Hot Gas Reheat Isn’t Free

In order to understand the benefits of MSP Technology used in Nautica Air System’s Dedicated Outside Air Units and Dehumidifiers it must be viewed through the MSP lens, where a clear picture will emerge. The engineering community is familiar with two main strategies for Dedicated Outside Air (DOA) equipment:

1. Standard Primary Air Cooling & Dehumidification with Heat/Reheat: Primary control is typically selected when the DOA A/C unit is provided as a stand-alone system serving a dedicated space without a separate primary building heating cooling system. In this scheme, supply air (comprised of a mix of outside air and return air) is discharged at various conditions to meet the desired room condition, typically 55-75°F dry bulb temperature, at a dew point that ensures 50% ± 15% relative humidity in the space.
2. Space Neutral Air Cooling & Dehumidification with Heat/Reheat: Space Neutral control is typically selected when the DOA A/C unit is provided as a parallel system to a separate primary building heating and cooling system. In this scheme, neutral supply air (comprised of 100% outside air or a high percentage mix of outside air and return air) is discharged at a constant condition equivalent to the desired zone conditions, typically 70-75°F dry bulb temperature and 50%±15% relative humidity based upon the time of year.


It is important to note that “Space Neutral” has two forms – sensible (temperature) and latent (humidity). Nautica Air System’s focus with their MSP Technology is only on the latent form, while Strategy #2 attempts to address them both. However, as we are addressing herein, controlling to a “Space Neutral” temperature condition results in excessive use of primary system cooling to compensate for DOAS hot gas reheating. Nautica Air System’s argument is that there should be a third strategy that effectively blends the two strategies into an energy-efficient hybrid.

Strategy #2 emphasizes neutral air, and requires the use of reheat (hot gas, electric, gas, steam, hot water, etc). In these systems, reheating is initiated when the supply air temperature falls below a pre-set dry bulb temperature. Energy is wasted whenever reheating is introduced to a zone that requires cooling. For this reason, a decoupled DOAS unit employing reheat wastes energy. This (decoupling) control strategy ignores zones that are in cooling mode. Rather than contributing to reducing the zone cooling load, systems employing reheat emphasize neutral air as it relates to dry bulb temperature. This practice lowers overall system energy efficiency. Please note that here we are talking about reheat, not heating.

Nautica Air System’s MSP Technology uses outdoor-air as the source for air reheating. A single continuous airstream is first pre-cooled (Figure 1) before passing through a cooling coil and is then re-heated. Other systems simply employ a hot gas energy source (Figure 2) for re-heat  and provide no pre-cooling. Learn more about the fundamentals of MSP and how it works by clicking here.

Heat, when added to cold and dehumidified outdoor-air supply to a building, will naturally increase the load in zones that are cooling. This is true of systems employing hot-gas reheat as well as outdoor-air reheat. The difference is that systems employing outdoor-air reheat, such as MSP Technology, have the added feature of outdoor-air pre-cooling. This feature dramatically reduces the dehumidification load and subsequently the total “system” load.

MSP Technology uses re-heat energy to drive the synergistic function of pre-cooling and vice versa. Hot gas reheat systems (see Figure 2) do not have a pre-cooling feature and thus have increased BTUH load, compressor horsepower and electrical consumption as per example in Table 1. In the MSP coil every BTUH of reheating is matched to a BTUH of energy saving pre-cooling. It is important to recognize that the sensible heat exchange between the incoming and outgoing air requires no mechanical assistance and is simply a function of the highly-efficient MSP coil. Pre-cooling and dehumidification by regenerative thermal exchange are essentially “free” and involve no additional equipment.

The goal of any Dedicated Outside Air System (DOAS) is to provide properly conditioned outside air while balancing capital cost, equipment complexity, and energy efficiency (lifecycle cost). Based upon the arguments above as well as field experience, Nautica Air Systems has found that it is much more cost effective to use intelligently staged compressors coupled with hot gas bypass as opposed to digital scroll compressors that can dynamically unload. The reason? The MSP coil provides significant “free” pre-cooling that reduces compressor horsepower by 20% – 40% (this effect is demonstrated in the graphic below). Also, when the cooling load requirements diminish the available BTUH for reheating with hot gas also begins to diminish. Therefore, the addition of digital scroll compressors and other means for “efficient” capacity control reduces the BTUH available for reheat. This tends to happen under cooler outdoor conditions when reheat could offer the most benefit.

When viewed from a lifecycle cost perspective, these reductions in compressor size more than compensate for the small inefficiencies associated with hot gas bypass (which is used only as a capacity fine tuning mechanism with the different stages of the compressor). From a capital cost and equipment complexity standpoint the MSP coil with staged compressors and hot gas bypass beats the significantly more expensive digital scroll compressors while achieving extraordinary energy efficiency.

By using staged compressors along with hot gas bypass to fine tune capacity control, the MSP Coil’s dew point is allowed to fluctuate below the dew point set point to a level that allows the compressors to optimize performance. This means that the Nautica Air Systems unit is capable of shouldering a portion of the latent cooling load required in a space. In the graph below we show the hours spent during a specific temperature (°F) discharge range throughout the months of the year in Jackson, Mississippi.

<55 55 – 59.9 60 – 65.9 66 – 70.9 71 – 75.9 Business Hours MSP Op. Hours
January 0 19 31 0 0 230 50
February 0 14 18 0 0 200 32
March 0 17 35 6 0 220 58
April 0 18 52 40 1 210 111
May 0 6 43 117 24 230 190
June 0 0 47 76 63 210 186
July 0 0 29 112 77 220 218
August 0 0 29 158 43 230 230
September 0 0 48 79 70 200 197
October 0 4 43 44 0 230 91
November 0 38 38 1 0 220 77
December 0 18 26 0 0 210 44
Full Year 0 134 439 633 278 2,610 1,484

Looking at the cooling effect, the supply air discharge temperature from the Standard Efficiency MSP coil is able to shoulder a large amount of the primary parallel system’s cooling load for a significant portion of the operating hours. MSP Technology is able to provide energy efficient supplemental cooling and dehumidification while, during the winter months the MSP coil will operate less and will understandably discharge lower temperature air. In winter, or high humidity, lower dry bulb conditions, when the amount of reheat through the MSP unit is too little, the engineer could design the primary parallel system so that the decoupled room heating and cooling system can adjust the mode of conditioning to maintain set point or Nautica Air Systems could integrate a gas-fired or SCR-fired electric heater for heating the entering air.

Through the MSP lens, “Decoupling” means indoor humidity is controlled by the DOAS unit, while indoor temperature is controlled by the building air conditioning units. Neither function can overpower the other. They work separately, yet in harmony. “Neutral air” means neutral dew point. Temperature control is the job of the building temperature control system. DOAS equipment employing MSP Technology is designed to regulate supply air dew-point, while dry bulb temperature “floats” naturally, in relation to outdoor air temperature.

When MSP Technology is used to treat outdoor air in a DOAS application, the combined DOAS and building systems operate more efficiently than Hot Gas Reheat systems.

McNellage & Associates are absolutely passionate about energy efficient dehumidification and outside air systems. Every year we work with owners, architects, engineers, and contractors all over the Gulf Coast to design thousands of CFMs of this equipment. We would love to work with you and your team! Click here to contact us and learn how to make your HVAC systems starting saving you thousands of dollars a year in energy costs.

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