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Subaru
1-3-2-4
Premium Member
join:2001-05-31
Greenwich, CT

Subaru

Premium Member

HVAC fan setting

I have a question on the HVAC fan setting on a commercial setting are they just recycling the air? Or does it vent outside?

This building has 9 AH's and I was just wondering.. I did turn them all to fan and went above and it did seem like it had some positive pressure.. but not 100% sure.

Not a great picture with the phone..


ArgMeMatey
join:2001-08-09
Milwaukee, WI

ArgMeMatey

Member

The commercial units that I've worked with have all had pneumatic-controlled fresh air/makeup air dampers. Sometimes those dampers are on a tstat so they don't pull in as much cold air in the winter, and also there are freeze stats that close the damper completely if there's a risk of freezing the hot water coil.

You would have to open the control panel and see how the air lines are connected. Often there are pressure gauges showing which dampers are being pushed or pulled, but you might have to know if the dampers are spring return, too.

We used to have parties in a cafeteria ... usually the fresh air was minimal for cafeteria use, but for parties, I would set the controls to fully open the fresh air & exhaust dampers. It was easy to tell what was going on because the controls were clearly marked and I could see the damper vanes open as I turned the control.

Subaru
1-3-2-4
Premium Member
join:2001-05-31
Greenwich, CT

Subaru

Premium Member

you know it could be on a damper but would something like that be noticeable? it was rather warm in the space below but I'd say the air temp from the vents was about 60 or so.. However above the tiles it was a tad chilly.

Msradell
Premium Member
join:2008-12-25
Louisville, KY

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I certainly agree that's almost commercial HVAC units operate but by looking at the picture provided it doesn't look like there's any penetrations through the roof so the unit may be 100% recirculation.

Subaru
1-3-2-4
Premium Member
join:2001-05-31
Greenwich, CT

Subaru

Premium Member

Yeah i was checking and I did not see anything for the roof.. which btw above the drop ceiling it's about another 10-15 Ft before you get to the roof.

ArgMeMatey
join:2001-08-09
Milwaukee, WI

1 edit

ArgMeMatey

Member

In a well-designed system the fresh air intake is located far from exhaust sources such as exhaust fans, plumbing vents, truck docks, diesel generators and so on. So, it could be ducted from any reasonable spot at the perimeter of the building or the roof.

Ideally in a big box you would draw your fresh air opposite or 90 degrees from the parking lot, and you would exhaust air near the truck dock or above the roofline. It's often easier to draw air from a sidewall to avoid roof penetrations.

I would note any louvered openings and ventilation fans and figure out why each one is there. Then you can check equipment to see if it appears to be working as designed.

In a building with automatic doors, balancing is important. If things are out of whack with too much positive pressure, the doors won't close properly. Not enough positive pressure and the cashiers are gonna complain about cold & hot air inrushes.

leibold
MVM
join:2002-07-09
Sunnyvale, CA
Netgear CG3000DCR
ZyXEL P-663HN-51

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said by Subaru:

I have a question on the HVAC fan setting on a commercial setting are they just recycling the air?

That is a complex topic.

Local codes for office buildings may have rules on the minimum amount of air that needs to be replaced with fresh outside air (to prevent oxygen depletion) and where such rules are present 100% recirculation would not be legal.
In a building where I was involved in the HVAC design discussions, the air replacement requirement (5%/hour) was implemented by exhausting the bathrooms directly outside and to have a pressure controlled outside air inlet into the air handler that would maintain a slight positive pressure in the building. I was told that this is the common way to do this (recirculating bathroom smells into the offices is probably not a good idea).

John Galt6
Forward, March
Premium Member
join:2004-09-30
Happy Camp

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You need to look for venting to the exterior, either on the roof or the walls. That will be the easiest way to figure out what is going on.

Ignore the air temp as it is possible to condition completely toxic air.

Building that have 100% re-circ will have low O2 concentrations...you can tell in a short period of time that the air is not fresh.

I have been to such buildings for service calls...it seems like everyone has had a 30 point IQ drop.