Short answer
Greater Cincinnati Water Works reports 2025 averages of 126 mg/L (7 grains per gallon) at its Richard Miller Plant and 140 mg/L (8 gpg) at its Charles M. Bolton Plant. Both are hard by the USGS scale. Expect sediment in tank heaters and scale in tankless units, so plan a yearly flush.
Key facts
- Greater Cincinnati Water Works' Richard Miller Plant, which treats Ohio River water and supplies about 88% of GCWW's drinking water, averaged 126 mg/L of hardness (7 grains per gallon) in its 2025 report.[1]
- GCWW's Charles M. Bolton Plant in southern Butler County treats groundwater from 13 wells, supplies about 12% of GCWW water, and averaged 140 mg/L (8 grains per gallon).[1]
- The U.S. Geological Survey classes water with 121 to 180 mg/L of hardness as calcium carbonate as hard.[2]
How hard is Greater Cincinnati Water Works water?
Greater Cincinnati Water Works (GCWW) serves the entire City of Cincinnati and most of Hamilton County[1]. It runs two treatment plants, and they start from different water:
| Plant | Source water | Share of GCWW supply | 2025 average hardness | USGS class[2] |
|---|---|---|---|---|
| Richard Miller Plant (east side of Hamilton County) | Ohio River surface water | About 88% | 126 mg/L (7 gpg) | Hard |
| Charles M. Bolton Plant (southern Butler County) | Groundwater from 13 wells | About 12% | 140 mg/L (8 gpg) | Hard |
All figures come from GCWW's water quality page, which reports 2025 data[1]. The USGS classes 61 to 120 mg/L as moderately hard and 121 to 180 mg/L as hard[2]. Both plants land in the hard band.
An older, undated GCWW FAQ listed 137 mg/L for Miller and 147 mg/L for Bolton. The 2025 report figures above are newer. If you see the higher numbers on other sites, that is likely where they came from.
GCWW does not publish which plant serves which neighborhood on that page. It says Ohio EPA treated Miller and Bolton as one distribution system for regulatory purposes in 2025[1]. Mason is also on GCWW water[6], but which plant serves it is not stated in the sources we read.
What about Fairfield, Hamilton and Middletown?
Several Butler County cities run their own water systems:
| City | Utility | Published hardness | Source |
|---|---|---|---|
| Fairfield | City of Fairfield | 2025 daily average 132 mg/L | [3] |
| Hamilton | City of Hamilton | 130 mg/L (2025 report) | [4] |
| Middletown | City of Middletown, lime-softening plant | No figure found | [5] |
So across the metro, the published figures cluster between 126 and 140 mg/L. That is about 7 to 8 grains per gallon (1 gpg is about 17.1 mg/L). Middletown softens its water with lime at the plant[5], but we found no published hardness number. Call the city for the current figure.
What does 7 to 8 grains per gallon do to a tank water heater?
When hard water is heated, part of the calcium and magnesium drops out of solution. In a tank, it settles on the bottom as sediment. Plumbers commonly link a thick layer to three problems:
- trap water under it that boils in small bursts, causing the popping or rumbling sound many owners hear,
- insulate the water from the burner on a gas tank, so more fuel goes up the flue,
- bury the lower heating element on an electric tank.
Many plumbers suggest draining a few gallons from the tank's drain valve once a year, or on the schedule in your owner's manual. In a tank that has never been flushed, sediment can clog the drain valve. If the valve will not close fully after a flush, have a licensed plumber replace it.
The anode rod also matters. It corrodes so the tank does not. A look at it during the yearly flush tells you how fast yours is wearing. See the anode rod guide.
What does it do to a tankless water heater?
In a tankless unit, scale coats the narrow passages of the heat exchanger. A 2012 guide prepared for the U.S. Department of Energy's Building America program says scale can restrict flow and shorten the unit's life, and that manufacturers may require a softener above about 8 to 14 grains per gallon (136 to 239 mg/L)[8].
Rinnai's SENSEI manual gives one maker's thresholds[7]:
| Rinnai's label | mg/L | Grains per gallon | Rinnai's guidance |
|---|---|---|---|
| Slightly hard | 61 to 120 | 4 to 7 | Softener or scale cutter recommended; flush residential units once a year |
| Moderately hard | 121 to 160 | 7 to 9 | Softener or scale cutter recommended; flush residential units once a year |
| Very hard | 161 to 200 | 9 to 12 | Strongly recommended |
Miller water at 126 mg/L and Bolton water at 140 mg/L both fall in Rinnai's moderately hard row[1][7]. Rinnai also lists 200 mg/L as the maximum total hardness for these units and says damage from water quality is not covered by the warranty[7]. Cincinnati water is well under that limit, but the yearly flush still applies. Other brands publish their own limits, so read your manual.
When scale builds up, many units post a code. See how to descale a tankless water heater for the codes and steps.
A simple hard water plan for a Cincinnati home
| Heater | What to do | How often |
|---|---|---|
| Gas or electric tank | Drain and flush sediment; look at the anode rod | Yearly, or per the manual |
| Tankless (gas or electric) | Descale the heat exchanger with isolation valves and a pump kit | Yearly, per Rinnai's guidance for this hardness band[7] |
| Any heater, with a softener | Keep flushing on schedule; softened water slows scale but does not end checks | Per the manual |
If you are buying a tankless unit, ask the installer to add isolation valves. They make the yearly flush a short job instead of a repiping job.
When to call a pro
- A tank drain valve that is clogged, leaking or will not close after a flush.
- Loud rumbling that continues after a flush, which can mean heavy sediment or a failing tank.
- A tankless unit showing a scale code with no isolation valves installed.
- You want a softener sized to your water. A licensed plumber can test your tap and size it to your household.
More questions
Which GCWW plant serves my house?
GCWW's water quality page does not map plants to neighborhoods, and Ohio EPA treated Miller and Bolton as one distribution system in 2025. Miller supplies about 88% of the water, so most homes get mostly Miller water. Call GCWW to confirm your address.
Is 7 or 8 grains per gallon bad for a tankless water heater?
It is within the range Rinnai's SENSEI manual calls slightly to moderately hard. Rinnai recommends a softener or scale cutter at that level and a yearly flush for residential units. Other makers set their own limits, so read your unit's manual.
Do I need a water softener in Cincinnati?
Not for a tank heater to work, but it cuts scale. A 2012 Department of Energy guide says some tankless makers may require a softener above about 8 to 14 grains per gallon. Bolton water sits at the low edge of that band.
Sources
- Water quality reports (2025 data, page updated March 26, 2026), Greater Cincinnati Water Works, City of Cincinnati. Accessed 2026-10-14.
- Hardness of water, U.S. Geological Survey, Water Science School. Accessed 2026-10-07.
- Fairfield drinking water, City of Fairfield, Ohio. Accessed 2026-10-07.
- 2025 Consumer Confidence Report (linked from hamilton-oh.gov), City of Hamilton, Ohio. Accessed 2026-10-07.
- Water Treatment, City of Middletown, Ohio. Accessed 2026-10-06.
- Utility providers, City of Mason (Imagine Mason). Accessed 2026-10-06.
- SENSEI RX Residential Installation and Operation Manual (100000839), Rinnai America. Accessed 2026-10-07.
- Measure Guideline: Transitioning to a Tankless Water Heater (IBACOS, September 2012), U.S. Department of Energy, Building America program. Accessed 2026-10-07.
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