Hmm, you know, I never gave the sacrificial anode even a passing thought before your post. I do drain the bottom of the tank periodically, whenever I think of it, which might range from weekly to once a season, depending.
I was just looking at mine; oil fired type: I can't see any evidence of a sacrificial anode that could be changed - guess I'll move on to zero's link and see what I can see.
Good post,
Pop
"Paul Franklin" wrote in message news: snipped-for-privacy@4ax.com... : On Fri, 18 Nov 2005 07:48:42 -0500, "Robert11"
: wrote: : : >Hello: : >
: >Never had the opportunity to look inside one, but as we just had a new wter : >heater installed, I became curious. It's the typical upright cylindrical : >kind, gas fired. : >
: >Was wondering: : >
: >Is the unit designed to keep a certain water level always in the tank, or is : >it designed to trap an air bubble at the top ? There isn't any float : >regulator inside, to the best of my knowledge. : >How does this work ? : >
: >If it truly fills totally with water, isn't the internal water pressure, : >with no elastiucity, a problem ? : >
: >How do they maintain such a good (hopefully) seal at the top of the tank ? : >I imagine that the tank is fabricated from rolled steel, with a top and : >bottom fastened on somehow. How is this done and sealed ? : >
: >Any pix on the web showing internal construction ? : >Couldn't find any. : >
: >Thanks, : >Bob : >
: Having recently "deconstructed" one (never mind why) I can tell you : how they were constructed 10 years ago or so...I doubt they are much : different now, except for the flame arrest combustion chamber. : : The tank sides are formed from steel about 1/8 inch thick. There is a : welded side seam, and the domed top and bottom pieces, also steel, are : welded to the sides. Both with dome up, by the way. Threaded openings : for the drain, inlet, outlet, sacrificial anode, and temperature and : pressure relief valve are provided by steel blocks about 1/2 inch : thick welded to the appropriate locations, with threaded holes. The : entire inside of the tank is coated with "glass". This isn't a thick : coating, but more of a very thin layer that I suspect is done the way : porcelain is done: a fine power is applied and then baked in a large : oven to melt and adhere the powder into a continuous film. This is : done for corrosion protection, and it is the failure of this coating : that usually leads to water heater failure. : : For gas fired heaters, in the center of the tank is another, smaller : diameter tube (about 3-4 inches in diameter. This runs all the way : through the tank, from top to bottom, and provides the path for the : flue gases to flow from the burner to the flue. This tube is also : steel and is welded top and bottom as well. Inside this tube, which : is outside of the tank, but inside the tank too (if you can follow : that!) is a spiral steel baffle that is designed to slow down the flow : of flue gasses and transfer as much heat from them to the tank as : possible. : : In operation, the tank is completely full of water, at full water : pressure. The thick steel walls are more than capable of withstanding : normal water pressure and more. The T&P relief valve is designed to : prevent the pressure from rising high enough to cause tank failure, as : might happen if the burner control failed and the heat source did not : turn off. Another poster mentioned the use of external expansion : tank. This is not intended to prevent the tank from failing, rather : it is intended to prevent the internal pressure from rising high : enough (during normal operation) to either cause the T&P relief valve : to open, or to cause premature failure of washers and seals in : plumbing fixtures. An external expansion tank is really only needed : if there is a check valve or equivalent in the path of the cold water : inlet such that water cannot flow back into the water mains or the : well pressure tank. As the water is heated, it expands, and if it : some can't flow back out the inlet to the water source, then internal : pressure will increase, sometimes significantly. This is the situation : that calls for an external expansion tank, which uses an air bladder : to provide expansion space. : : The outside of the tank is covered with spray applied foam insulation, : more or less depending on the efficiency and cost of the heater. The : foam insulation is wrapped with the thin sheet steel outer jacket that : you see when you look at the heater. : : The sacrificial anode is a metal rod, usually zinc I think, sometimes : aluminum, that screws into one of the openings on the top of tank and : extends into the tank. It's whole purpose in life is to extend the : life of the tank. The glass coating on the tank inevitably has : pinholes, if not at first, eventually. These pinholes provide a place : for corrosion to start, and once started, it proceeds rapidly until : you have a leak. The metal of the sacrificial anode is chosen so that : it corrodes first, before the steel, thus preventing the tank from : corroding. Checking the sacrificial anode every few years and : replacing it when it is nearly used up is the best way to extend water : heater life, especially if you have soft water that is aggressive to : the steel. The other way to prolong life is to periodically drain out : the sediment that builds up on the bottom : : : :