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Warehouse Air Changes

D

DTBarch

SAWHORSE
Joined
Nov 1, 2010
Messages
78
Location
Phoenix, AZ
Topic under discussion is the required air changes per hour (ACH) for a warehouse space in Phoenix, AZ. I'm an architect, not an engineer but my consultation with engineers on this topic has yielded a wide range of answers, and google search is even less productive. For the record, I'm speaking about leased warehouse facilities, not warehouses that are owned, occupied and operated by the same entity. The owner/landlord may have a records storage tenant now, but could lease to a manufacturing tenant in the next 5 years, then metal parts storage facility for the next, and so on...

ASHRAE appears to list the recommended air changes for warehouses as a range of 6 to 30 changes per hour. That is a massive spread in the range. In a warehouse served by evaporative coolers, trying to get to 30 air changes would take an insane amount of specialized evap cooler equipment, and in reality, most leased evap-cooled warehouse space in the Phoenix metro area are providing no more than 3 to 5 air changes per hour. Just look at the google aerial view of the warehouses in Phoenix. Very very few of them have that amount of equipment on the roof.

Here's a real life example in Phoenix. An existing 64,000sf warehouse building is currently served by 12 evap coolers that each push 20,000cfm. After crunching all the miscellaneous factors in, the air change calculation yields 7.5 air changes per hour existing. Just to get to 14 air changes, one might think a conservatively prudent midpoint, the owner or tenant would have to add another 12 evap coolers to the building.

The threshold for IMC Table 403.3.1.1 OSA compliance is much much lower at 0.06cfm per sf. A 64,000sf warehouse only requires 3,840 cfm of OSA to be compliant. Each evap cooler in our example provides 20,000cfm and there are 12 existing on the roof, so obviously, OSA is not an issue, but recommended air changes is a whole other story.

So, my questions are:

1. What is a realistic recommendation for number of air changes in a warehouse space in Phoenix, AZ? For you practicing mechanical engineers out there, what do you use as your guideline here? If the space described above was not cooling as effectively as the tenant would like in the summer, is the recommendation they need to add 12 more units to get to the the midpoint of the max recommendation of 30?? Adding 12 more units is a very costly alteration, and comes with a LOT more annual maintenance. The space is "code compliant" so there's no mandate here, right? The only required calculation required by the AHJ is an OSA calc which is an easy hurdle with evaps. We don't have to demonstrate compliance with ACH rates.

2. Given the extreme range in the ASHRAE recommendation, is it prudent to use ASHRAE recommended ACH rates as a basis of design for new evap cooled warehouse space, and if so, again where in the 6 to 30 range do you start?

Thank you in advance for your feedback...
 
Last edited by a moderator:
@ = @ = @ = @

DTBarch,

Which energy code has the city of Phoenix officially
adopted, ...the IECC, or ASHRAE [ i.e. - if you use ASHRAE
and the city of Phoenix uses the IECC, then what ] ?

IMO, ...since the lower number of " 6 " ACH would be the
lesser cost, I would start there.........Also, you may want to
to create a spreadsheet with the various ACH and their
associated costs............If the tenant wants to have more
cooling, then you can provide them your costs matrix
and let them decide how much more cooling do they
want, or can afford ?

@ = @ = @ = @
 
Last edited:
I think the issue is what is the code minimum for the the empty space that need to be provided by the building owner, what is the tenant improvements that may need additional HVAC, how the lease assign to improvements for the occupancy.

If I have warehouse storing dry goods what HVAC do I need, compared to storing medical supply that nee to be kept below 80 degrees?
 
DT ... are you asking about a spec building, or build to suit? If it is a new build spec building, tenant not identified, i would build to minimum and let the tenant add more cooling as needed.
 
north star said:
@ = @ = @ = @

DTBarch,

Which energy code has the city of Phoenix officially
adopted, ...the IECC, or ASHRAE [ i.e. - if you use ASHRAE
and the city of Phoenix uses the IECC, then what ] ?

IMO, ...since the lower number of " 6 " ACH would be the
lesser cost, I would start there.........Also, you may want to
to create a spreadsheet with the various ACH and their
associated costs............If the tenant wants to have more
cooling, then you can provide them your costs matrix
and let them decide how much more cooling do they
want, or can afford ?

