Expansion Tanks & Tankless Water Heaters

Anyone see a reason to put an expansion tank (or other thermal expansion control) on a home with a tankless water heater? The code doesn’t provide an exception, but I don’t see a reason for it.

I’ve looked at a few different installation manuals for tankless and have not seen one required. I would have to think that since it is only heating water as it travels through the unit you don’t have the expansion issues you do with a tank full of hot water.

Exactly my thoughts.

The water still expands in the system but is not subject to flashover to steam with a large volume like a water heater tank is.

never seen an exp tank on electric or gas, on demand, heaters

Out here we don’t even use expansion tanks on tank water heaters. In 11 years I’ve only seen two, both installed by plumbers who moved here from back East.

Why is that?

They are required on newer construction ‘out here’ I have see thousands in the past 12 years. Carlsbad, San Clemente, fallbrook etc are loaded with them

I have no idea.

I haven’t seen any in new construction either, but it’s been a long time since I’ve done an inspection in San Clemente or Fallbrook.

I do a lot of new construction in Del Mar Heights, La Jolla, 4S Ranch, Chula Vista… not a single one.

Something else comes to mind, I was told that when a back flow check valve is installed (now required here in Bend) to stop homes water from backing up into City water, that the back pressure can cause extra pressure on the lines/connections/fixtures etc…
Any thoughts on this?

It is required to stop the homes water from backing up into the city water, that is being caused by the back pressure which can also cause extra pressure over the lines or to the connections. These are somethings that strikes my mind.