Air conditioned warehouse with roof peak vents?

[markk-pe]

I perform energy audits for numerous facilities at Kennedy Space Center in Florida. I've seen some strange set ups so far, but the current facility I am auditing has me stymied. The facility was apparently not originally designed to be air conditioned because it has roof vents on the peak of the roof and they are using a 25 Ton package unit with flex duct under one of the roll up doors. While not very efficient, it seems to cool the space so the occupants are happy with it (and they don't want to spend the money to install proper duct work).

When the Facility Manager was walking through the facility with me I asked him why he didn't seal off the roof vents. He replied that the air conditioner blows in along the bottom of the warehouse and has a return a few feet above the floor. He stated that hot air rises, so the hot air would go out the vent and the cool air will stay low in the facility. His contention is that having the vents helps conserve energy, the hot air can escape but when it is cooler outside cool air can enter.

I'm only doing a Level 1 Energy Audit, so there is no time to build a facility model (I have to complete 1 facility a week). I was wondering if anyone on this forum has experience with this type of space conditioning. Does the Facility Manager's evaluation make sense? Or would sealing off the vents conserve energy?

 

[markk-pe]

Attached is a picture of the HVAC set up for the warehouse. There is an office space in the east end of the warehouse with a mezzanine for storage above it. The FM says the HVAC does not cool the mezzanine very well in the summer. The west end of the warehouse if just open space, about 40 feet to the roof peak.

 

[dbill74]

Sounds like it's naturally ventilated. In Florida, indoor temperature may get up to 80-85 F. The 25T HVAC unit is probably there to control humidity and keep mold from forming (a real problem in Florida). Depending on the contents of the warehouse, nothing wrong with it.

Back before there was mechanical ventilation, natural ventilation was how people cooled their houses. If you ever look at old houses (built in 1800s) you will notice cupolas on top. The cupola acts the same as the roof vents allowing hot air out the top, as the hot air escapes it creates a draft that draws cooler air in through windows and doors on the lower levels cooling the house. Another strategy these old houses used to stay comfortable was to use high ceilings, 16-20 feet. The same principle drives the weather around the world.

Compared to cooling the entire warehouse to a comfortable 75F or even a warm 80F, what they have uses A LOT less energy.

Only real advantage of using rigid duct versus flex is a reduction in friction losses resulting in less work for the fan. If it is limited to less than 100 feet, I wouldn't fret over it.

As for the mezzanine getting warm, as long they aren't up there on a daily basis and the contents aren't damaged if it gets a little warmer, then all is good.

 

[willard3]

Hot air rises is all you have to know. Movie theaters only cooled the first 10' and ventilated the higher spaces; this is a common approach.

 

[markk-pe]

I appreciate the replies, they make sense. I'm just struggling to understand. For example, if I go to the 2nd floor in my house and remove the attic door, I feel heat coming from the attic into my home. The attic has a ridge vent and soffit venting, so hot air should be rising instead of coming in?

 

[chicopee]

For your house, instead of ridge vents, turbine ventilator could be a first option; second option, electrically operated exhaust fans.

 

[GroovyGuy]

Are the roof vents provided with dampers that can be adjusted from near slab? I assume that they are, so why not just close the dampers?
Do the AC units have provision for supplying any MUA?
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[MintJulep]

If hot air is leaving at the roof peak then air must be coming in to replace it.

The effect on energy will be all about where the air comes in.

Stratification can be your friend.

If you have a natural convection driven ventilation system that is above the occupied space and even it's coolest temperatures are above the occupied space then they will stay segregated.