Tankless Water Heater Scale Prevention: Softener, Conditioner or Flush?

Tankless Water Heater Scale Prevention: The Short Answer

Every tankless water heater on hard water needs a plan for scale, but not every one needs new equipment. Start with two facts: your water's hardness number and what your heater's manual says about water quality and flushing. If your water is within the manual's limits, flushing the heater on the schedule it gives may be all the protection you need. If it isn't, add treatment in front of the heater. A water softener takes the hardness out. A salt-free conditioner leaves the minerals in but keeps most of them from sticking. Either way, keep flushing.

Which one fits depends on how hard your water is, whether iron or sediment is in the mix, and whether you want soft water at every tap or just a protected heater. The rest of this guide walks through each choice, what the testing behind them actually shows, and where the equipment should go.

Why Tankless Heaters Are So Prone to Scale

Hard water carries dissolved calcium and magnesium. According to the U.S. Geological Survey, when hard water is heated in a home water heater, calcium carbonate can come out of the water as solid deposits, and that scale can shorten the life of the equipment and raise the cost of heating water.

A tankless heater doesn't store hot water. It heats water in a heat exchanger as it flows through, so every gallon you use passes over those hot surfaces. A guide to gas-fired tankless heaters from the Department of Energy's Building America Solution Center notes that in hard water areas, scale can build up on the piping in the heat exchanger. That cuts the heater's capacity and efficiency, and in extreme cases it can cause the heater to fail completely.

You usually won't see it happening. What you notice is a heater that seems to struggle: less hot water than it used to deliver, or trouble holding a steady temperature. Your manual lists the symptoms and service codes for your specific model.

Step 1: Find Your Hardness Number

Hardness is reported two ways. Labs and utilities usually give it in milligrams per liter (mg/L) or parts per million (ppm) as calcium carbonate. Softener sizing uses grains per gallon (gpg). Penn State Extension puts one grain per gallon at about 17 mg/L, so divide a mg/L result by 17 to get a close gpg figure.

The categories are the USGS's general guidelines. The gpg column is our conversion using that 17 mg/L figure, rounded, so treat the edges as approximate.

Where to get the number:

• City water: look for a hardness line in your utility's annual water quality report. Our guide to reading your water quality report explains the units and dates. Not every utility lists hardness, so call and ask if yours doesn't.
• Well water: you'll need to test it. Here's how to test water hardness at home. If you're testing a well anyway, check iron and manganese too, because they change which treatment will work.

Step 2: Check What Your Heater's Manual Allows

Your heater's installation and owner's manuals are the rulebook here, not a generic chart. Look for three things:

1. A water quality section. If the manual has one, note any maximum hardness and any limits for other minerals it sets for the heater.
2. The flushing or descaling instructions. These give the interval and the method the maker wants you to use.
3. The warranty terms. Read how water quality and maintenance are treated before scale becomes a problem, not after.

Can't find the paper copy? Search the maker's support site for your model number. Then compare the manual's limit with your hardness number. That comparison, not the brand of your heater or your neighbor's advice, tells you whether flushing alone is enough.

Softener, Conditioner or Flushing: How They Compare

Here's how the three approaches line up. They aren't mutually exclusive: flushing belongs in every plan, and treatment decides how much scale there is to flush.

Water Softener

A softener trades hardness minerals for sodium as water passes through a resin bed, then flushes the captured calcium and magnesium out of the resin with salt brine during regeneration. (Our explainer on how water softeners work covers the chemistry.) Because it actually removes the minerals, it gives you the strongest protection, and the rest of the house gets softened water too.

The trade-offs are real. The University of Minnesota Water Resources Center notes that softening can extend appliance life, prevent scale on fixtures, cut energy use in water heaters and reduce the cleaning products you need, but the salt discharged during regeneration adds chloride that harms plants and animals in lakes and streams. A softener also needs floor space, a drain and regular salt.

Salt-Free Scale-Prevention Conditioner

Salt-free conditioners don't soften water. The best-studied type, template-assisted crystallization, uses treated media to convert dissolved hardness into tiny scale-resistant crystals instead of removing it, as the University of Nebraska-Lincoln explains. The minerals stay in your water. What changes is how readily they form scale.

Two limits matter before you choose one:

• The water has to be fairly clean going in. The University of Minnesota says these systems generally need relatively clean input water, and a filter may be needed first for iron, manganese and sediment. The Crystal Quest® Eaglesorb ES3 anti-scale cartridge, for example, calls for pretreatment when iron, manganese, copper, oil, hydrogen sulfide or phosphate are present, or when chlorine is above 3 ppm.
• Very hard water can outrun it. For our Salt-Free Water Conditioner, hardness of 25 gpg or higher may call for pretreatment or additional media.

For a closer look at where salt-free systems shine and where they don't, see do salt-free water softeners work.

Flushing (Descaling) the Heater

Flushing removes scale that's already there. The Building America guide describes the usual setup: isolation valves, often sold as a flush kit, let you close the heater off from the rest of the plumbing, and a small pump circulates a mild acid solution such as vinegar through the heat exchanger. It also notes that many manufacturers recommend a professional cleaning every year.

Flushing doesn't prevent anything, though. Scale starts rebuilding as soon as hard water flows again, and the testing below suggests a descaled heater may not get all of its efficiency back. Use the method, solution and interval in your manual, follow its shutdown steps for gas or power before you start, and if you're not comfortable doing that, have a plumber do it.

