needs are.

Most tankless units use gas for heating the water due to the extremely high BTU input required to quickly transfer energy to the water in the heat exchanger pipes of the unit.  Electric models do exist and there is generally no difference in operational costs.  However, electric tankless water heaters require a lot of electrical capacity.  These units commonly need 150 Amps at 240 Volts.  For some homes, that is their entire electrical service.  So expect to pay a lot of money for electrical work.  You may need to increase your service capacity.  And you will need to run a very high amperage double circuit to the water heater.

Even though they last nearly twice as long as conventional tank-style water heaters, tankless heaters still need periodic maintenance.  Actually, my opinion is that they require annual maintenance.  The primary issues affecting the performance of your tankless water heater are the buildup of scale inside the heat exchanger plumbing, and any accumulation of debris on the cold-water inlet filter.  Scale is the result of the dissolved minerals in hard water coming out of solution.  The hardness of your water will affect how quickly this scale builds up. 

For this maintenance, your system should be installed with two service valves; one at the cold water input and one at the hot water output of your water heater.

See the diagram below for examples of hot and cold service valves.

(diagram from Rheem Manufacturing)

By performing the following annual maintenance, you should avoid most issues with your tankless water heater:

  1. Clean the combustion air intake filter (if installed)

  2. Drain the internal plumbing of the water heater

  3. Remove, clean, and then reinstall the cold-water inlet filter

  4. Flush the internal plumbing of the water heater to clear it with a descaling kit

Before we talk about the annual maintenance in more detail, there is maintenance task recommended for traditional tank-style units that is not required for most tankless units.  Most (if not all) tankless water heaters do not have a sacrificial anode rod that needs replacement every five to six years.  There is no anode rod because there is no tank that needs protection from corrosion.

Annual Maintenance

For all of the steps above, I recommend following the manufacturer’s maintenance instructions of your tankless water heater manufacturer.  For step #4, Flushing the internal plumbing, you will also need to follow the instructions of the kit manufacturer.  I also provide a summary of this procedure below.

Water heater flush

The purpose of using the flush kit is to remove the built-up of scale inside the heat exchanger plumbing of your tankless water heater.  This is accomplished by circulating a weak acid solution through the system for a short period of time (about 45 minutes) to dissolve the scale inside the pipes.  Be sure to put this recurring annual maintenance in your electronic calendar or task/reminders list.

Please see the article labeled "Step-by-Step Tankless Water Heater Flush" for an idea of the general procedure to flush the water heater.

Flush kits

These kits run from $120 to $180 and are required to perform this annual maintenance.  After you have made the initial investment, all you need in the future is the acidic descaler solution that costs about $25.  You can search for a flush kit on the internet that meets your needs.  Here are two examples:

Home Depot:



There are also several informative videos on YouTube on how to flush your tankless water heater.  I really like Matt Risinger’s channel.  Here is one he posted:

Example diagram of a tankless heater flush kit setup:

V1=hot water shutoff, V2=cold water shutoff, V3=hot water service valve, V4=cold water service valve. D1,2,3= supply and return hoses with circulation pump. (diagram from Rheem Manufacturing)


Tankless water heaters offer many advantages, but do not buy one thinking you are going to save money in the long run. Instead, buy one for reasons of longevity, endless hot water, space saving, or lower carbon footprint. But even though they last longer, they require descaling at an appropriate interval consistent with your water hardness. Invest in a descaling kit and you are investing in the worry-free operation of your water heater for years to come.

© 2019 Tim Wiley All Rights Reserved

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