Short answer
Columbus Water & Power reported 2025 average hardness of 98 to 125 mg/L by plant, or 5.7 to 7.3 grains per gallon, after lime softening at the plant. That is moderately hard to hard: enough to build tank sediment and tankless scale over time, but well under the 200 mg/L limit in one major tankless manual.
Key facts
- Columbus Water & Power's 2025 annual average hardness was 125 mg/L (7.3 gpg) at the Dublin Road plant, 98 mg/L (5.7 gpg) at Hap Cremean and 123 mg/L (7.2 gpg) at Parsons Avenue.[1]
- Columbus softens its water at the plant by adding soda ash or caustic soda and hydrated lime to remove the calcium and magnesium that cause hardness.[1]
- Rinnai's SENSEI tankless manual lists 200 mg/L as the maximum total hardness for its units and recommends a softener or scale cutter and yearly flushing from 61 mg/L.[4]
How hard is Columbus water?
Columbus Water & Power (CWP), formerly the Division of Water, runs three water plants. Its 2025 report lists hardness for each one[1]:
| Plant | 2025 annual average (mg/L) | Range (mg/L) | Grains per gallon |
|---|---|---|---|
| Dublin Road | 125 | 122 to 132 | 7.3 |
| Hap Cremean | 98 | 85 to 118 | 5.7 |
| Parsons Avenue | 123 | 122 to 127 | 7.2 |
Hardness is dissolved calcium and magnesium, stated as mg/L of calcium carbonate. One grain per gallon is about 17.1 mg/L[2]. The system draws on the Scioto River, Big Walnut Creek and ground water pumped from sand and gravel deposits of the Scioto River Valley. The Parsons Avenue plant is the one that uses ground water[1].
On the U.S. Geological Survey scale, 61 to 120 mg/L is moderately hard and 121 to 180 is hard[3]. So Hap Cremean water is moderately hard, while Dublin Road and Parsons Avenue are just over the line into hard. Our Ohio study puts the Columbus system average at 115 mg/L, tied for 18th hardest of the 24 Ohio cities we could rank[2].
Doesn't Columbus already soften its water?
Yes, and many people do not know it. Step 5 of the city's treatment process is softening: soda ash or caustic soda and hydrated lime are added to remove the calcium and magnesium ions responsible for hardness[1]. That is why Columbus water is far softer than the raw well water in some Ohio cities. It is not soft water, though. The plants still deliver about 6 to 7 grains per gallon.
What about the suburbs?
Many suburbs buy Columbus water. Our hardness study lists Gahanna, Hilliard, Reynoldsburg and Upper Arlington on Columbus water, and reads Dublin as served mainly by Hap Cremean and Grove City by Dublin Road, based on the CWP service map[2]. Other nearby cities run their own plants:
| City | Utility figure | Source |
|---|---|---|
| Westerville | Averages 128 mg/L (7.5 gpg) | City Water FAQs, undated[7] |
| Delaware | 121 mg/L (range 103 to 142), 2025 | City of Delaware[8] |
| Newark | 119 mg/L, 2025 | City of Newark[9] |
What does this hardness do to a tank water heater?
When water is heated, some of the minerals drop out. In a tank, they settle on the bottom as sediment. Sediment under a gas burner can make popping and rumbling sounds, and heavy buildup wastes energy. At Columbus levels this builds slowly, but it builds.
Rheem gives tank water heaters a typical life of 8 to 12 years[6]. Hardness is one factor among many, alongside the anode rod, water pressure and how much hot water the house uses. We found no study that ties a specific Columbus hardness level to a specific number of years, so be wary of anyone who quotes one.
Practical steps:
- Flush the tank on the schedule in the owner's manual.
- Have the anode rod checked during service visits on older tanks.
- Note the heater's age from its serial number so you can plan a replacement before it fails.
And a tankless water heater?
Tankless units are more sensitive. Scale coats the narrow passages of the heat exchanger. A guide prepared for the U.S. Department of Energy's Building America program says scale can restrict flow and shorten a tankless unit's life, and that manufacturers may require a softener above about 8 to 14 grains per gallon (136 to 239 mg/L)[5]. All three Columbus plants are below that range[1].
Rinnai's SENSEI manual sets its own bands[4]:
| Rinnai band | mg/L | Rinnai guidance | Columbus plants in this band[1] |
|---|---|---|---|
| Slightly hard | 61 to 120 | Softener or scale cutter recommended; flush residential units once a year | Hap Cremean (98) |
| Moderately hard | 121 to 160 | Softener or scale cutter recommended; flush residential units once a year | Dublin Road (125), Parsons Avenue (123) |
| Maximum for the unit | 200 | Water quality damage is not covered by the warranty | None |
So in Columbus, plan on a yearly descale for a tankless unit, and read your own model's manual, since each maker sets its own limits[4].
When to call a pro
- A tank that pops, rumbles or leaves grit in faucet aerators and has never been flushed: a plumber can flush it or tell you if it is too far gone.
- A tankless unit showing a scale or service code: see the descaling steps, or have it serviced.
- You are choosing between tank and tankless and want the softener question answered for your plant's water.
- Any leak from the tank body itself, which cannot be repaired.
More questions
Do I need a water softener in Columbus to protect a water heater?
Not as a rule. Columbus already softens at the plant, and its 98 to 125 mg/L sits below the 136 to 239 mg/L range where a DOE guide says tankless makers may require one. Some tankless makers still recommend a softener or scale cutter at this level, so read your unit's manual.
Which Columbus plant serves my house?
It depends on where you live. The Columbus Water & Power report includes a service area map for the Dublin Road, Hap Cremean and Parsons Avenue plants. Homes near the boundaries can get blended water.
How often should I flush a tank water heater in Columbus?
Follow the schedule in your heater's manual. With moderately hard water, many owners flush once a year. A heater that pops or rumbles when heating often has sediment on the bottom.
Sources
- 2025 Drinking Water Consumer Confidence Report, Columbus Water & Power. Accessed 2026-10-14.
- Ohio water hardness by city (2026), waterheaterfacts.com. Accessed 2026-10-14.
- Hardness of water, U.S. Geological Survey, Water Science School. Accessed 2026-10-07.
- 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.
- Water heater lifespan: when to repair vs. replace (Mar 25, 2026), Rheem Manufacturing. Accessed 2026-10-06.
- Water FAQs, City of Westerville. Accessed 2026-10-07.
- 2025 Water Quality Report, City of Delaware, Ohio. Accessed 2026-10-07.
- 2025 Consumer Confidence Report, City of Newark, Ohio. Accessed 2026-10-07.
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