Acidic Water and Your Hot Water Heater: How Low pH Destroys Plumbing (and How to Stop It)

Caller "We're on well water. I tested it myself and got a 6.2 pH. I've bleached the well, I have several filters in place, even a carbon filter. I've replaced two water heaters already."

Aidan "6.2 pH. You have to get that up to neutral. Neutralizer in first, then the iron filter, then the softener."

That was a caller from North Carolina this summer. Two water heaters, a bleached well and a row of filters, and nobody had ever fixed the water going into the heater. Your water heater is the most expensive appliance connected to your plumbing, and on an acidic well it is the appliance the acid hits hardest.

Is it really the acid?

Acid is one of five well water problems that ruin water heaters, and the fix is different for each one. Before you buy anything, match what you are seeing to the rows below; the acid row is first because that is what this page is about. A caller from Pennsylvania in August had changed the anode rods, then replaced the whole heater thinking it was "all hogged up with the iron," and still had the smell. The cold water stank too, so it was never the heater. He needed an iron filter.

Only a test separates these. A hardware-store pH strip tells you today whether you are under 7. The certified lab test tells you the pH, the hardness, the iron, and how much copper is already coming off your pipes; of the lab tests my customers send in, about one acidic tap in six had more copper than the EPA allows. Text me the report and I will tell you which row you are in.

If you have not tested yet: certified lab test, $199

Test the water before the next heater

One certified lab test, mailed to you, run by an accredited lab, gives you the pH, hardness, iron and copper that say which row above is yours, and what to put ahead of the heater.

Get the certified well water test

53 contaminants. Results reviewed by Aidan, in plain English. On city water, use the city water test.

Why the water heater fails first

The heater fails first because corrosion runs faster in hot water, and the heater is where the hottest water in the house sits all day. What I tell people who call me with a low pH: heated acidic water is about five times more aggressive than the cold water side. The water did not get more acidic. Heat speeds up the reaction between the acid and the metal, and the heater makers say the same thing in their own manuals: A. O. Smith lists "hotter water temperatures" among the things that eat the anode rod faster. Your cold copper lines see the same water at 50 or 60 degrees for a few seconds on its way to a faucet. The tank holds it at 120 to 140 all day and all night.

Two minutes on camera: what acid does to the copper, the water heater and the fixtures, and how soon.

In the video I put it this way: one of the main household appliances affected most by acid water is your hot water heater. As the water passes through, it starts to wear away, especially the bottom of the heater, and it eats away your elements. I have seen houses where the bottom blew out of the hot water heater and flooded the basement. Depending on water usage and the pH, it can be six months to 18 months before you start seeing real damage to your plumbing and water-using appliances. That is why the heater is on my inspection list for anyone who suspects acidic water: blue-green stains, pinhole leaks, small leaks at the hot water heater, and anything acting up in the dishwasher or washing machine.

What acid does inside the tank

Acid eats the anode rod (the protective rod inside the tank) first, then reaches the steel through flaws in the glass lining, and the tank rusts through at the bottom. Every tank heater is a steel tank with a glass (enamel) lining baked onto the inside, and a metal rod hanging in the water that is there to corrode so the tank does not. The makers call it a sacrificial anode. A. O. Smith's manual puts the sequence in one sentence: "Once the anode rod is depleted, the tank will start to corrode, eventually developing a leak."

1. The anode rod goes first. A. O. Smith says to inspect it after at most three years and every year after that, and to replace it when most of the rod is under 3/8 inch thick or a third of its steel core wire is showing. Acidic water and hot water are both on its list of what wears the rod faster.
2. Then the steel, through the lining. With the rod gone the acid works on the steel wherever the glass lining has a flaw. Rust-colored hot water at this stage is the tank itself, not the well; if the cold side runs orange too, it is iron in the water (the iron row in is it really the acid).
3. Then the bottom leaks. The first sign is a puddle at the base of the tank, and by then there is no repair. Neither A. O. Smith nor Bradford White covers a tank whose rod was taken out, and Bradford White's warranty also excludes rusty water, odors, and lime in the tank.

