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Sizing & Selecting HVAC Equipment for Proper Humidity Control

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The leading causes for moisture/mold problems in buildings are related to envelope problems (i.e., leaks through roofs, walls, foundations, windows, and doors), minor and catastrophic piping failures (plumbing and appliance leaks), sewer backups, and floods. Sound maintenance and due diligence are required by building owners/operators to ensure that structural components have not deteriorated and that minor concerns do not become major problems. Of lesser notice – however, perhaps more insidious – are the humidity and moisture problems that result from incorrectly-applied and/or improperly-sized air conditioning systems.

Given all that is known about the negative impacts of oversized mechanical equipment (see Table 1), it is surprising that equipment oversizing is common in U.S. homes and buildings. Given recognized industry procedures for calculating heating and cooling loads, it is surprising that latent loads (caused by the infiltration of warm moist air, internal latent sources, etc.) are often inappropriately considered. Yet, surprising or not, equipment is still routinely installed with total capacities that are 50 – 100 – 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 as to why designers and contractors specify equipment that is too large for the application. The reasons range from load calculations not being undertaken, to incorrect observance of procedures, to reliance on inappropriate rules of thumb.

  • No load calculations:
    Prior experience is used: A contractor, utilizing his years of experience, feels comfortable quantifying cooling and heating loads by comparing one building against similar buildings completed in the past. Yet, this guess 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.

    Simple replacement of “like” for “like”: When replacing existing equipment, some contractors simply size the replacement equipment at the same tonnage as the existing unit. However, this assumes that the original equipment was properly sized. Additionally, this ignores whether building functions have changed (activity type, occupancy level, intensity of appliance and equipment load, increased electronics). Also, this approach ignores whether substantial upgrades were made to the building (improvements in lighting, insulation, more efficient appliances / equipment, better windows, etc.) and whether latent loads have changed substantially since the building was originally constructed (i.e., more plants, fountains, Jacuzzis, indoor pool, etc).

  • Incorrect observance of procedures:
    Mistakes in the load calculation: Even if the designer uses industry-recognized load calculation procedures, simple computational errors, input errors (when utilizing computer programs), or incorrect building detail assumptions result in erroneous loads.

    Use of safety factors: Many building designers and contractors routinely add a safety factor – as a “just in case” insurance – upon completion of a load calculation – when they complete a load calculation; perhaps they think this is a correct approach since they have been doing it for years and have had relatively few “non-cooling” callbacks. However, it cannot be reiterated enough – once a correct load calculation has been performed, there is no reason to add a safety factor. Doing so leads to oversized equipment.

  • Use of obsolete and inadequate “rules of thumb”:
    Estimating shortcuts: Over the years, 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 ft2/ton),
    • airflow to ton relationships (such as, 350 – 450 CFM/ton), and
    • relationship of latent load to full load (such as, latent load = 30% of total capacity).
      However, “rule-of-thumb” shortcuts generally result in greatly miss-sized HVAC equipment in various building applications.

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

  1. Establish building design and criteria requirements
  2. Determine the design loads
  3. Do not arbitrarily increase load (safety factor)
  4. Verify system capabilities
  5. Evaluate latent loads

Step 1:Establish Building Design and Criteria Requirements

Before undertaking a load calculation, it is important to ascertain the type of HVAC systems that are compatible for a building and its use. This includes determining special space requirements or occupant needs or expectations:

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

Appearance issues, architectural design concerns, and building constraints also have an impact on the type of system to be selected and on how the mechanical equipment can respond to the design and building requirements. In older buildings without central air-conditioning, finding space for ductwork can be challenging. As such, ductless mini-splits, high-velocity/small duct systems, or chilled water systems might be considered.

The overall building budget and system budget also impact the type of system, zoning, and capabilities of the equipment to be selected. The fuel types available to the site, and relative costs, are also considerations.