@ = @ = @ = @
Click to expand...
Phoenix uses IECC, but I don't believe there is anything in the IECC that dictates air changes. My issue is that there is a massive gap between IMC ventilation requirements, and ASHRAE air change recommendations for warehouse space, which can easily trigger conflict if those technicalities filter down into landlord / tenant dialogue. In today's market, evap coolers installed are approx $25k per unit installed after plumbing, electrical and structural, so complying with ASHRAE recommendations can easily be hundreds of thousands of dollars, but the Code does not require any of it. A quick google search for air change recommendations yields the ASHRAE range, so there can easily be a perception that ASHRAE is "code", which can manipulate a tenant's expectations.
 
TheCommish said:
I think the issue is what is the code minimum for the the empty space that need to be provided by the building owner, what is the tenant improvements that may need additional HVAC, how the lease assign to improvements for the occupancy.

If I have warehouse storing dry goods what HVAC do I need, compared to storing medical supply that nee to be kept below 80 degrees?
Click to expand...
Absolutely, but medical supply, or any other commodity that needs to be kept under 80 degrees will never happen with an evaporative cooled warehouse space in the Phoenix summer months, so that use will always require an air conditioned solution. That is becoming more and more common in the projects that I see, but this issue is specific to evap cooled space and the sub-set of tenant uses that are compatible with temperatures over 85 degrees
 
mtlogcabin said:
My first question would be will the roof support the additional equipment that will need to be added?
Click to expand...
MT yes, that is always a factor. Our typical evaps operate at just over 1,000lbs which is fairly easy to deal with structurally, even in a panelized roof system, but you're right, structural is a necessary component of any new RTU work.
 
e hilton said:
DT ... are you asking about a spec building, or build to suit? If it is a new build spec building, tenant not identified, i would build to minimum and let the tenant add more cooling as needed.
Click to expand...
This is an existing building that has had probably 7 different tenants over it's life. The current tenant is unhappy with the effectiveness of the existing evap coolers, which is NOT uncommon in Phoenix because the summers are Fire, and there are natural limits to evap cooling that render them relatively useless in July and August when monsoon humidity enters the desert. In that tenant - landlord dialogue, there is a slippery slope down to the ASHRAE recommendations...
 
e hilton said:
Depends on lease terms. Was it rented as-is? Did LL promise certain conditions?
Click to expand...
Yes, rented as is with no promises. This isn't really a leasing question I'm asking, just attempting to solicit engineering feedback on the basis for the ASHRAE recommendations for 6 to 30 air changes vs. what the mechanical code requires. In this case, to illuminate the issue, if we provided 30 air changes via evap coolers, it's 3,840 cfm required by the IMC versus 960,000 cfm to meet ASHRAE recommendations. Cost wise, that's $25,000 versus $1.2m in equipment 1st cost. Why is there such a vast difference on this? Seems like there should be a reasonable explanation...
 
IMC required air changes are just to provide adequate air for occupants to breathe and remove some odors. I believe you have to add 10 CFM per occupant to 0.06 CFM per SF to get the total required ventilation air, then divide it by the zone air distribution effectiveness. This will not provide temperature control. Additional ventilation and exhaust are required if there are processes in the building that create additional odors or high humidity.

Sizing evaporative coolers for comfort depends on many factors, including design outdoor & indoor temperatures, building insulation, internal heat gains, solar gains if there are windows or skylights. That's why there is such a wide range of air changes
 
As you pointed out, ASHRAE does not offer firm guidance, just a wide ranging value for air exchanges. Add to that the varying opinions from the engineers you mentioned. Seeing as no silver bullet exists as in a specific target value I’d say if your client is within the wide ASHRAE range (6-30ach to be verified) move on to the next ASHRAE target such as ventilation rates in breathing zones. Does the facility meet that requirement and or other more clearly defined target values.
 
Not my area of expertise, however working in the fabrication industry, my understanding ACH is about air quality for employee breathing, and though EVAPS can help perform that task, they are not necessarily the most efficient way, nor cost effective way.

On the flip side, your OP is about hammering down the correct number of ACH from what I have been reading here. Thus, maybe a forum specific to mechanical contractors might be a better fit for the question since ASHRAE range is so large.
 
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