Replacing a spent rod does what the manual says it does: it extends the life of the tank. It does not change what is coming in from the well, so the new rod is on the same clock as the old one. And if your hot water smells like rotten eggs, that is often the rod reacting with the water, not acid; the rotten egg smell in hot water page sorts that out.

Is hot water acidic?

No. Heating water does not make it acidic. The pH reading of pure water drops as it warms, from 7.0 at room temperature to about 6.6 at the 120 to 140 degrees a water heater runs, but the water is neutral at every point on that line. Heat shifts the pH scale, not the water. The familiar scale, where 7 is neutral, is only exact at room temperature; warm the water and the neutral point slides down with it.

So if your well water is acidic, it does not get more acidic in the tank. If anything the hot side reads a touch higher, because heating water drives off some of the dissolved carbon dioxide that made it acidic in the first place. An earlier version of this page said the opposite, that the tank made the water more acidic. That was wrong and I have fixed it. One buyer in 2022 tested his hot tap and got 6.2; that number came from his well, not from his heater. What the heater changes is not the acid. It is how fast the acid works, which is the section above.

What the heater makers require

The tankless makers put a number on it: water between pH 6.5 and 8.5, or the water-damage parts are not covered. The tank makers give no pH number; they exclude water damage and require the anode rod to stay in. These are from the installation manuals and warranties, not from me. Read that against a well at pH 6.0. On a tankless you are outside the maker's written limit the day it is installed, and the heat exchanger that does the heating is the part they will not replace. On a tank heater there is no number to fail, just a rod the acid eats faster than the inspection schedule expects. Whichever heater you own, the makers are telling you the same thing I do: fix the water before it reaches the heater.

How an acid neutralizer protects the heater

An acid neutralizer is a tank of calcite (crushed limestone) that the water passes through as it enters the house, so every drop reaching the heater is already at about pH 7. It goes after the well pressure tank and ahead of everything else: neutralizer first, then an iron filter if you need one, then the softener, then the heater. The water comes up through the calcite, dissolves a little of it until it reaches neutral, and moves on. More acidic water dissolves more calcite; water that is already neutral dissolves almost none, so it regulates itself. The non-backwashing model I recommend for most homes has no chemicals to add, no electricity and no drain line. Once it is in, the acid stops reaching the plumbing and the heater, and that shows up in the retests: the pH 6 buyer quoted below had his copper checked by a lab afterward and it came back under the limit. It is never too late to put one in, but it does not put steel back on a tank that has already thinned, so a heater that is already leaking still gets replaced. The next one is the one you save.

Which tank depends on your pH. Plain calcite handles water down to pH 5.5 in the 2.5 cubic foot tank and 6.0 in the 1.5. Below that the tank ships with a calcite and FloMag blend mixed to your number; the lowest well we have treated with the blend was 3.67. The Clack non-backwashing tanks are $1,195 for the 1.0 cubic foot, $1,295 for the 1.5, $1,395 for the 2.0 and $1,495 for the 2.5, calcite included; the blend adds $200. Two trade-offs to know about. The calcite adds 3 to 6 grains of hardness to the water, which is why we sell the neutralizer and softener as a pair. And the calcite dissolves, so you check the level once a year and add bags every 18 to 36 months depending on the tank size and your pH; servicing an acid neutralizer walks through it.

What ships: the tank, the Clack valve with the bypass already on it, and the calcite in separate 50-pound bags that go in at install through the fill funnel. Plan on a licensed plumber; what I tell callers is that a plumber who can put in a hot water heater can put in one of these, and it should not take him more than an hour or two. I support your plumber by phone at 800-460-5810.

Ask Aidan

Heater on its way out?

Text me your pH, or a photo of the stain or the heater, and I'll tell you whether the water is the cause and which tank fits.

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