Step 2: Determine the Design Loads

A. Building construction parameters
With an understanding of the building requirements and the type of system to be used (i.e., central ducts, non-ducted, heat pump, air conditioning with gas heat, chilled water, etc.), the next step is to perform a rigorous load calculation (i.e., ACCA’s Manual J® for residential buildings, and Manual N® for commercial buildings). It is critical that the building construction parameters are carefully evaluated and that assumptions on the related building details are carefully verified. Examples of such deliberations include:

Building Envelope: How tight is the building envelope? Were sound construction practices observed? Were high quality materials with good insulation values used in the wall, ceiling, window, and door components?

Solar orientation:
The direction the building faces has a large impact on the loads experienced by various rooms. Buildings with large amounts of glazing (windows, skylights, etc.) – especially when concentrated in the southern or western exposures – may have profoundly different loads than equivalent buildings oriented north or east.

Glass type and shading:
When using “rated” glass, the window values from the National Fenestration Rating Council (NFRC) should be used, and not the values for generic glass. The NFRC ratings will yield a closer approximation of the actual loads. Additionally, factoring in internal and external window shading has a significant impact on the resultant loads.

Insulation type/level: The effectiveness of the insulation in the walls, around the windows, in the ceiling, and basement/slab areas effects the amount of heat gain/loss within a building. A sound estimate of “R- value” for existing buildings, or judicious use of information provided on the drawings of new structures, needs to be made to ensure proper load determination. Correct determinations of the degree of effectiveness of wall sections, doors, windows, insulation, etc., will yield a more accurate load. Wrongly assuming that a building is of average construction when it is of much better construction will result in oversized equipment.

Duct tightness/location:
The location of the ductwork, and its level of insulation, has a great impact on the building load. As an example, a leaky, poorly-insulated duct located in an unconditioned space can result in a need to double the equipment size to satisfy the occupied space conditions. However, observing sound duct procedures, which minimize the introduction of warm, moist air to the system, will result in a marginal impact on the load requirements. Even better is placing the ducts within the conditioned space as this minimizes the effect of any duct leaks as well as heat loss/gain to the delivered airstream.

B. Design Conditions
The design conditions to be observed are very important. 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: 70°F (21°C) at 30% relative humidity
  • Summer Cooling design point: 75°F (24°C) at 50% relative humidity

The key point here is establishing the temperature differentials for use in the cooling and heating load calculations.

C. Full Load Versus Part Load
Once a load determination has been performed, it is important to pay attention to the sensible and latent loads resulting from the calculation. It needs to be recognized that the load calculation is based on the peak load conditions (the 1% design day). For summer cooling, this generally occurs on a very sunny, hot day, and the peak sensible condition results from the peak dry bulb observed. However, what happens during the evening when the sun sets? What if it is raining? It is quite reasonable to expect a number of summer evenings where the outdoor condition may be in the low 80°Fs and the relative humidity is 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). Hence, just when latent removal capability may be needed most, it is least available. Fixed capacity equipment, being oversized for a part-load condition, easily satisfies the thermostat and cycles off long before moisture removal can be effected. The issue here whether it is preferable to be a little warm on the very hottest day, or have poor humidity control from excessive cycling almost all of the rest of the time. Multi-stage or variable-speed capacity equipment help address this issue as long as their part-load latent capacity matches the part load building requirement.

One solution is for designers to rerun the load calculation for a representative nighttime, high humidity condition (say, 83°F and 95% relative humidity). Since all the building parameters (i.e., measurements, orientations, and construction factors) have already been entered into the computerized calculation method, 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 (i.e., the peak latent load occurs with the peak wet bulb conditions). The peak dry bulb condition should be used to size the needed capacity of the equipment. However, the second run allows the ability to assess that the selected equipment can also handle the peak dew point latent load. If it cannot, then other HVAC combinations – or the use of auxiliary dehumidification equipment – should be considered.

Step 3: Safety Factor – NOT!
Once a load calculation has been determined, and the sensible and latent loads established, let’s not ruin the good work by arbitrarily adding safety factors. Routinely adding “just-in-case” safety factors of 25 – 50 – 100% is not an acceptable alternative for undertaking a proper load calculation. Other ways to ruin an otherwise good load determination include:

  • Overly conservative assumptions on the building construction details. Purposely using “loose” or “conservative” design criteria to increase the calculated cooling requirement is unnecessary and counterproductive for obtaining the proper loads.
  • Failure to observe room and building diversity factors. The required capacity is not necessarily the sum of the peak individual room loads. Buildings with large levels of solar glass load – especially if these windows are predominantly on one side of the structure – will have rooms with large loads that peak at different times than other rooms.
  • Upsizing equipment in the belief that bigger is better. This is also a problem with customers who think getting a bigger unit for nearly the same money is good value. Contractors need to carefully explain the benefits of using properly-sized equipment.

