T&P valves means temperature and pressure. The T&P valve should be left on the tank so that the insertion probe part of the T of T&P is functional.

The potable water expansion tank belongs on the cold water pipe before the tank.

The T&P valve is inplace and works fine. The expansion tank is just teed in ahead of it.

What are the reasons the expansion tank belongs on the cold water side?

If he added a t to pipe in the expansion tank, then he extended the distance that the reief valve is inserted into water, thus an extended temp probe T&P would be in order. Since the standard is 5" and the longer ones are 8", with a T and nipples, I doubt even an 8" would be inserted as far as a factory 5" would be. What's the difference? Reaction time and temp.

Your heater is provided with a properly certified combination temperature - pressure relief valve by the manufacturer.

This along with installation manuals stated that the potable water expansion tank belongs on the cold water side so are you looking for a blessing on your install and alterations or just wish to challenge manufacturers recomendations on installation?

Actually the configuration is very different than you have assumed. The hot water heater is 2 indirect heated tanks (Superstor SS-119) using a boiler (McLean Weil PPG6) to heat the water. At the top of the hot water tank is a threated fitting where a pressure relief valve can be added. The installer removed the plug in this fitting, installed a tee followed by 150 degree pressure relief valve. To the tee an Amtrol ST-12 expansion tank was installed. The expanding water goes to the expansion tank. The pressure relief valve opens if the presssure exceeds 150 psi. There is no T&P valve or probe as you have described. This is all handled elsewhere separately using electronic controls.

The entire installation was done a reputable plumbing company and passed all inspections.

I know that the expansion tank manufacturer says to install the expansion tank on the cold water side. I know the instructions says this. The question is why should it be on the cold water side? And why not on the hot water side? Anybody know?

"The question is why should it be on the cold water side? And why not on the hot water side? Anybody know?

Because hot water will shorten the life of the bladder in the thermal expansion tank.

Expansion tanks are designed to be installed on the cold water side of the WH as detailed in their installation instructions.

"Because hot water will shorten the life of the bladder in the thermal expansion tank."

I could understand that except that the Amtrol ST-12 is the exact same expansion tank we have in our hydronic closed system that runs at a temperature in the range of 120 to 170 degrees at all times. Amtrol specifies the ST-12 as an expansion tank for hyrdronic systems with a working operating temperature of 150 degrees and a maximum of 200 degrees. That means the bladder is designed for that range which is much the same as the hot water temperatures. Curious.

It certainly helps to share all info when asking question. When hot water tank is mentioned who in their mind when guess you have an indirect?

You mentioned you have a T&P valve earlier so if it is truly a T&P it will have a short, approx 5" probe out the end of the threads. However now that I know the size of the boiler (heat source), I doubt the correct one was used on the indirect tanks to match the BTU's of the boiler, so that does not meet code even though many inspectors are not educated enough to catch that.

If you were mistaken in calling it a T&P valve and it truly is a pressure relief valve only, you inspector missed this important issue also.

Justalurker as the manufacturers are correct, one other reason is energy efficiency. Why capture and store hot water when expanding cold water captured in tank is less heat loss. They are not the same tank, potable water expansion tanks have a poly type liner to keep active fresh water from corroding the steel tank wall. This liner is similar to a milk jug, which doesn't hold up as well with hot water. Every time it empties, the hot water having warmed the liner will try to suck it with the leaving water in the tank. Not so with a boiler tank, as it is a closed system, less entrained oxygen and no liner present.

I have seen potable water taanks used on heating systems, especially with oxygen issues, like non-barrier radiant tubing, but these dont see the higher pressures or large pressure fluctuations as a domestic water system does.

I received an answer back from the manufacturer as shown below. It makes perfect sense. Given all the uninsulated copper pipe in the hot water system in my unheated mechanical room and crawl space, I don't think this reason will affect my configuration much so I don't plan to move the tank to the cold water side. Maybe one day when I insulate the pipe, I'll move the expansion tank too.

"We recommend the cold water side installation because the tank is not insulated.
When the expanded volume enters the tank, it will cool down. With the tank on the cold water side, this expanded volume will pass through the water heater before going out to the faucets."

"Manufacturers tend towards how instructions rather than why instructions.
For the why questions contact the manufacturer and ask for the philosophy department. ;-)"

I understand what you mean. Most installers don't need the why, just the how. In my case, the why means something to me. The bladder in the tank has failed and so I have to install a new tank. The question is should I do the 20 minute replacement in the current location or cut pipe and sweat new fittings to move it to the cold water side. The installation was done by a reputable licensed plumbing company and I am not licensed so I usually defer to their judgement. I like the 20 minute answer but will do the more involved one if there is a good reason. I called the local building department and they said the installation meets code. Good. That's one reason to leave it where it is. That led to asking the "why". Now that I know the answer, as shown in my previous post, I can leave it where it is, for now. No harm. No foul.

Thanks for everyone's help and feedback.

"It certainly helps to share all info when asking question. When hot water tank is mentioned who in their mind when guess you have an indirect?"

I do agree completely with this. Usually I find, though, that the more detail given, the less likely to get an answer to the question being asked. The additional detail allows a responder to go off in directions not related to the post. It's a tough call. In this case, whether the tanks were indirect or direct, as well as the T&P valve situation was not germaine to the question which was "why do manufacturers say to put the expansion tank on the cold water side?

"You mentioned you have a T&P valve earlier so if it is truly a T&P it will have a short, approx 5" probe out the end of the threads. However now that I know the size of the boiler (heat source), I doubt the correct one was used on the indirect tanks to match the BTU's of the boiler, so that does not meet code even though many inspectors are not educated enough to catch that"

I was wrong when I said T&P valve. Sorry. It didn't affect the question I was asking though.

