Question about powered exhaust

[moose412000]

I'm a residential tech, but starting to get into some light commericail. My company did a job with a couple Lennox package rooftops.(Sorry, no model# with me now). Units are equipped with econimizer and a power exhaust below the econimizer. Wondering if anyone can give me some insight on the purpose and benefit of the exhaust.

Thanks

[damedi07]

it can do a number of things......for instance if the unit is hooked up to a CO2 sensor it can kick the power exhaust on in the level gets too high. or say if the unti is economizing it may kick on the power exhaust once the economizer opens to a set point, say 50% for instance.

[maxster]

if there was a smoke condition after a fire within,the fire department would "key in" the exhaust to purge the space 100% their call to start.during normal occuppied times if your bringing in OA to cool the space in the winter your ballooning the space with it.with the dampers in an OA mode and the return almost closed the exhaust would do its job by pulling the return air up from the space and dumping it out...the unit supply fan would still be running pulling in OA to cool the space with your exhaust discharging the heated RA.this set-ups on all brands is mostly on the economizing side during fall/winter rarly during your summer months...maybe a cool morning or if the building sytem call for purge of the space prior to occuppied times

[nineball]

For the most part power exhaust is used to relieve building pressure while the unit is economizing. When the econimizer is bringing in OSA you are acutally filling the building up with air much like a balloon so you need to remove the extra air that you are bringing in or the doors to the building will be held open. Some buildings you see will not have power exhaust instead they will have barometric relief dampers on the R/A side to remove building pressure.

[pat461]

If the return air is pulled from a high ceiling , say 18ft , it could be 10 to 20 degrees hotter than @ 5ft. exhausting that air and bringing in cooler air thru the economizer will save you a good bit of money.

I've seen smokey facilities kick out the smoke thru powered exhaust.

[heatingman]

above and beyond what the others said, exaught fans normally come on when the economizer opens beyond a restable point say 50 %. there is dual function 1 presure relief since adding air to building, and 2 removal of hot air via the return during econ mode since the econ damper opens to outside and closes to return air, causing no removal of room air w/o exaughst.