Cylindrical Water Heater Construction Questions

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
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Robert11 wrote:

The tank is completely filled with water at service pressure and there is no air cushion.

Yes, that is why an expansion means is specified and usually required for most homes.

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Hi George:
Thanks for reply.
I have an expansion tank on my hot water heating system, =but to the best of my knowledge i don't see any for the hot water system (the cylindrical hot water heating system).
House is 30 yrs old. Just replaced the hot water heater, but nobody ever said anything about an expansion tank for it . Should there be ?
What types are used for this purpose ? I don't think I've ever seen one for the hot water system ?
Not to sharp with this stuff; the feed water is of course common to both systems; does the one expansion tank on what I take to be (only) for the heating system help the hot water system in this regard ?
Bob
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wrote:

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
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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
: 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 : : : :
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wrote:

Here ya go: http://home.howstuffworks.com/water-heater.htm
-zero
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