Water softeners and tankless water heaters

Why tankless heaters are sensitive to scale

A tank heater warms 40 or 50 gallons slowly, and scale mostly settles to the bottom of the tank. A tankless heater does the opposite: it heats water instantly as it rushes through a compact heat exchanger with narrow passages. Heat is exactly what makes calcium carbonate come out of solution, so those passages are where scale forms.

As scale builds up, it insulates the heat exchanger and narrows the flow path. The symptoms tend to show up gradually:

• Hot water that takes longer to arrive or runs lukewarm at high demand
• Reduced flow at the faucet when the heater is running
• Scale or flow-related error codes on the unit's display
• The burner or elements working harder for the same temperature

With hard or very hard water, which is common across the Albuquerque area (see how hard local water gets), this can happen within the first year or two if nothing is done about it.

What manufacturers ask for

Every brand is different, so the manual for your model is the real authority. That said, the pattern is consistent across the major manufacturers:

• A maximum water hardness. Several major brands list a ceiling of around 200 mg/L, roughly 12 grains per gallon, in their installation manuals. Above that, they call for water treatment.
• Regular descaling or flushing, commonly once a year and more often in hard water. Some units have a maintenance reminder you can set.
• Warranty language about water quality. Many warranties exclude damage caused by scale or by water outside the stated limits. It's worth reading that section before something goes wrong rather than after.

If you don't have your manual, the model number on the unit's label will find it on the manufacturer's website.

Setting up a softener for a tankless home

A softener solves the problem at the source: if the calcium and magnesium are removed before the water reaches the heater, there's nothing left to form scale. A few details make the difference:

1. Placement. The softener goes before the water heater on the main line, so all the hot water is softened. On a private well, it goes after the pressure tank.
2. Flow rate. Tankless homes often run several fixtures at once. The softener's valve and tank need to handle your peak flow without a pressure drop, so the softener is sized for flow as well as hardness.
3. Capacity. Sized to your household and hardness so it regenerates efficiently. Our cost guide shows the sizing math.
4. Keep the service valves accessible. Most tankless installs have isolation valves for flushing. Make sure the softener install doesn't bury them.

A salt-free conditioner is sometimes suggested for tankless heaters. It can reduce scale, but it doesn't remove hardness, and results vary. Check whether your heater's manufacturer accepts it as meeting their water requirements. Our softener vs. salt-free comparison goes into the differences.

Descaling: still needed?

With properly softened water, far less scale forms, and many owners find their heat exchanger stays clean. Still, follow the maintenance schedule in your manual. It may be a warranty condition, and periodic flushing also clears sediment and keeps you familiar with the unit.

A typical descale looks like this (always defer to your manual):

1. Turn off power or gas to the unit.
2. Close the hot and cold isolation valves.
3. Connect a small pump and hoses to the service ports, with a bucket of the manufacturer's descaling solution or plain white vinegar.
4. Circulate it through the heat exchanger for the time the manual specifies, often around an hour.
5. Flush with fresh water, clean the inlet filter screen, close the service ports and restore the valves and power.

If that sounds like more than you want to take on, a plumber familiar with your brand can do it quickly.

A note on tank-style heaters

Tank heaters benefit from soft water too, with less sediment at the bottom and less of the popping and rumbling that comes with it. One thing to know: tank heaters have a sacrificial anode rod that protects the tank from corrosion, and it's commonly recommended to check it periodically, since water chemistry, including softened water, affects how quickly it's used up. Your heater's manual will say how and when.

If you're comparing systems, the water softeners we install use metered valves sized for your household, and our free water test tells you your hardness before you decide anything.

Sources

• U.S. Geological Survey, Hardness of Water (why heated hard water forms scale)
• Manufacturer installation and owner's manuals for your specific tankless model (hardness limits, maintenance intervals and warranty terms vary by brand and model)