How hard is Chicago water, and what does it do to a water heater?

How hard is Chicago's water?

The City of Chicago's Department of Water Management (DWM) draws from Lake Michigan and treats it at the Jardine and South (Sawyer) water purification plants^[1]. Its 2025 chemical analysis report (the full-panel test DWM publishes apart from the annual water report) showed hardness of 141 to 143 mg/L as calcium carbonate at the plant outlets and in the distribution system for the first quarter of 2025^[1]. That is about 8.3 grains per gallon, using 1 gpg = 17.1 mg/L.

Measure	Chicago, Q1 2025	What it means
Hardness, mg/L as CaCO3	141 to 143[1]	Calcium and magnesium dissolved in the water
Grains per gallon	About 8.3	The unit most softeners and water heater manuals use
USGS class	Hard (121 to 180 mg/L)[2]	Not "very hard," which starts above 180[2]
Rinnai tankless limit	200 mg/L maximum[3]	Chicago is below it

Because the whole city draws from one lake through two plants, the figure is fairly uniform across Chicago neighborhoods^[1]. DWM's annual Consumer Confidence Reports are the place to check for newer data^[6].

What does 8 grains per gallon do to a tank water heater?

Hardness minerals come out of solution when water is heated. In a tank, they settle as sediment on the bottom. A.O. Smith says most tank heaters need flushing at least once a year, and more often in hard water^[4]. Rheem says removing sediment improves efficiency and extends tank life, and gives a tank lifespan of 8 to 12 years^[5].

What you notice in a Chicago basement:

• Popping or rumbling when the burner runs, from water trapped under sediment. See popping and rumbling noises.
• Slower recovery as sediment insulates the bottom of the tank.
• A drain valve that clogs during a flush.

Many web pages say hard water cuts tank life in half. We have not found a manufacturer source for a specific number, so treat that claim with caution. What the makers do say is to flush on schedule^[4][5].

What about tankless water heaters?

Tankless units heat water fast through a small heat exchanger, so scale builds there. Rinnai lists 200 mg/L as the maximum total hardness for its SENSEI tankless line and says damage caused by water quality is not covered by the warranty^[3]. Chicago's 141 to 143 mg/L sits below that ceiling^[1], which means a tankless heater is a reasonable choice here, as long as you keep up with descaling. See how to descale a tankless water heater.

Do you need a water softener in Chicago?

It depends on your goals, not on a hard rule. At about 8 gpg, city water is classed as hard^[2] but under the tankless limit cited above^[3]. Many plumbers suggest starting with the maker's flush and descale schedule and adding a softener only if you also see scale on fixtures and glassware or your unit's manual calls for one. Softened water can wear a tank's anode rod faster, so check the anode more often if you add one. See anode rod.

A simple maintenance plan for Chicago water

Heater type	Task	How often
Gas or electric tank	Flush sediment	At least yearly, more often in hard water[4]
Gas or electric tank	Inspect anode rod	Yearly[4]
Tankless	Flush and descale heat exchanger	Per the manual and when the unit shows a scale code[3]
Any	Note the install date	So you can plan replacement before a failure

For the full schedule, see water heater maintenance.

When to call a pro

• A tank older than about 8 years that rumbles loudly or leaks at the base: have a licensed plumber judge repair versus replace^[5].
• A tankless unit shows a scale or service code you cannot clear: a licensed plumber can descale it and check the inlet filter.
• You are adding a softener or changing the water line at the heater: that is plumbing work for a licensed plumber.