Tuesday, October 6, 2009

Water Heaters 101

So, if you read a recent post of mine, you might have gotten the impression that I had some water heater issues in my house. I did. Let me tell you all about it. I've included some chemistry to entertain those so inclined.

Several months ago as I was giving my children a bath, I noticed the water was kind of rust colored. I did some investigating and determined the colored water was coming from my water heater. "Crap!" I said. My water heater was probably rusting.

All water heaters will eventually fail. They are made of metal and since they are filled with water, the metal will eventually oxidize (i.e. rust). That is what happened to my water heater.


So let me ask you this: does water conduct electricity?

The answer is no. Water does not conduct electricity. Ions (positively and negatively charge ions such as sodium and chloride) dissolved in the water conduct electricity.

The oxidation of metal (such as the rusting of metal) involves the removal of electrons from the metal to something like oxygen. In the case of the rusting of iron the following reaction represents what is happening:

4Fe(s) + 3O2 ----> 2Fe2O3

This reaction converts a nice structural metal such as iron into a really lousy solid such as iron (III) oxide (Fe2O3).

To avoid this undesired reaction, water heaters contain what is called a sacrificial anode that gets oxidized more easily than the iron. The metal of choice is usually aluminum. Why aluminum you ask? Well, it gets oxidized more easily than iron. This ability is measured by what is called reduction potential.

Oxidation is the lose of electrons. If something loses electrons, something must gain those electrons. That process is called reduction. Reduction is the gain of electrons. To simplify and generalize things we chemists write these processes as what are called "half-reactions" and by convention (for easy comparison) we write them all as reduction reactions. For example the reduction of iron (III) cation and aluminum cation are shown:

Fe+3 + 3e– ----> Fe(s) E°= –0.036 V
Al+3 + 3e– ----> Al(s) E°= –1.66 V

This is the conventional way of writing ALL half-reactions, but this is NOT what is happening in the water heater. The reverse of these reactions (i.e. oxidation) is occurring and should be written like this:

Fe(s) ----> Fe+3 + 3e– E°= +0.036 V
Al(s) ----> Al+3 + 3e– E°= +1.66 V

The E° is the potential, in a sense it is a way of writing how easily the reaction occurs. Without getting into ∆G° and Faraday's constant suffice it to say that since aluminum metal has an oxidation potential of 1.66V, it will react in preference to the iron which has a lower oxidation potential.


I hoped that the rust was just because the anode was bad and that the tank had not begun to corrode to terribly (I know, I was delusional). So, I went down to Home Depot and got a new Al anode and proceeded to replace the old anode. I got me a 1 1/16 inch socket and with great difficultly remove the old anode. Except, there was nothing left. Just the big bolt that screws into the top of the tank. The Al was totally gone. Not a good sign. I put the new on in and hoped that was the end of that knowing full well it would not be.

A few weeks went by until on Saturday evening, my wife asked why the floor in the laundry room was wet. Crap. The structural integrity of the water heater had been compromised. Thankfully, it was a slow leak, but one that could not be ignored. On Sunday, I went to Lowe's to buy a new water heater. Here's where I made a big mistake.

In Minnesota there are a lot of rules to make sure no body is ever in danger of getting any type of injury. And, there are apparently a lot of rules when it comes to water heaters. By law in MN, a gas water heater must be power-vented (i.e. with a noisy fan on top of the water heater) out of the home. As far as I know, this law is only about 7-8 years old. Most water heaters that need to be replaced are the conventional kind. These water heaters vent up through a vent in the roof thanks to the updraft of the hot air.

The cost of a power-vented water heater is about $800. The cost of a conventional one is about $450. hmmmmm. I thought I would save some money and go conventional. It thought I could vent it out of the vent that goes out of the side of my house that was currently being used by the power vent of the broken heater.

Long story short: it doesn't work. I bought the conventional water heater, hauled it into my basement, cut it out of the box, installed it, filled it with 50 gallons of water and fired it up. It took 2 minutes for me to realize, I had just made a $450 mistake. This water heater was not going to work. This was Sunday night. I shut off the gas and hoped I could find a fix.

The next day I called a plumber to see if anything could be done. In summary, he said,"no you freakin moron!" though he was much more polite than that. I spent all day feeling sick to my stomach because I had just made a $450 mistake. I was mad at myself, and I felt sorry for my family.

Then I had an idea. In all of my construction projects I've done over the years, I've returned a lot of unused stuff. Stuff that no store would have to accept. So, I called Lowe's to see if I could return the water heater that I had installed in my basement. The lady said, "probably." That was good enough for me. That Monday night, I went to a different store and bought the kind I needed. I hauled it into my basement and drained and unhooked the wrong one. I installed the new one only to find out I needed a different water connector. No problem, but I'd have to wait for Tuesday to get it. We'd be without hot water for a few more hours, but it would work. I hauled the wrong one to my garage.

A few days later, my dad and I hauled the wrong one back to Lowe's and those suckers nice people took it back. I was nervous that they wouldn't, but I was upfront with why I was returning it. The guy that helped me actually seemed sympathetic.

So, I learned a lot about gas water heaters. The whole thing ended up putting me $1000 deeper in the hole, but this is what we do for the modern conveniences of life, and it could have been a lot worse.

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