"If you were mistaken in calling it a T&P valve and it truly is a pressure relief valve only, you inspector missed this important issue also."

The temperature and pressure valve is elsewhere in the system. It is quite a complicated system. It does meet code. Nonetheless, the T&P valve error I made had nothing to do with the question.

"Justalurker as the manufacturers are correct, one other reason is energy efficiency. Why capture and store hot water when expanding cold water captured in tank is less heat loss. They are not the same tank, potable water expansion tanks have a poly type liner to keep active fresh water from corroding the steel tank wall. This liner is similar to a milk jug, which doesn't hold up as well with hot water. Every time it empties, the hot water having warmed the liner will try to suck it with the leaving water in the tank. Not so with a boiler tank, as it is a closed system, less entrained oxygen and no liner present. "

Yes, you are right. This is the reason the manufacturer gave me this morning.

"I have seen potable water taanks used on heating systems, especially with oxygen issues, like non-barrier radiant tubing, but these dont see the higher pressures or large pressure fluctuations as a domestic water system does. "

I agree. The pressure fluctuations are very different as are the oxygen issues. I actually have the same model tank on the hot water system, the hydronic system and fire sprinkler system. The manufacturer certifies that tank for all 3 uses. The local building department approves it for all 3 uses too.

Again, thanks to everyone for your help.

Elsewhere in the system?

Every heated storage vessel must have Temperature & Pressure protection, plain and simple. If they are elsewhere, how do they protect the indirect as it is heated, especially in the event of someone closing valves.

Failed aquastat on tank, boiler firing or better yet over-firing and tank isolation valves are inadvertently closed and you have a bomb.

Ever seen the video of the water heater fired with its T&P valve plugged? It was a missle!

That is code and no ifs, ands, or buts.

Install manual states there is a factory installed T&P valve with long element sized for the BTU size and capacity of the tank.

Tell us again why you dont have a T&P on tank and why you have something elsewhere?

Interesting, Superstore drew the exp tank on the cold water inlet line also.

And you trust your installer still? Can he read or at least interpret pictures?

Here is a link that might be useful: Superstore install manual

Here is an excellent explanation of why it is located on the cold supply side: Tim Carter's, Ask the Builder

"Here is a link that might be useful: Superstore install manual"

Your explanation makes sense if we were talking about an SSC-119, which is what your link is for. As I said, we have two model SS-119 units. They are quite different than the SSC models.

"Here is an excellent explanation of why it is located on the cold supply side: Tim Carter's, Ask the Builder"

I read this before I posted my original question because the explanation didn't say why on the cold side instead of the hot side. Did I miss it?

Here's the installation instructions for Watts potable thermal expansion tanks... http://www.watts.com/pdf/1915356.pdf

I believe they manufacture and sell more thermal expansion tanks in the US than all other manufacturers combined. Watts specifies the installation on the cold side.

You might call them and ask for an engineer to explain why.

SSC's replaced the SS models as your tank is at least 5 years old. Install instructions remained the same as did ANZI standards regarding T&P valves

In addition your existing tank most likey failed due to missaplication and possibly even missing relief valve that would have functioned versus burst the inner bladder and/or vessel.

You are in denial and wish to play with you and your families safety.

you missed it. he clearly explains why..
http://www.askthebuilder.com/B102_Water_Heater_-_Expansion_Tanks_shtml_
the expansion tank is installed on the cold water side of a
water heater...it is code..the umc..[uniform plumbing code]
there are no arguements,ifs ands or buts. the code says itSHALL be...

"SSC's replaced the SS models as your tank is at least 5 years old. Install instructions remained the same as did ANZI standards regarding T&P valves
In addition your existing tank most likey failed due to missaplication and possibly even missing relief valve that would have functioned versus burst the inner bladder and/or vessel. "

You are in denial and wish to play with you and your families safety."

I don't know if the SSC replaced the SS but the SS did not come with a T&P valve and the installation instructions were not the same. See http://www.jtgmuir.com/manufacturers/superstor/ifwhbrochure.pdf plus the install instructions on the HT web site.

The expansion tank was replaced by the manufacturer under warranty. It was defective.

The pressure relief value was working properly. That's how we knew the expansion tank had failed.

As indicated in a previous post, the manufacturer recommends the installation of the expansion tank on the cold water side because the tank is not insulated and therefore it would be more energy efficient to put it on the cold water side. Otherwise there is no difference.

"you missed it. he clearly explains why..
http://www.askthebuilder.com/B102_Water_Heater_-_Expansion_Tanks_shtml_
the expansion tank is installed on the cold water side of a
water heater...it is code..the umc..[uniform plumbing code]
there are no arguements,ifs ands or buts. the code says itSHALL be..."

I read the entire article you referenced. I see where it says to install it on the cold water side but I don't see where it says why to do that and I see no reference to the umc you mentioned. Could you post the entire paragragh you are seeing in that article that says why and cites the code section.

Can you site the actual section number in the umc and or uniform plumbing code you mentioned that says expansion tanks on potable hot water systems must be on the cold water side. That would help greatly to go back to the plumber and building inspector.

They just have to be on the cold side. That's how they work....you want to prevent hw expansion/backflow/contamination of the cold side.

The reason the expansion tank should be located on the cold water side is to prevent bacterial growth. Warm water temperature mixing within the volume of water contained in the expansion tank sets up conditions for bacterial growth.