# Is San Francisco water hard or soft?

> San Francisco water is soft, high in pH and treated with chloramine. What that does inside galvanized, copper and PEX pipe, and how Daly City and SSF differ.

URL: https://hilltoprepipesanfrancisco.com/guide/san-francisco-water-and-your-pipes/

Learn

The chemistry of Hetch Hetchy water, read for what it does to pipe: why old steel here clogs with rust rather than scale, what chloramine means for material choice, and why the answer changes a little across the county line.

Last Updated: October 30, 2026

Soft. SFPUC reports average hardness of 62 ppm in the water it delivers to San Francisco, with a range of 8.1 to 112 ppm, a pH adjusted to about 9.3 for corrosion control, and chloramine as the disinfectant. For pipes, that means mineral scale is rarely the problem here; corrosion of the metal is. [Hilltop Repipe San Francisco](https://hilltoprepipesanfrancisco.com/) offers and manages repiping in [copper](https://hilltoprepipesanfrancisco.com/copper-repiping/) and [PEX](https://hilltoprepipesanfrancisco.com/pex-repiping/) across San Francisco, Daly City and South San Francisco, with the work carried out by licensed and insured contractors, and material choices start from this chemistry.

Key takeaways

- SFPUC water averages 62 ppm hardness and a pH of about 9.3, adjusted for corrosion control.
- Old galvanized pipe in San Francisco clogs with its own rust, not with mineral scale.
- Chloramine is the disinfectant, so tubing and fittings should carry ratings for it.
- Tap sampling puts lead and copper well under federal action levels; household plumbing is the listed source of both.
- Daly City blends in local groundwater, and South San Francisco's supplier blends in much harder groundwater.

## What is in the water SFPUC delivers

SFPUC publishes its water quality data every year. The figures that matter for pipe are the ones that govern scale and corrosion.

| Measure | Range | Average | What it means for pipe |
| --- | --- | --- | --- |
| Hardness (as calcium carbonate) | 8.1 to 112 ppm | 62 ppm | Soft; little mineral scale forms inside pipe or water heaters |
| Alkalinity | 8 to 131 ppm | 64 ppm | Low buffering, one reason the pH is adjusted |
| Calcium | 3.1 to 29 ppm | 16 ppm | Little calcium available to deposit as scale |
| pH | 9 to 9.7 | 9.3 | Raised for corrosion control; slows metal leaching into the water |
| Total dissolved solids | 24 to 197 ppm | 95 ppm | Low mineral content overall |
| Chloramine residual | under 0.1 to 3.6 ppm | Highest running annual average 2.6 ppm | The disinfectant; below the 4.0 ppm regulatory maximum |

These are system figures. Water at a particular tap varies with the season, the source blend and the building’s own plumbing.

## Where it comes from

Hetch Hetchy Reservoir supplies up to 85% of the water SFPUC delivers each year. That supply is exempt from filtration and is treated with ultraviolet light and chlorine, adjusted in pH for corrosion control, fluoridated and chloraminated. Other sources make up the rest. In 2025, for example, about 0.42% was groundwater from a deep aquifer under northern San Mateo County and western San Francisco, blended into the supply.

The supply is low in dissolved minerals, averaging 95 ppm in total, which is why the water is soft. It is also why SFPUC raises the pH. Soft, lightly buffered water at a low pH would be more aggressive toward metal pipe, so the utility adjusts it upward to slow corrosion in the city’s mains and in household plumbing.

## What San Francisco water does inside pipes

Hard water deposits calcium and magnesium as scale, which narrows pipes and coats water heater tanks. San Francisco’s water does little of that. What it doesn’t stop is the metal itself corroding over decades.

