Watt's 210

What can you all tell me about a Watt’s 210?

Try here:

http://www.nachi.org/bbsystem/viewtopic.php?p=114309

The Watts valve is simply a gas-shutoff valve that is activated by high temperature water.

If there is a Watts valve on the heater, there must also be a pressure relief valve on the system. Also, many jurisdictions do not allow their use, except by “special permission.”