What the Testing Shows (and What It Doesn't)

Two studies get quoted a lot on this subject, often without their fine print.

Softened versus hard water in tankless heaters. In a study for the Water Quality Research Foundation, Battelle ran five instantaneous gas water heaters on unsoftened well water of about 26 gpg and five on softened water. According to the Water Quality Association's summary, the units on hard water had to be delimed at the equivalent of 1.6 years of service. Their average efficiency had dropped from 80 percent to 72 percent, and after deliming it recovered only to about 77 percent. The units on softened water performed well through the whole test.

Keep the context in mind. The foundation is the research arm of the water treatment industry's trade association. The test was accelerated, so "1.6 years" is a calculated equivalent rather than time in a real home, and the well water also carried iron. The takeaway that holds up: very hard water scales a tankless heater quickly, and descaling doesn't fully undo the damage.

Salt-free devices in a heated-water test. Researchers at Arizona State University tested softener alternatives for the WateReuse Research Foundation using a German protocol that runs a water heater and measures the scale that forms; their webinar slides describe twenty-day runs. Per the project summary, template-assisted crystallization reduced scale formation by more than 88 percent. Electromagnetic and electrically induced precipitation devices managed about 50 percent.

That's encouraging for the technology, with limits. It was a short lab test, not years inside a tankless heat exchanger. It used Arizona waters. And it tells you about a type of device, not any particular brand, ours included.

A more cautious federal view. A 2013 Pacific Northwest National Laboratory review for the Department of Energy, written before the Arizona State results were released, concluded that for tankless heaters installed where water is hard, where there are very few other options, a softener may be the only viable way to improve performance. It also pointed out that the Battelle test used extremely high water temperatures, use rates and hardness. Read together, the evidence favors a softener when you want the most protection, supports salt-free media as a real but less proven option, and keeps flushing in the plan either way.

Which Option Fits Your Home

Once you have your hardness number and your manual's limits, most homes land in one of these spots:

• Your hardness is within the manual's limit. Flush on the manual's schedule. Keep the isolation valves accessible and working. Revisit the decision if your water source changes or your utility's report shows hardness climbing.
• You're over the limit, you want soft water everywhere, or your water is very hard. A whole-house softener gives the most protection and the side benefits of soft water. If your water also carries iron or manganese, plan for them with whoever sizes the system. Keep flushing on schedule.
• You're over the limit, your water is otherwise clean, and you'd rather skip salt. A salt-free conditioner is a reasonable fit, especially if a drain is hard to reach or you'd rather not handle salt or drink softened water. Don't skip the flushes, and look at the flush water: if scale is still coming out heavily, the treatment isn't keeping up.
• You're choosing between the two and can't decide. Our softener vs. conditioner comparison goes deeper on cost and everyday feel. If you want to see what hard water does to the rest of your appliances, see how hard water damages appliances.

Where the Treatment Goes

Placement decides what gets protected, and sizing decides whether the heater still gets enough water.

Whole House or Heater Only

A system at the point of entry, where water comes into the house, treats everything downstream, including the cold line that feeds the heater. That's the usual spot for a softener, and it's how most people install a whole-house conditioner. A cartridge on the heater's cold-water inlet protects just the heater and costs less, but your fixtures and other appliances still get untreated water.

Either way, treatment has to reach the water before it enters the heat exchanger, so a heater-only cartridge belongs on the cold-water line that feeds the heater, not on the hot outlet.

Match the Flow Rating to the Heater

Every treatment device has a flow rating, and it has to keep up with the most hot water your household draws at once. Find your heater's maximum flow in its manual and compare:

• The Crystal Quest® Whole House Water Softener is listed at 9 to 11 gallons per minute of service flow for the 1.5 cubic foot, 45,000 grain model and 10 to 13 gpm for the 2 cubic foot, 60,000 grain model.
• The Eaglesorb ES3 anti-scale cartridge is rated at 1.5 gpm in the 2-7/8 by 9-3/4 inch size, 2.5 gpm in the 2-7/8 by 20 inch size, and 3 gpm in the 4-5/8 by 9-3/4 inch size. If your heater's peak draw is higher than the size you're considering, that cartridge will hold the flow back, so look at whole-house treatment instead.
• For the Crystal Quest® Salt-Free Water Conditioner, ask us to confirm sizing for your heater's flow before you buy.

Keep the Flush Valves Usable

Install the treatment so the heater's isolation valves stay between it and the heater, and within easy reach. You'll still be flushing, and a softener or conditioner that blocks access to the valves makes that job harder than it needs to be. If your heater went in without isolation valves, have them added when the treatment goes in.

Frequently Asked Questions

Will a salt-free conditioner lower my hardness test result?

No. Because template-assisted crystallization converts hardness rather than removing it, a hardness test after the conditioner reads about the same as before. Judge it by the heater instead: steady hot water and less scale coming out when you flush.

Are magnetic or electronic descalers a good substitute?

The evidence is weaker. The University of Nebraska-Lincoln calls magnetic water treatment unproven, with no scientific consensus on whether or how it works. In the Arizona State testing, electromagnetic and electrically induced precipitation devices cut scale by about half, well short of the salt-free media type. For a heater whose manual has a firm hardness limit, that's a thin margin.

If I install a softener, can I stop flushing?

Don't stop on your own. The units on softened water performed well in the Battelle testing, but your manual's maintenance schedule still applies, and a flush on schedule is cheap insurance against scale you can't see.