expansion tank on hot water heater

In the five homes I've owned over the last 30 years, I have never had an expansion tank on the hot water heater. My daughter just bought a home (new construction) and it has one. Did something change? Do I really need one? It seems to me that any expansion of the water in the tank would just be pushed out the cold water inlet and pressure wouldn't increase. What am I missing?
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Jmagerl wrote:

Places with high city water pressure will have a Pressure Reducing Valve to lower it. By design, these valves block flow to the street (some exceptions). Also, many localities now require backflow preventer valves (check valves). All these situations are required to have an exp tank today.
http://www.stateind.com/expansion/expansion.htm
Jim
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It depends on what's upstream of the cold water inlet. If there's a checkvalve before there's a pressure tank, then you ought to have an expansion tank.
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Good points. So... since I am on a well the pressure tank will accept the expansion. Differenrt question: I was in the store the other day looking at replacing my brothers hot water heater. In the replacement kits, there were things called "heat traps". THey screwed into the inlet and outlet of the water heater. What are these? and do they in any way make me need an expansion tank? (I don't have them on my heater). I'm expecting my heater to go soon (20+ years old)

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Jmagerl wrote:

Heat traps are small ball check valves. They prevent hot water "migration" (small circulating flow) out the Cold and Hot supplies when no water is being drawn. It is a small energy-saver/. Chances are you won't need an exp tank on your well; wait and see.
Jim

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Why do you need a ball valve? Wouldn't a simple U bend do the trick?
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default wrote:

These are actually easier to install: http://www.perfectioncorp.com/water/heattrap.html since they go straight in-line from the heater threaded bosses.
Jim
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Speedy Jim wrote:

I don't see how it would....Unless they were quite tall and very well insulated.
Those "heat traps" use lightly loaded check valves to counteract the small pressure differential caused by the difference in specific gravity of hot and cold water. What they do is keep the hot water from moving up and out of the water heater and into the piping and cold water "sinking down" into the tank from those same pipes during idle periods.
When a hot water faucet is opened, mains pressure easily pushes those valves open and flow proceeds normally.
Hydronic heating systems sometimes have similar valves to prevent a similar kind of thermosyphon flow from keeping the heat in the boiler from reaching the baseboard units when the circulator pump isn't running, since water will still flow through a stopped centrifugal pump. They used to be called "gravity valves", and may still be.
Jeff

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The hot water in the tank rises to the top of the U, and stops. it's not going to to down the far side, because the water downstream is colder and denser than the water in the top of the U. you might get some convection between the top of the U and the heater tank, but that ought to be it.
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snipped-for-privacy@uri.edu wrote:

Yes, I agree now. I forgot that there's typically no "return line" as there is in a "gravity operated" thermionic hot water heating system.
And if there *was* a return line it'd be there for the purpose of providing "quick hot water" at the faucets via convection, exactly the opposite of the energy savings this thread is about, so you wouldn't want to install anything to interfere with the intended action.
Would you agree that a short U-bend wouldn't stop water from circulating when there was a return as in a heating system?
Jeff
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I was wondering the same thing. If these things are ball valves, where does excessive pressure go in the tank when the water heats? I have those on my own water heater, but it has never tripped the pressure release valve, so I dont worry about it. The only thing I could figure is that they release the pressure at a certain point ????
Mark
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snipped-for-privacy@UNLISTED.com wrote:

Yes. The tiny ball will lift open with very little pressure; there is just enough force holding them closed to prevent the thermal siphoning.
Jim
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I had a hot water heater replaced in January. The instructions said I may need an expansion tank. Water was leaking from the pressure relief valve. Sure enough, had to install an expansion tank. No more leaks.
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