Tankless water heater and hard water in Monterey Park

Updated Oct 30, 2026 · 5 min read

What Monterey Park's report shows

The city's 2025 water report says the supply comes from production wells in the Main San Gabriel Groundwater Basin, plus water purchased from San Gabriel Valley Water Company, which pumps groundwater from the Main Basin and the Central Basin. Minerals, not anything dramatic, are the story.

Hardness is reported two ways. In the city's own wells it averaged 356 mg/L, which is 20.8 grains per gallon, with results from 340 to 380. In the water the city buys from San Gabriel Valley Water Company, the average was 246 mg/L (14.3 grains), with a range of 91 to 335. Two homes a few streets apart can see different water depending on which source feeds the line. Our Monterey Park water quality report has the full 2025 numbers.

The report's pH averaged 7.3 in the city wells and 7.7 in the bought water. Hardness is not a health issue, and the report does not present it as one. It is a maintenance fact for the equipment that heats the water.

What Rinnai's own manual says about hard water

Rinnai's installation and operation manual for its residential tankless models has a water quality table, and total hardness is the first line: up to 200 mg/L. The manual says that if you install the heater where water is known to be hard or to cause scale, the water must be treated and the unit may need a more frequent flushing schedule. It also says scale buildup is caused by hard water and can be accelerated by a high temperature setting.

The same manual describes what happens as scale accumulates in the heat exchanger: when it starts to affect performance, the controller shows an LC diagnostic code, and the heat exchanger needs to be flushed to prevent damage. Some models also have a Service Soon (SS) indicator you can switch on in the parameters to prompt a flush. And the manual is blunt that damage and repair caused by scale in the heat exchanger is not covered by the maker.

Put the pieces next to each other. Both averages, 356 and 246 mg/L, are above the manual's 200 mg/L line, and the city wells are close to double it. A tankless heater in Monterey Park should be planned as an appliance that gets descaled, not as one that never needs attention.

Tankless versus tank in water like this

A tank heater and a tankless unit meet the same minerals in different places. The tank collects sediment on its floor, which you drain. The tankless unit has thin passages inside its heat exchanger, where even a modest crust matters, and you cannot drain that crust out. You dissolve it.

The right-hand column's last row is a design decision made at install time. If the home does not already have service valves under a tankless unit, adding them is the first job, because the flush procedure depends on them.

What a descale actually involves

Rinnai's technical bulletin for flushing its heat exchangers gives the outline. Close the hot and cold shutoff valves, connect a circulating pump outlet hose to the service valve on the cold line, and run a drain hose from the hot line's service valve into a bucket. Put four gallons of undiluted food-grade white vinegar in the bucket, open the service valves, and circulate for at least an hour at three to four gallons per minute. Then rinse with clean water for five minutes, and clean the inline filter at the cold inlet.

That bulletin is for one family of units, so treat it as the shape of the job and not as a script for every heater. Some models also have an interval written into the manual and a service reminder that can be switched on. Reading the manual for the exact model on your wall is the safest first step. When we look at a tankless unit that has gone without a flush in water like this, the first things we check are the inlet filter and whether service valves were ever installed.

Monterey Park service for this

Treating the water, not just the heater

The manual's first answer to hard water is treatment, and Rinnai sells a scale control system that feeds a blend of control compounds into the supply. A softener is the other common route. Monterey Park homes with a tankless heater and very hard tap water are the textbook case for a softener, and our guide to sizing a water softener for Monterey Park shows how the grains-per-gallon figure turns into a capacity.

Softening swaps calcium and magnesium for sodium, so look at that row too. The city reported sodium averaging 25 mg/L in its wells and 24 in the bought water. If the household watches sodium, it is part of the decision.

For the heater itself, our water heater service page describes tank and tankless work, and the water softener page covers the treatment side.

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