Expansion tank for hot water heater

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Old 06-06-01, 12:06 PM
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After installing a new hot water heater, the pressure relief valve kept popping at the end of each heat cycle. I was told that I needed to add an expansion tank to get rid of this problem. Why does the new tank require this when the old tank did not?
 
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Old 06-06-01, 12:11 PM
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Do you know if you have a pressure reducing valve on the water service?

The new water heater, did it come with the nipples that screw into the top of it?
 
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Old 06-06-01, 12:26 PM
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Expansion tank for hot water heater

There is a pressure reducing valve that was there for the old tank as well. On the top of the tank there is a compression fitting for the 1/2inch pipe coming out. It is the same as was there before.
 
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Old 06-06-01, 01:56 PM
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Does the PRV valve look like this? iF so what side is this valve on, cold or hot water side of the heater?

Not sure what the 1/2" pipe your talking about, both inlet and outlet from for the water heater should be 3/4".
 
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Old 06-07-01, 05:07 AM
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The inlet and outlet are 3/4 in. on the tank. Both have been reduced to 1/2 as all the pipes on the system are 1/2 in. I have the normal pressure relief valve on the tank that is 150lb pressure and 210 degrees. It is a normal working system, and yes there is a pressure regulator on the inlet side reducing the pressure from the street to 50psi. All I wanted to know is why I have to add an expansion tank to prevent the pressure relief valve from popping each time the water heats up.
 
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Old 06-07-01, 07:47 AM
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That prv valve that is on your inlet side of the tank is causing a closed system, meaning when the water heats up there is no place for the hot water to expand to, so it builds pressure, once the pressure reaches 150lbs the pop-off trips, until it relieves the pressure back below 150lbs, the only way to stop this is by installing a expansion tank.

If there was no prv valve then the expansion can be sent backout the water service, the prv valve stops the flow of expansion back into the cold water line.

You will need to install the expansion tank on the hot water side of the tank.

See picture below.



[Edited by Plumber2000 on 06-07-01 at 12:06]
 
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Old 06-07-01, 09:03 AM
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I know that all that you have said is true. I knew that before. The question is that since all of that was there before the new installation of the new tank this pressure valve didn't pop. Now it does with the new tank. Why is it doing it with the new tank and didn't do it with the old tank? Nothing changed but the tank.
 
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Old 06-07-01, 09:19 AM
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Plumbing is strange, things do happen, it could be the old T&P was seized up and it could never trip, and upon the new one getting installed, indeed showed you of a protential hazard. So maybe this was a good thing replacing your old heater.

You sure don't want the new heater to rupture and explode.

Think about what it will do if this ever happened. The T&P is a safety device and if there to prevent a major disaster from taking place.

I have never seen but I have heard of water heaters exploding do to excess pressure or tempature, and believe me it's like a bomb.

Time to put that expansion tank in, and sleep good at night.

[Edited by Plumber2000 on 06-07-01 at 02:12]
 
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