### Galvanized steel

Galvanized pipe is steel coated in zinc. In San Francisco, the zinc wears away first, then the steel underneath rusts from the inside. The rust builds up on the pipe wall until a 3/4-inch line carries the water of a much smaller one, and the threaded joints, where the wall was cut thinner, start to leak. In hard-water cities scale adds to that buildup; here it is mostly the pipe’s own corrosion. That is why galvanized pipe can look fine on the outside while the inside is nearly closed, and why rusty water often shows after the water has been off or sitting. The [galvanized pipe replacement](https://hilltoprepipesanfrancisco.com/galvanized-pipe-replacement/) page covers the fix, and the [rusty water guide](https://hilltoprepipesanfrancisco.com/guide/rusty-water-from-pipes/) covers other sources of discoloration.

The utility’s pH adjustment slows metal from leaching into the water, but it cannot undo decades of rust already inside old steel.

### Copper

Copper does not rust, but it can develop pinholes and leaking joints, and there is no single cause. Water chemistry, flow velocity, sediment, workmanship at the joints and electrical conditions can all contribute, so soft water alone does not explain a pinhole. One leak does not prove the system is failing; several on the same run are a stronger sign. The [copper pinhole leaks guide](https://hilltoprepipesanfrancisco.com/guide/copper-pinhole-leaks/) goes through the causes.

### Where copper meets steel

When copper is joined directly to steel, the two metals form a galvanic couple and the steel corrodes faster at the joint. San Francisco’s code requires a dielectric or insulating fitting, or a brass nipple at least 6 inches long, at every copper-to-steel connection. That matters most in a [partial repipe](https://hilltoprepipesanfrancisco.com/partial-repiping/), where new copper meets old steel that stays.

## Chloramine and pipe materials

SFPUC uses chloramine, a combination of chlorine and ammonia, as the disinfectant that stays in the water all the way to the tap. Every material in contact with drinking water is exposed to it continuously.

For PEX, the relevant number is the tubing’s chlorine resistance rating, which PEX makers publish for continuous hot and cold use. A repipe scope should name a product and fitting system rated for the conditions. California’s code also requires all PEX installed in the state to have at least 30-day UV protection and sets a stronger standard for PEX in continuously recirculating hot water. For copper, chloramine is part of the water chemistry that can play a role in pinholes alongside the other factors above, and no single cause has been established. Neither material is universally better in San Francisco water; the [PEX vs copper guide](https://hilltoprepipesanfrancisco.com/guide/pex-vs-copper/) sets out the trade-offs.

## What the tap tests show

Every three years SFPUC samples taps in homes for lead and copper. In its 2024 round at 50 sites, the 90th percentile lead level was 4.9 parts per billion against an action level of 15, and the 90th percentile copper level was 72 parts per billion against an action level of 1,300. SFPUC lists internal corrosion of household plumbing as the typical source of both. In other words, what small amounts appear come from the building’s own pipe, solder and fixtures, not from the city’s mains. A lead test costs $25 per tap through SFPUC, and the [lead pipes in San Francisco guide](https://hilltoprepipesanfrancisco.com/guide/lead-pipes-san-francisco/) covers what the utility has removed and what remains on the owner’s side.

## Does the water need softening or filtering

For pipe protection, softening is not the answer in San Francisco. The water is already soft, and a softener does nothing for the rust inside galvanized pipe, which comes from the steel, not the water’s minerals. Questions about taste, chloramine or a household filter are a separate decision from the plumbing. If brown water is the concern, the source matters more than any treatment: the [are galvanized pipes safe guide](https://hilltoprepipesanfrancisco.com/guide/are-galvanized-pipes-safe/) covers what old steel does to drinking water.

## Daly City and South San Francisco

The chemistry changes a little across the county line.

**Daly City** blends two sources: SFPUC Hetch Hetchy water for about 60% of average daily demand and groundwater from Westside Basin wells about 300 feet deep for about 40%. The city fluoridates the well supply. Its 2025 report lists no known lead service lines in the city’s system and a 90th percentile lead level under 5 parts per billion. The [Daly City page](https://hilltoprepipesanfrancisco.com/service-areas/daly-city/) covers repiping there.

