Is this silver round thing a pressure reducing valve? Looks a whole lot like the pressure reducing valve I see on the main water supply, but ive never seen one on a water heater before, only the much bigger diaphragm-expansion tank deal.
It looks like a small thermal expansion device. I’ve never seen one like this before so I’m not familiar with it.
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Martin is correct as usual. I ran across one like that some months ago. It is made by Zilmet if I recall
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Just found it as you were posting. Thanks!
Yep, Just a mini version of what I’m used to seeing. That makes sense as there isn’t much water to expand in a tankless at any given point. Does that mean that tankless water heaters installed with the bigger ones are overkill?
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It does not. Every expansion tank manufacture has their size for a certain water heater size.
A 40 gallon water heater may require a different size expansion tank than a 75 gallon water heater.
I’m talking about tankless water heaters. I commonly see the bigger expansion tanks installed on tankless. Would a tankless water heater ever reasonably need an expansion tank like the one all the way on the right in that above picture? I’m just curious now that I’ve seen a variation.
That depends on your jurisdiction and the requirements from the water heater manufacture. Requirements for a thermal expansion device on a tankless water heater has changed over the last couple of years.
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According to the manufacturer, it depends on the water volume. This device controls 2 gallons volume. What does a tankless hold? I dunno.
http://zilmetusa.com/Thermal-ZEP1
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total guess, but I cant imagine any tankless heaters hold much more than that. the volume of the entire enclosure cant be much more than 5 or 6 gallons.
Spendy little guy. 
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I agree Michael. When I flush a 200,000 btu tankless water heater I loose between 1/2 to 1 gallon of vinegar in the coil.