What is it? Pressure Relief?

[Vern Heiler]

I found two of these in the crawlspace of home heated by three 50 gallon water heaters. I did not find any kind of thermal expansion device anywhere, including in the toilet tanks.

I think there where expansion tanks hanging from the bottom of the 'T' at one time? Don't know why they have been removed.

What is the device at the top of the 'T'?

[Jerry Peck]

I'm thinking atmospheric breaks when winterizing.

Would do no good for thermal expansion or anything else if that valve were turned off.

[Harry Rezaei]

I might be incorrect, but the top of the T looks like an Automatic Air Vent..?
And maybe it can also be a Water Hammer Air Chamber?

:confused: