HVAC "hard pipe" w/mechanical damper ?

1958 house, slab foundation, HVAC supply ducts are hard pipe in slab done at time of pour (NOT typical in Texas).

Supply vents are therefore in floor throughout home.

Each vent has a mechanical damper control (spring loaded) with a felt-type product to seal supply pipe when there is no air flow.

Can anyone help with some history of such a control? I’m just curious to learn more. It worked great … will work better once vents are cleaned real good and “guck” is removed from hinge points, etc., etc.



Nolan; we use a VAV box (variable air volume) in commercial A/C it is a rubber bladder used to control volume. have not seen any such animal as you describe in my area especially in residential someone must of had a brain Fa**

I can clearly picture a post-war housing development in Columbus, OH of 1,000’s of identical homes with that exact same damper setup. I have no additional information on those dampers. I do seem to remember some issues with the underslab ductwork permitting entry of radon gas.


I’m gonna guess that you may be looking at a “Fire-damper”. I’ve designed a number of Residential Monster’s in the past. A number of them had sub slab duct installations. We used galvinized, spiral, pvc and designed in sheetmetal sleeves to house fire dampers. Arch./Engr. went a little overboard, but we designed and installed them, and they worked fine.
It’s just a guess, but if they are FD’s, recommend having the fuseible links checked or replaced by a knowledgeable HVAC Contractor


You are exactly right we are looking at fire dampers. Over the past 11 years I have designed and installed about 400 of such underground systems. I designed and installed one about 8 years ago in a house that Michael Jordan owned on Hilton Head Island, South Carolina (although I never seen him their). Over the course of the last 11 years I have only known of one of those systems to have problems and that was because of a faulty foundation.