Brand new construction. This is the cold water supply valve for the H2O Heater. When the heater heats up, condensation comes out of the plastic tube. What should I call it and why is it there? Thank you in advance for all your help!
it is an expansion valve and takes the place of an expansion tank-----they waste at little water to relief the pressure----hence the little drain tube attached
I have never seen an expansion tank on a hot water tank so why an expansion valve. They are required on heating boilers due to the check valves in the water replacement/supply line. Is there a check valve somewhere in the supply line? Does it also have the regular TPR valve?
When water is heated it expands. If the inlet is not blocked by a check valve, pressure reducing valve, or other device, the increase in volume simply travels back into the source. If the inlet is blocked, this increase in volume will cause an increase in pressure, sometimes to dangerous levels and the Temp/Press valve may open.
A thermal expansion tank can be installed in the line that will absorb the increase in volume, preventing the relief valve from discharging unnecessarily.
Know the theory, Raymond, but do you see many or have seen any expansion tanks on hot water tanks unless they were being used for space heating purposes?
Brian A. MacNeish asked:
Space heating has nothing to do with when expansion tanks or expansion valves are required or should be installed on water heaters.
Plumbing Codes such as the IRC require an expansion tank or similar device on any tanked water heater where there is present a backflow perventer, a pressure-reducing valve, or other flow restricter on the cold-water supply.
Charles Buell has this one correct.
We’re starting to see and install more and more of them every day now around here.
I believe because the water company started putting in check valves to keep hot water from backing through their meters.
That’s what I found on this one last week, with gas compression fittings and no TPR valve. I couldn’t get a pic from the other side. This was a '93 W.H.
We very rarely have any of the highlighted items installed on cold supply lines here. I’ve been inspecting for 23 years and have only seen a few pressure reducing valves installed. I know why an expansion tank should be installed but is not required practice here yet, maybe due to so little need and no failures yet.
Brian
You need to get out of your corner of the world. 
Thanks for all your input! The point is, it is an expansion valve. Why it is there, we’re not sure (Yes, the heater has the appropriate TPR valve.) Maybe it’s some local code thing (Lakewood CO) But it seems to be functioning as intended. At least now I know what to call them if/when I see it again! Thanks!
It’s more relaxing down here. Some of the housing is the oldest in Canada which makes for interesting inspections. The first house I owned and renovated is over 200 years old now, had some re-cycled ( doing this around 1800???) roof boards in it and is still being lived in by the family I bought it from (who bought it back from me).
A lobster fisherman lives 600 feet down the road from me. Beautiful scenery around St. Margaret’s Bay. My kids (when small) and I had 6 different swimming holes/white sand beaches within 10 km. Cirque de Soliel is in town now (40 km away). Halifax is more like a big town than an impersonal city. My son’s girlfriend (from London, ON) couldn’t believe the friendliness (“I was just standing in line and someone started up a conversation!!”). No alarm system on the house.
No! I think I’m going to stay here!!! The quality of life is so cheap!!
Brian where do u reside, your profile is void of facts?
I have been to PEI and really like the innocence. Something you don’t see to much off. Everyone genuinely friendly.
My company logo is of a Mill circa 1849 which I onced owned. I wouldn’t move back to the big smoke for all the tea in China.
I always said if I was going to live in or near a city, it would be San Francisco or Halifax.