What is this?

[Matthew Barnicle]

It was on the 2nd floor of a 6 story condo building with condensing unit on the roof. This device was attached to the liquid line adjacent to the air handler. My guess is that it prevents too much or regulates pressure since the condensing unit is so high up????

[Richard Moore]

It's a "normally closed" solenoid valve. The valve opens and allows flow to the apartment's air-handler when called for by the thermostat.

No expert on this, but you are probably right about the height and the "weight" of the liquid refrigerant being a factor although it doesn't appear to be a "pressure reduction" valve. I suspect it could screw the system up if the evaporator was left "open" when the whole system wasn't running.

[Dom D'Agostino]

From Emerson's site, it looks exactly like yours:

http://www.emersonclimate.com/flowcontrols/pdf/Solenoids.pdf

[Matthew Barnicle]

The unit has it's own condensing unit on the roof, so I don't know why they would need a solenoid for the refrigerant other than because of the height issue which may effect the pressure.

[Markus Keller]

It could be a new improved version of a line trap. If you have more than (I think it's 30" roughly) of vertical run on a refrigerant line you need to install an anti-siphon trap. Most hvac guys just install a copper trap, maybe this is technology at work. Just my guess.