**South San Francisco** is served by Cal Water’s Bayshore District, which blends SFPUC water with local groundwater. Cal Water’s report lists that groundwater’s hardness at 287 to 461 ppm, averaging 384, against 62 for the SFPUC supply, with a pH averaging 8.5. Where a home receives a larger share of well water, scale on fixtures and in water heaters is more plausible than it is in San Francisco. Cal Water reports no known lead service lines in its systems. The [South San Francisco page](https://hilltoprepipesanfrancisco.com/service-areas/south-san-francisco/) covers that city’s permits.

## Pressure is a separate question

Water chemistry explains how pipe ages; it does not explain low pressure at the meter. San Francisco is served through about 13 pressure zones fed from reservoirs and tanks at different elevations, so static pressure varies from street to street. California’s code requires a pressure regulator and an expansion tank where static pressure exceeds 80 psi. A walk-through measures pressure at the meter before anyone blames the pipe, and the [low water pressure guide](https://hilltoprepipesanfrancisco.com/guide/low-water-pressure/) explains why.

For a repipe scope that names materials suited to the water at your address, call or send the form.

## Questions about this guide

### Does San Francisco water need a softener?

Not for hardness. SFPUC reports average hardness of 62 ppm, low enough that mineral scale is not what clogs pipes here. A softener does nothing for the rust inside old galvanized pipe, which comes from the steel corroding. Owners in parts of South San Francisco served partly by hard groundwater have a different case to weigh.

### Is chloramine in San Francisco water a problem for PEX?

Not when the tubing is rated for it. SFPUC disinfects with chloramine, and PEX makers publish chlorine and chloramine resistance ratings for their tubing. The scope should name a product whose rating suits continuous use in chloraminated water. San Francisco's code places no restriction on PEX for water distribution.

### Is South San Francisco water harder than San Francisco water?

It can be. Cal Water serves South San Francisco with a blend of SFPUC water and local groundwater, and its report lists the groundwater's hardness at 287 to 461 ppm, averaging 384, against an average of 62 for the SFPUC supply. What reaches a given tap depends on the blend in that part of the system.

## Related service

### Copper repiping

New hot and cold supply in rigid copper tube, joined by solder or press fittings, with dielectric transitions where it meets steel.

Copper repiping

## More guides

- Learn [What causes pinhole leaks in copper pipes?](https://hilltoprepipesanfrancisco.com/guide/copper-pinhole-leaks/)There is no single cause. Water chemistry, flow speed, workmanship, sediment and electrical conditions can each thin a copper wall until it opens, and where the leaks appear is the most useful clue to which one is at work.
- Learn [Are galvanized pipes safe for drinking water?](https://hilltoprepipesanfrancisco.com/guide/are-galvanized-pipes-safe/)Galvanized steel is not lead pipe, and San Francisco's tap results are low. The real questions are what the pipe was once connected to, what its rust carries into the water and how much life the steel has left.
- Learn [Why is rusty water coming from my faucet?](https://hilltoprepipesanfrancisco.com/guide/rusty-water-from-pipes/)Brown water has three usual sources: the utility's mains, the water heater and old galvanized pipe. Which taps, which side and how long it lasts tell them apart before anyone opens a wall.
- Learn [Are there lead pipes in San Francisco?](https://hilltoprepipesanfrancisco.com/guide/lead-pipes-san-francisco/)San Francisco's lead story is mostly history and small parts: lead service lines removed decades ago, short lead connectors on old galvanized lines, and household fixtures and solder. What SFPUC reports, and what an owner can check.
- Decision Guide [PEX vs copper for a repipe: which should you choose?](https://hilltoprepipesanfrancisco.com/guide/pex-vs-copper/)Both materials are code-approved in San Francisco. The real differences are in how each one travels through a building, what it costs, how it is joined and what has to change around it.