Home & Garden Columns

About the House: The Mysterious, Eternally Exploding Water Heater

By Matt Cantor
Thursday October 08, 2009 - 12:23:00 PM

Periodically I see something that is so stupid that I have to reset my stupid gauge, re-evaluate the relative stupidity of all the other things I see, and give all the other builders, handy-folk and homeowners a little more slack because they didn’t perform this particularly odious act. As you may have surmised, I was recently privy to such an act and it is both fascinating and incredibly stupid. So without further ado, I present for you the mysterious, eternally exploding water heater. 

The scene is an REO in a relatively poor neighborhood near the Oakland-Berkeley border. A lot of young couples are making their starts in these marginal neighborhoods and I say bully for them. The houses need saving, the neighborhood need revitalization and an infusion of new blood, and I need the cash. 

This house was all right but nothing particularly special. But with love and the energy of these two extraordinarily beautiful and talented people, it will no doubt be a hot ticket as the years roll by. I racked up item after item as I proceeded through my job and then got to the water heater. It had been, I noticed, turned all the way down to what is often labeled as the vacation setting (perhaps the makers of these things are under the impression that you earn enough to take vacations, but that’s another subject).  

A vacation setting is, essentially, a stand-by mode. The pilot is still lit; The main controls are in the on position but no heating will take place. Actually, these make a lot of sense and we would all do well to put our water heaters in the vacation setting when we’re away for a few days (of course, with an on-demand water heater, they are always in this setting but for the few moments when you ask them to shower you). 

Obliged, as I am, to investigate things, I turned the water heater dial up to the standard, non-numerical setting on the dial and listened. The water heater went “shhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhFWUMP” with what sounded like a small explosion at the end. This was about five to seven seconds—a long time in the ignition business. I don’t usually wait this long to hear a pilot ignite the burner without shutting something down, but I was just hoping against hope that I would hear that burner ignite and was a bit in the headlights of the thing as I waited for what I suspected was coming. Afterwards, I left the unit on but kept a close eye on it. When the heating stopped, after a long while, I intended to turn it off and started looking it over. 

Something was very wrong and I figured it must just be a defective unit that somehow involved the pilot’s inability to efficiently and immediately ignite the gases as they left the burner. 

In fact, that was just exactly the case but not due to a manufacturing error or anything that had failed in the unit. For reasons I cannot explain, I decided to take a look at the specification tag on the unit and there, in bold Times New Roman, stood the word I need to see: Propane. Never seen it—in more than 20 years of doing this. This unit, installed in the city on a CNG system (natural gas) was built for use with propane. I figure it must have been brought down from Sonoma or Marin by some stoned plumber who never stopped to ask a vital question. 

Propane and natural gas are not the same. They seem similar but they are different in one enormous way that affects a lot of things. They are not the specific gravity (whaaahu?). One is lighter then air (natural gas or “CNG”) and one is heavier (propane) and this is a huge matter; just ask anyone who lives on a boat. 

When hooked up to propane, a propane water heater will release gas from its burners and the propane will fall from the burner and meet a pilot light that is located just below the burner, thus igniting it quickly, and therefore safely. 

When hooked up to CNG, a natural gas water heater will release gas from its burner that will immediate rise, meet the pilot light above the burner, and burst into diaphanous blooms of blue heat. 

But when CNG is released from the burner of a propane water heater, the gas will rise and meet empty space above the burner inside the water heater as it fills downward from the top, eventually meeting the pilot light located below the burner. Thus the long wait and the small explosion that nearly blows the door off the thing every time it comes on.  

And that was the thing I shared with my client. Left as it is, this unit would do this every time the water cooled down a bit and during every shower that lasts more than two minutes and several times every night (good God). Given the variable dynamics of this behavior, it is more than possible that one of those occasions would indeed blow the door off the combustion chamber, inflaming the nearby corn broom, followed by the house. 

Again, the placement of the pilot, above the burner or below, is critical. Now there are other variations in the make of the two different water heaters based on the differing temperature of combustion, density of the gas, etc. but clearly, this is the big difference. 

Natural gas, being lighter than air, has to be regarded with a special respect since it can fill up areas under floors or the tops of fireplaces where dampers are shut. Any upper surface can contain it and, when a sufficient concentration has been reached, it only takes a small spark to wake the thunder and fire gods simultaneously. 

Propane, for those who don’t live in the country, may be more dangerous because it will fall to the floor and fill up the bottom of the basement or even crawlspace before the unseen lake of potential energy meets a suitable catalyst and blows the house off its foundation. Crawlspaces in country propane casas are usually, and certainly should be vented at the very lowest point to allow for the escape of the gas. On boats, it’s no joke. A propane leak fills the boat from the bottom up, which can destroy the vessel or just ruin your day, if you’re lucky. 

As long as we’re on the subject of flammable gases and their specific gravities, it’s fun to introduce one more member of this family, and that is gasoline (which slowly becomes gaseous at room temperature). Gasoline fumes, like propane, are heavier than air and will accrue on the floor of your garage (or wherever you store your car or a leaky can of gasoline). For this reason, the building codes have long demanded that sources of ignition, such as water heaters and clothes dryers be, elevated well above the floor (18 inches is usually cited in the code books) so that they cannot ignite this low-lying blanket of dynamite. 

Take a look at your garage and imagine what would happen to a fluid poured on the floor and that’s what gasoline fumes will do. They’ll act just like a fluid on the floor except that you won’t see it. A nice secret is that all the water heaters sold in California are now of a special type that we call FVIR, or Flame Vapor Ignition Resistant, and these are designed to prevent the disaster I’ve just described. Their “combustion chamber,” the little cavity containing burner and pilot, is sealed to prevent ignition of gasoline fumes nearby and a complex intake system allows flammable gases to burn up inside the unit without getting back out and lighting the rest. A very amazing invention and, again, the only thing sold in California for residential tanked water heating today. 

This job never fails to make me laugh, surprise me and make me scratch my head. I have no idea what the next 10 years will bring, but as long as I don’t get blown up in the process, I’m dyin’ to see what’s next.