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