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    Sizing & Selecting Unitary Equipment for Proper Humidity Control

    March 1, 2004
    Moisture problems that result from incorrectly applied and/or improperly sized air conditioning systems are a growing concern among contractors, engineers,

    Moisture problems that result from incorrectly applied and/or improperly sized air conditioning systems are a growing concern among contractors, engineers, and building owners.

    Even though the HVAC industry is besmirched with litigation stemming from moisture-related indoor air quality problems, the leading causes of moisture/mold problems in buildings are envelope failures, minor and catastrophic piping failures, sewer backups, and floods.

    However, HVAC contractors play an important role in moisture control, through the design and installation of mechanical systems. Given all that's known about the negative effects of oversized HVAC equipment, it's surprisingly common in U.S. homes and buildings.

    It's also surprising that latent loads caused by the infiltration of warm moist air or internal
    latent sources are often inappropriately considered. Yet, surprising or not, equipment is still routinely installed with total capacities that are anywhere from 50 to 200% greater than needed. This article briefly explains why oversizing occurs, and outlines steps for properly selecting HVAC equipment for building applications.


    There are a number of explanations why designers and contractors specify equipment that is too large for an application. The reasons range from load calculations being totally ignored to incorrect application of the accepted procedures.

    Load Calculations Not Performed

    1. Sometimes, prior experience is used to "guess" the load: A contractor, utilizing years of experience, feels comfortable quantifying heating and cooling loads, and compares one building against similar buildings completed in the past. Yet, this assessment ignores new construction materials and methods that have resulted in tighter, more energy efficient structures. Additionally, it minimizes the increased expectations that today's occupants have for comfort and healthy environments.

    2. Simple replacement of "like" for "like": Many contractors simply size the replacement equipment at the same tonnage as the existing unit, assuming the original equipment was properly sized. This approach ignores whether building functions have changed such as: activity type, occupancy level, intensity of appliance and equipment load, or the increased use of electronics. Also, this approach ignores whether substantial upgrades were made to the building itself, such as: improvements in lighting, insulation, more efficient appliances, or better windows. Additionally, latent loads may have changed substantially since the building
    was originally constructed, because of the introduction of more plants, fountains, Jacuzzis, indoor pools, etc.

    3. Estimating shortcuts: Numerous rules of thumb and other load estimating shortcuts have been devised to determine equipment selection.

    These include:

    • floor area per ton of cooling, such as 500 to 600 sq.ft./ton
    • airflow to ton relationships, such as 350 - 450 cfm/ton
    • relationship of latent load to full load, such as latent load = 30% of total capacity.

    However, "rule of thumb" shortcuts generally result in greatly mis-sized HVAC equipment in various building applications.

    Accepted Procedures Incorrectly Applied

    1. Mistakes in the load calculation: Industry-recognized load calculation procedures may be followed; however, simple computational errors, input errors when utilizing computer programs, or incorrect building detail assumptions result in erroneous loads.

    2. Using conservative assumptions: Some contractors may purposely adopt conservative practices that increase the load. For example, ignoring internal and external window shadings, automatically assuming that the ducts in older buildings are leaky or unsealed, using worst case compass orientation, and treating kitchen and bath fans as if they were part of the engineered ventilation load.

    3. Use of overt safety factors: Many building designers and contractors routinely add a safety factor -- as a "just in case" insurance -- upon completion of a load calculation. Throughout the years, they become conditioned to think they're following a correct approach because they may have relatively few non-cooling callbacks.

    Once the sensible and latent loads are established, an additional safety factor can ruin a good calculation. Routinely adding "just-in-case" safety factors of 25%, 50%, or 100% is unacceptable for nearly all unitary applications. Not only does the larger equipment size result in equipment cycling (and hence a warmer average coil temperature), it also may result in too much airflow. This would increase the coil temperature and decrease latent capacity.

    It can't be reiterated enough -- once a correct load calculation has been performed, there is no reason to add safety factors. Doing so leads to oversized equipment.


    Rigorous heat gain/heat loss procedures are necessary to ensure that equipment is properly sized for varied applications. Below are the proper steps for sizing and selecting HVAC equipment.

    Step 1: Establish building design and criteria requirements

    Determine the type of HVAC systems, i.e., ducted vs. ductless equipment, gas-fired furnaces vs. electric heat pumps, unitary vs. water chillers, etc., that are compatible with a building and the building's application, before starting a load calculation. This includes determining special space requirements, occupant needs, and occupant expectations:

    • Duct location and level of sealing and insulation
    • Ventilation or filtration needs for asthmatics
    • Special occupant comfort and health needs.

    Step 2: Determine the design loads

    The next step is to perform a rigorous load calculation using an approved load calculation program such as ACCA's Manual JÆ for residential buildings or Manual NÆ for commercial buildings.

    Carefully evaluate the building construction parameters and verify any assumptions about the details. For example:

    Building envelope:

    • Envelope tightness
    • Vapor barriers
    • Materials and insulation values.

