What's This Thing?

[George Potter]

I know I'm going to be feeling a little silly as soon as someone tells me the answer, but I'm stymied. I saw this the other day attached to the duct in a new home. It appears to be some sort of counterweight, and if it is, its purpose escapes me. The "puck" slides along the rod and looks like it should be able to be locked into position anywhere along the rod. In this case, it was just loose, sitting at the bottom. Anyone? Anyone? Bueller? Anyone?

[Erby Crofutt]

It's a static pressure-air flow damper assembly with a counter weight.

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[Jim Luttrall]

Yes, the weight should have been adjusted by the installer and locked in place, but I doubt it would make a difference in that position if it works.
This was on a zoned system with two or more damper motors, right?
The standard use around here is on two story house supply ducts. The unit you are seeing would be installed in a bypass duct from the supply side to the return. This allows the full air flow through the system without throttling the ducts down and causing excess air flow through the one zone that is calling.

[Scott Mitchell]

Erby & Jim have it right. This is basically an inexpensive way to control hvac flow to typically a two story home. Most don't operate very efficently or for long.

I bet their are two thermosts, but only one hvac unit.

#1 It's in the attic duct work, # 2 Out of sight, out of mind. # 3 Nobody but the inspector even knows it's there or how to check it's operation. # 4 It will never be serviced or checked for proper operation. # 5 Most Hvac service tech's won't have a clue what is does or even find it.

Scott Mitchell

[Paul Johnston]

How would the system work with 2 thermostats and one unit?

[Michael Thomas]

Most don't operate very efficiently or for long...#1 It's in the attic duct work, # 2 Out of sight, out of mind.

I can't check then for "efficiency" but I always check them for "operation", and a I now report them when I see them, and note that their inspection should be part of normal yearly HVAC maintenance.

[Michael Thomas]

How would the system work with 2 thermostats and one unit?

http://www.netropolus.com/Pictures/2ZoneHVAC.gif

[George Potter]

Man-oh-man! Inspection News guys are the best! Thanks for all the insight.