This is a new home, townhouse. this valve is mounted above the hot water tank, tank has a standard TPRV, this looks like a shutoff with a drain, drain line runs into the drain pan, is it a pressure relief as well? Can anyone tell me more? Lucky my client didn’t ask! 
John Kogel
www.allsafehome.ca
Pressure relief valve. Serves the same function as an expansion tank. That’s my guess.
John was it on the cold side.(I hope)?
Check out this cool site
http://www.apollovalves.com/photolibrary/photo.asp
Thats the brand at your job.
Can you make out the model number from your save?
According to the catalog: Combination Water Heater Shutoff and Thermal Expansion Relief Valve
I’ve never seen one before
Bob, Chuck…did you click on the link in post #3? Conbraco & Apollo are made by the same company.
Yeah I noticed.
Saving that site.
Not I. Skipped right past it. I don’t usually click links without descriptions (not a critique, just a habit), else I would have known you already had it covered down inside the .pdf.
I wish someone had described my ex wife ahead of time.
Thanks everybody. Alternative to expansion tank, got it! 
Could have used one of those in this tankless installation.
Nah, expansion tank is better.
In theory, a relief valve set at an appropriate pressure would work.
However, you then have have 3 problems. You now have to deal with
sending the released water somewhere. It wastes water. And the
valve could eventually fail to close completely. An expansion tank
is the better solution, which is why it’s used.
go here for some good stuff on this subject.
Acknowledged. Thanks, Robert.
I did an inspection yesterday and water was flowing at a rate similar to an AC condensate discharge out of one of those valves. At what point is it a problem? How long should the water flow and how often???
