# Commercial HVAC Question **URL:** \ **Category:** Factual Questions\ **Created:** [November 18, 2004, 3:25am UTC](https://boards.straightdope.com/t/commercial-hvac-question/275410 "2004-11-18T03:25:37Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [November 18, 2004, 3:25am UTC](https://boards.straightdope.com/t/commercial-hvac-question/275410/1 "2004-11-18T03:25:37Z")
Don’t most buildings have a one-pass system? In other words, isn’t it a common practice to exhaust 100% of the air instead of routing a runs of return duct work back to the air handler? Or, is this a design preference? (Esp. nowawday, wouldn’t the money saved on energy by using return ductwork offset the cost of time and materials of running the ductwork itself? - Jinx 😕 ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [November 18, 2004, 5:26am UTC](https://boards.straightdope.com/t/commercial-hvac-question/275410/2 "2004-11-18T05:26:26Z")
Not in my own experience. The cost of operation would be significantly greater, were the system constantly drawing outside ‘unconditioned’ air, as opposed to filtering and recirculating what is already inside. Ductwork costs can be reduced by use of architectural details as plenums, but those savings may be offset by increased cost of wiring in those spaces, as plenum rated cable is much more expensive. ---
**Author:** ![Tripler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripler/32/21807_2.png) [@Tripler](https://boards.straightdope.com/u/Tripler)\ **Post date:** [November 18, 2004, 6:10am UTC](https://boards.straightdope.com/t/commercial-hvac-question/275410/3 "2004-11-18T06:10:48Z")
**Jinx** , no, not really. It’s much more efficient to recondition “return” air and pull a fraction of the “supplied” air from the outside than it is to do a ‘one-pass’ (a lot less windier in the building, too). Take a typical office building, set at a 68_F interior and (say it’s a hot day) a 100_F. The supply will crank out 68\* air. There’s typically not enough heat energy (from machinery, people, and fixtures) to heat that 68\* temp interior air to 100\*, so your return air may be at . . . say 75\*. For simplicity’s sake, you only need to expend energy to cool that 75\* air down to 68\* and filter it, rather than cooling 100\* air down to 68\*. Plus, you don’t need to move _as much_ air, as it’s already somewhat cooler and conditioned. Psychrometrics is not a difficult science, but rarely do things work out perfectly. I always overdesigned my systems, knowing they were industrial, they would be in a hot region (the Middle East), and the doors of said hangars might be open for extended periods of time. Most systems I’ve installed _do include_ an outside air supply duct fit on the return, to introduce fresh air into the working/living space, but then again, it’s a fraction. But if you absolutely _wanted_ a 30-Ton unit for your home, I can make that happen for you. Tripler I’d have to charge you extra for the penguins, though. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [November 18, 2004, 11:53am UTC](https://boards.straightdope.com/t/commercial-hvac-question/275410/4 "2004-11-18T11:53:59Z")
Oh, perhaps the place I am thinking of must have had a large return plenum above the ceiling… Thanks, that makes a lot more sense, economically… - Jinx ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [November 18, 2004, 12:47pm UTC](https://boards.straightdope.com/t/commercial-hvac-question/275410/5 "2004-11-18T12:47:31Z")
The fresh air intake is also filtered. Frequently by HEPA type filters to reduce the effects of allergens and other obnoxious particluates on builing occupants.