    Solar orientation:

    • Direction the building faces affects the various room loads
    • Buildings with large amounts of glazing (windows, skylights, etc.)

    Glass type and shading:

    • For "rated" glass, use values from National Fenestration Rating Council (NFRC).

    Insulation type/level:

    • Make sound estimates of R- values for existing buildings
    • Use information provided on the drawings for new structures.

    Duct tightness/location:

    • Where possible, place the ducts within conditioned space to minimize the effect of any duct leaks.

    Step 3: Verify design conditions

    Outdoor design conditions should be the 1% cooling dry-bulb design point for the specific geographic location where the building is located. The indoor design conditions should be based on customer needs and requirements. As a default, designers should observe the following nominal indoor design conditions:

    • Winter heating design point: 70F at 30% relative humidity (RH)
    • Summer cooling design point: 75F at 50% RH

    STEP 4: Examine Full Load vs Part Load

    Pay attention to the sensible thermal comfort and latent moisture loads. Recognize that the load calculation is based on the peak load conditions (sensible plus latent). For summer cooling, this generally occurs on a sunny, hot day. The peak sensible condition occurs at the peak dry bulb condition. However, what happens during the evening when the sun sets? It's quite reasonable to expect a number of summer evenings having outdoor conditions near 80F and relative humidity of 100% -- it's raining!

    Since the design methodology results in equipment sized for peak dry bulb temperature (the hot, sunny afternoon), the equipment is quite oversized when operating at non-peak, part load duty (the other 99% of the time). Constant volume unitary equipment, often oversized for a part-load condition, easily satisfies the thermostat and cycles off long before much interior moisture is removed.

    To get a good handle on part load performance, one solution is for designers to rerun the load calculation for a reasonable nighttime high humidity condition, such as 83F and 95% RH. Because all the building parameters have already been entered into the computer, rerunning with a revised outdoor condition is relatively quick.

    This new run will result in a lower total tonnage capacity, but is likely to indicate a much higher latent load. Use the peak dry bulb condition to size the needed capacity of the equipment. The second run helps you make sure the selected equipment can also handle the peak dew point latent load.

    Use care when selecting equipment to satisfy the load requirements. As an example, if the load comes out to be a 31,500 Btuh requirement, nearly all contractors select a 3.0 ton (or greater) unit rather than a 2.5 ton unit. However, when moisture control is an issue, it's better to be 10% undersized than 10% oversized.

    Step 5: Verify system capacities

    Observe all manufacturers' sizing, selection, and application guidelines as you verify capacities and make the final equipment selection. It's crucial that the selected equipment has the capacity to handle the latent load at full load operation (generally, peak dry bulb conditions) and part load operation (generally, peak wet bulb conditions), in order to control moisture.

    Step 6: Evaluate your options if the selected equipment can't satisfy latent

    There are a number of options to consider if you determine that the selected unitary equipment can't satisfy the full-load and/or part-load latent requirements:

    • Modified control strategies that engage the cooling equipment based on humidistat demand. However, humidistat control is likely to require reheat to prevent overcooling in the conditioned space. Other control sequences can permit the evaporator to operate at a lower temperature.
    • Optimized equipment using multi-speed/variable-speed indoor fan units enable lower supply temperatures. Additionally, some permanent magnet fan motors allow the contractor to set a
      specific airflow. Reducing the airflow provides for increased opportunities to remove moisture from the air stream.
    • Hybrid equipment that uses wrap-around heat pipes or air-to-air heat exchangers to reheat the supply air leaving the dehumidifying coil.
    • Optimized evaporator coils with split-face capability, increased coil rows, or wider fin spacing help to remove moisture.
    • Whole building dehumidification equipment (perhaps interconnected to the primary fan and using the same duct system as the air conditioning system) can independently control building moisture loads.
    • Dedicated outdoor air systems can reduce the moisture loads that otherwise arise from the introduction of warm, moist ventilation air, because the outdoor air is separated from the
      indoor return air.

    Use Proper Strategies

    The correct application of equipment and operational strategies can handle challenging comfort conditioning requirements and provide for proper moisture control. However, many practices used in the past don't provide adequate moisture control in geographic areas of moderate-to-high outdoor relative humidity.

    The correct sizing and selection of HVAC equipment, appropriately controlled, are important steps to maintain proper humidity levels within buildings. This requires that the HVAC system is configured to meet both the sensible and the latent loads, not only at the design conditions (full load), but also over a broad range of off-design conditions (part loads). Additionally, buildings with large internal latent loads need special considerations. To truly control moisture -- as opposed to merely moderating it as a byproduct of the air tempering process -- requires dedicated equipment and controls.

    Glenn C. Hourahan, P.E. is the vice president of Research & Technology for the Air Conditioning Contractors of America (ACCA) in Arlington, VA. He has served on ASHRAE's Environmental Health Committee, Research Administration Committee, and is a current member of the Task Group on Residential and Small Buildings Applications (TG9.RSBA). He can be reached at 703/575-4477 or [email protected].