do you really need a thermal expansion tank

Hi, Is it on a closed system, where expanding water has no place to go?

Yours, Larry

Hi, It's a closed system, but it sounds like there could be a pressure tank involved for your booster pump to be feeding. If water can expand into that pressure tank, you should be good to go. Pictures?🌄

Yours, Larry

When you take 70 gallons of 68° water at 60 PSI and increase the temperature to 230° water at some higher pressure* you end up with 73 gallons of water. If this is a closed system, as Larry mentioned, and there is no air cushion to compress, then your extra 4 gallons of water will try to expand by making the 70 gallon tank bulge to become a 74 gallon tank. If the tank can't bulge to accommodate that extra 4 gallons and the tank bursts, that higher pressure water that is keeping the water from changing to steam will no longer hold that water pressure. That will allow the lower pressure water to FLASH into steam. That steam will rapidly expand to 1700 times its original size of when it was water. So now you have 104 gallons of 240° water FLASHING into 176,800 gallons of steam. That is known to the fire department as a BLEVE. (pronounced Blev - ey). (Boiling Liquid Expanding Vapor Explosion). So if your 70 gallon water heater plus the rest of the water in the piping system were to have a high limit temperature switch failure and the relief valve were to fail to open when the pressure reached 150 PSI, you could end up with a problem that your Expansion tank could have resolved.

* The higher pressure will be based on several factors and may be as high as 1000 PSI or more 

But if you had a failure of the relief valve because you did not follow the manufacturer's instructions that clearly state the the "owner of the relief valve MUST operate the relief valve at least once a year to verify that the passageways are clear from obstruction"  and you also fail to test the high limit switch as a result of neglecting to perform proper maintenance on a regular basis, then you could end up with a failure of two different safety systems on your water heater.   

Not knowing the relief valve isn’t operating properly can go unnoticed for years just waiting for the limit switch to fail years later.  So both things may not happen at the same time “all of a sudden”  The second failure will happen “all of a sudden” after years of no maintenance. When the first failure stays hidden. 

Now I know that domestic hot water is technically an open system. But if there is a check valve between the water main and the water meter, or just past the water meter, then the system is no longer open to the water main on the street. the other side of your system is connected to all the water faucets and taps that you use every day. That makes it an open system, until you go on vacation for 2 weeks and no one opens a faucet or valve for 2 weeks. That makes your open system a closed system when combined with the check valve at the water main. If that closed system has for some reason to have the water temperature go higher than it is designed to because of a failure of some man made part on your water heater, then you have the recipe for disaster.

But that only happens to "other people"… Right?

Just so you know, as far as I'm concerned… You happen to be "other people"

Hi, I'm thinking it's more of a "what if" or "worst case" scenario. Ed is right than essentially nobody checks the T&P yearly, and that could be a big liability to take on. But, the more obvious problem usually is thermal expansion. Water just gets bigger as it's heated. Where will that extra water go?

Yours, Larry

Never bypass a safety feature. Installing an expansion tank in a closed loop system is a safety feature. No matter how small the risk is someone will eventually pay the price.

If it's an open system, you do not need an expansion tank. If it's a closed system, then you do need an expansion tank. If your booster pump has an expansion tank associated with it, then that should cover the expansion. If not, then one should be added.