My God I wrote that a long time ago. And no there are not two Ians…I am one and the same. I just post more now over at Terrys, but still keep an eye on the stimulating debate over here! 
No Chuck… it’s over! Actually, the reason I felt the need to keep repeating myself (as I do here, again,now) is that it was obvious to me that supposedly professional people out there were either not willing or not capable of reading what was said and staying on issue without becoming a wise ***. I shall report… “Work done in a non professional manner”.
Absolutely!.. …
Bored?..digging up bones? Lol
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I know this is an old thread, but I just replaced my water heater and it caught my eye.
I won’t comment on code requirements in any particular location but follow the code. However, I think there are a few definitive answers related to this discussion.
Why would anyone install a second valve on the hot water outlet in addition to the cold water inlet? There are several good reasons. (1) A valve on both hot and cold legs avoids needing to drain the hot side of the piping (not a huge advantage since you will still have to vent the new heater, but it does save some time during draining). (2) If you have any single lever faucets you can’t use any of them during the heater replacement since a mid-position (i.e. not fully cold) will allow some cold water to migrate back into the hot water line, which can make soldering more than a challenge and (3) With a valve on the hot water side and a union you can isolate the heater and inspect/replace the anode rod as needed with very minimal draining. This is particularly helpful on water heaters where the anode rod requires removal of the hot water line, especially if you inspect the anode rod every three years like some manufacturers recommend. There may be more reasons, but I think that is enough.
Relative to over pressurization risk, having a valve on the inlet and outlet side of a water heater vs. just one on the inlet side adds no measurable risk to the probability of heater over pressurization. For practical purposes water is an incompressible fluid, heating will cause expansion, and any expansion in a close system will increase pressure. Assuming your piping is air free (i.e. no trapped air), you do not have an expansion tank (and many homes do not), all your end use valves (sinks, tubs, etc.) are closed and you have no leaky faucets, once you close the cold water inlet valve you have a closed system. Closing a second valve on the outlet just makes the closed system slightly smaller. If the heating source is still active, water will expand when it is heated increasing the pressure in the tank creating a possibility the TPR valve should lift on overpressure with either one or two valves. That is assuming your TPR valve works. Also, if your tank is already hot (i.e. at or near setpoint) the water is already expanded so there is almost no probability of over pressurization. Maybe we should be worried about the water cooling an contracting causing the water heater to collapse inward like a steam filled tin can in a science experiment (not really, but it seemed funny).
In conclusion: There are several very good reasons why two valves are better than one. Two valves do not realistically increase risk of over pressurization compared to one. Finally, regardless of one valve or two, turn off the heat source before closing any valve and the concern of over pressurization becomes nonexistent.
I think you know; I spent the extra couple of dollars to install two valves.
I know this is an old thread, but I just replaced my water heater and it caught my eye.
I won’t comment on code requirements in any particular location but follow the code. However, I think there are a few definitive answers related to this discussion.
Why would anyone install a second valve on the hot water outlet in addition to the cold water inlet? There are several good reasons. (1) A valve on both hot and cold legs avoids needing to drain the hot side of the piping (not a huge advantage since you will still have to vent the new heater, but it does save some time during draining). (2) If you have any single lever faucets you can’t use any of them during the heater replacement since a mid-position (i.e. not fully cold) will allow some cold water to migrate back into the hot water line, which can make soldering more than a challenge and (3) With a valve on the hot water side and a union you can isolate the heater and inspect/replace the anode rod as needed with very minimal draining. This is particularly helpful on water heaters where the anode rod requires removal of the hot water line, especially if you inspect the anode rod every three years like some manufacturers recommend. There may be more reasons, but I think that is enough.
Relative to over pressurization risk, having a valve on the inlet and outlet side of a water heater vs. just one on the inlet side adds no measurable risk to the probability of heater over pressurization. For practical purposes water is an incompressible fluid, heating will cause expansion, and any expansion in a close system will increase pressure. Assuming your piping is air free (i.e. no trapped air), you do not have an expansion tank (and many homes do not), all your end use valves (sinks, tubs, etc.) are closed and you have no leaky faucets, once you close the cold water inlet valve you have a closed system. Closing a second valve on the outlet just makes the closed system slightly smaller. If the heating source is still active, water will expand when it is heated increasing the pressure in the tank creating a possibility the TPR valve should lift on overpressure with either one or two valves. That is assuming your TPR valve works. Also, if your tank is already hot (i.e. at or near setpoint) the water is already expanded so there is almost no probability of over pressurization. Maybe we should be worried about the water cooling an contracting causing the water heater to collapse inward like a steam filled tin can in a science experiment (not really, but it seemed funny).
In conclusion: There are several very good reasons why two valves are better than one. Two valves do not realistically increase risk of over pressurization compared to one. Finally, regardless of one valve or two, turn off the heat source before closing any valve and the concern of over pressurization becomes nonexistent.
I think you know; I spent the extra couple of dollars to install two valves.