We occasionally get questions from building owners who have worked with contractors in other parts of the country and are surprised when we recommend controls sequences or equipment configurations that differ from what they have seen elsewhere. The explanation is almost always the same: Louisiana's climate is genuinely different, not slightly different but fundamentally different in ways that affect how HVAC systems need to operate and how controls need to be configured. As I described in our commissioning walkthrough post a few weeks ago, verifying humidity control is a critical step we never skip in this climate.
The Numbers Behind the Problem
Our design conditions for the New Orleans metro area and the surrounding region are typically cited as approximately 95 degrees Fahrenheit dry bulb with a 78-degree wet bulb coincident temperature. If those numbers do not immediately mean something to you, here is the translation: when it is 95 degrees outside on a Louisiana summer afternoon, the air contains enough moisture that the wet bulb temperature is 78 degrees. For context, Phoenix, Arizona at the same 95-degree dry bulb temperature might have a wet bulb of 60-65 degrees.
This matters because the wet bulb temperature is what determines how much energy your air conditioning system must use to actually dehumidify the air, not just cool it. The difference between sensible cooling (lowering the air temperature) and latent cooling (removing moisture) is central to understanding why Louisiana HVAC is harder than most.
In a dry climate, the latent load, the energy required to remove moisture from the air, might represent 20-25% of the total cooling load. In Louisiana, it regularly represents 40-50% of the total cooling load. That means nearly half of your cooling system's capacity is occupied with dehumidification rather than temperature reduction. This ratio affects every aspect of how the controls need to work.
Why Standard Reference Sequences Often Fail Here
Most ASHRAE reference sequences of operations and many manufacturer-supplied control programs are written with mixed-climate or dry-climate assumptions. They work well in a mid-range climate like Atlanta or a dry climate like Denver. They are often inadequate for southeastern Louisiana.
Supply air temperature reset is a prime example. In a dry climate, resetting the supply air temperature upward when cooling loads are low (raising the supply from 55 to 62 or 65 degrees on a mild day) saves energy with minimal risk. In Louisiana, a supply air temperature of 62 or 65 degrees may not be cold enough to remove adequate moisture from the return air, especially during shoulder seasons when cooling loads are low but outdoor humidity is still very high. Buildings that implement aggressive supply air temperature reset without humidity monitoring end up with indoor relative humidity creeping into the 65-70% range, which is both uncomfortable and a mold risk.
Economizer controls are essentially impossible to use for free cooling in Louisiana. Economizer logic in most climates allows the system to bring in 100% outdoor air when outdoor conditions are favorable, using cool outdoor air to meet interior cooling loads without running the compressor. In Louisiana, the outdoor air is almost never suitable for economizer free cooling during the cooling season. Our outdoor humidity is high enough that even on a 65-degree day in October, the moisture content of the outdoor air often exceeds what we want inside the building. Economizer sequences need to include enthalpy-based high-limit controls, not just temperature-based controls, and the setpoints need to be calibrated for our climate rather than copied from a generic sequence.
Dehumidification Sequences: Not Optional Here
In many parts of the country, dehumidification is a secondary concern managed passively by the cooling system. When the system cools the air, it removes moisture as a byproduct, and that is usually sufficient. In Louisiana, that passive approach is often not enough.
During mild weather, when cooling loads are low, a properly sized HVAC system runs short cycles. A system that short-cycles does not run long enough to remove adequate moisture from the air. This is the reason we often see buildings with comfortable temperatures but excessive humidity in October and November, when cooling loads are moderate but outdoor humidity remains high.
The controls response to this problem can take several forms. Active dehumidification sequences command the system to continue running the cooling coil at low supply air temperatures even when the thermostat setpoint is satisfied, prioritizing moisture removal over temperature. Some sequences use reheat to maintain comfortable discharge temperatures while still running the coil cold enough to condense moisture. More sophisticated approaches stage dedicated dehumidification equipment alongside the primary cooling system.
The specific sequence that works best depends on the building, the equipment, and the occupancy pattern. The wrong answer is to have no dehumidification sequence at all and simply hope the cooling system handles it.
Mixed-Air Temperature and Humidity Monitoring
In a standard BAS installation, you typically have return air temperature and outdoor air temperature sensors, and the system calculates or measures mixed-air temperature where these two streams blend before reaching the cooling coil. In Louisiana, we also strongly recommend mixed-air humidity sensors. Without humidity data at the mixed-air point, the controls system is making decisions about how much outdoor air to bring in without knowing what the actual moisture load from that air is. In our climate, that missing data matters.
Return air humidity sensors are equally important. Rising return air humidity is often the first indicator that the dehumidification sequence is not keeping up, and catching it early allows the system to respond before occupants feel uncomfortable or, worse, before moisture problems develop in building materials.
The Case for Dedicated Outdoor Air Systems
In buildings with significant ventilation requirements (high occupancy, code-required minimum fresh air), we frequently recommend considering a Dedicated Outdoor Air System, or DOAS. A DOAS handles all the fresh air ventilation for the building as a separate piece of equipment, pre-conditioning that air (cooling and dehumidifying it) to neutral conditions before delivering it to the zones. The zone-level HVAC equipment then handles only the sensible load for that zone.
The controls advantage of this approach in Louisiana is significant. The fresh air handling system can be specifically designed and controlled for dehumidification without compromising the zone temperature control. The zone equipment does not have to fight the latent load from ventilation air because that work is already done. The controls sequences for both systems can be optimized for their specific tasks rather than trying to do both jobs with a single system.
Louisiana's climate is demanding, and the buildings that handle it well are the ones whose controls systems are designed for it specifically, not adapted from a generic template.