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Do you need an expansion tank on a tankless with no main pressure regulator and no circulation line? I don’t think you do, a realtor I do a lot of work for had me check out a house he’s selling and a home inspector called it out. Nobody installs expansion tanks here on tank or tankless. #4 in the report
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You only need an expansion tank if the increased volume has no where to go. If the water in your heat source can expand and that pressure can go somewhere you don't need an expansion tank. Blue2 mentioned a recirc loop, if it has a check on the incoming cold then yes, it will need an expansion tank.

If there are no check valves, and there is no prv on the cold main to the house OR the prv has a built in bypass to allow excess pressure back into the city, you don't need an expansion tank. You're best to assume that any prv has no bypass, like anything mechanical it can fail and you shouldn't rely on it.

You probably don't need an expansion tank at this house, but if they're willing to pay for one just do it, pretty easy money right? 😉
 

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View attachment 132432 This is right out of the installation manual. It shows an expansion tank. I’d put one in.

both of those show an expansion tank after a check valve or backflip preventer. Even if there was a prv installed you would need one. If none of those situations are present then you would not need an expansion tank.
Like what was said previously, it’s not going to hurt anything if one gets installed and it is not needed.
 

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If you are going to be technical about it, almost any water heating system will benefit from an expansion tank. They aren’t required around here unless you have a closed system (checks, prv, etc.). Pressure isn’t static. It fluctuates as a function of the use of the local water system. At my house, at 3 AM water pressure with no prv can be as night as 175 psi. Even if the pressure fluctuates between 40 and 80 psi, the expansion tank will help absorb some (not all!) of that stress on the tank or heat exchanger. I think that in they are excellent for the longevity.

Most of the water heaters that I replace, I just work an expansion tank in to the cost of the install. I explain it thoroughly though.
 

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We don't need expansion tanks here because the expansion goes back into the city water. If you have a backflow preventer on commercial(mandatory) then you need an expansion tank because the water cannot expand into the cities' system.
 

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Does thermal expansion come into play with a tankless? It doesn’t fire until there’s flow so I don’t think so.
But if the AHJ says it gets an expansion tank then it gets an expansion tank. If not, tell the home inspector to kick rocks.
 

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Does thermal expansion come into play with a tankless? It doesn’t fire until there’s flow so I don’t think so.
But if the AHJ says it gets an expansion tank then it gets an expansion tank. If not, tell the home inspector to kick rocks.
You are on the right train of thought. Where it throws it off is systems that have a recirculating loop through the building, or even an internal recirculating loop such as the Navien 240a.
A properly installed recirculation system will have a check valve on the cold inlet, creating the closed loop that requires expansion tank.

Spoke with a navien rep and they said even if it is not required, the recommended an expansion tank on any of the npe-a models since they have the buffer tank inside of them.
 

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Did you guys notice in the diagram it shows an optional 8-10 gallon storage tank to prevent 'cold water sandwich'?

So they are almost recommending a tank to assist in a tank-'less' water heater.....Hmmmm.....a tank to assist tankless. That almost defeats the whole point of tankless. Afterall, tankless is supposed to eliminate a tank altogether!
But at least they throw that in there to suggest it to the plumber.
 

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Did you guys notice in the diagram it shows an optional 8-10 gallon storage tank to prevent 'cold water sandwich'?

So they are almost recommending a tank to assist in a tank-'less' water heater.....Hmmmm.....a tank to assist tankless. That almost defeats the whole point of tankless. Afterall, tankless is supposed to eliminate a tank altogether!
But at least they throw that in there to suggest it to the plumber.
Start calling them small tank systems lmao.
The companies that have internal recirculation solved that. Such as navien.
 
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