Care must be observed when selecting equipment to satisfy the load requirements. As an example, if the load comes out to be a 31,500 BTU/h 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 is better for HVAC equipment to be 10% undersized than 10% oversized. ACCA Manuals S® (Residential Equipment Selection) and Manual CS® (Commercial Systems and Equipment) provide sizing guidance for the spectrum of HVAC building equipment.

Note: The sizing of heat pumps in very cold climates is always an engineering balancing act. If the units are oversized, they have poor humidity control in the summer. But, if undersized for the winter heating load, it leads to enormous strip heating requirements during the winter.

Step 4: Verify System Capacities
In verifying capacities and making the final equipment selection, it is essential that all manufacturers’ sizing, selection, and application guidelines are observed. Additionally, as noted previously, be sure that the equipment can meet the sensible and latent cooling requirements without being oversized. However, for controlling moisture within the building, it is crucial that the selected equipment has the capacity to handle the latent load – at full load operation (peak dry bulb conditions) and part load operation (peak dew point conditions).

When discussing moisture control, it is common to refer to the sensible heat ratio (SHR), defined as the ratio of the sensible load to the total load, where the total load is the sum of the sensible and latent loads. However, there is a difference between equipment SHR (occurs at the evaporator coil) and application SHR (occurs in the conditioned space).2 As the heat gain decreases, the space sensible load decreases while the space latent load generally remains essentially the same. This results in a decreasing space SHR requirement (say, 0.75 going to 0.65; or lower). This means that the latent removal requirement is a higher percentage of the total load. Yet, as the total load decreases, the latent capabilities of most air conditioners are unable to track the changing SHR requirement of the space.

The likelihood of a mismatch between equipment SHR and space SHR illustrates the need for appropriate design analyses to ensure that the HVAC system meets both sensible and latent loads at full and part load conditions. This is absolutely critical when outdoor air ventilation (generally, warm and moist air) is required and/or if the HVAC system operates with continuous fan while the compressor cycles.3 Other factors also exist that could contribute to excessive humidity levels and a mismatch of equipment and room SHRs. These include:

  • Improper space pressurization (increases infiltration of moist outdoor air),
  • High thermostat set points during unoccupied periods,
  • Improper system operation.

These factors need to be considered and addressed to ensure that the latent removal capability of the equipment is not overwhelmed.

Consideration of latent load requirements separate from sensible cooling load requirements is not all that far-fetched. In many parts of the U.S., it is relatively common for slow-moving, low-pressure weather patterns to move in, and 70°F to 75°F temperatures at 80%+ relative humidity conditions are experienced for extended periods. Therefore, with no sensible load, and no call for cooling, air conditioners provide no moisture reduction.

Step 5: Considerations if Selected Equipment Cannot Satisfy Latent Requirements
There are a number of options that can be considered if a “standard” equipment selection is unable to satisfy the full-load and/or part-load latent requirements. Equipment manufacturers offer innovative options and approaches for modulating moisture:

  • Modified control strategies that engage the cooling equipment based on humidistat demand. This could lead to a requirement for reheat to prevent overcooling in the conditioned space. Other control sequences can permit the evaporator to operate at a lower temperature.
  • Optimized equipment that utilize multi-speed/variable-speed indoor fan units and compressors to enable longer system runtimes. Slower speed, reduced airflow delivery leads to longer operation periods … which provides for increased opportunities to remove moisture from the air stream.
  • Hybrid equipment that use wrap-around heat pipes, desiccant materials, or enthalpy control. The intent is to wring-out as much moisture as possible before the air reaches the primary cooling coil.
  • Optimized evaporator coils with split-face capability or increased number of coil rows to better remove moisture. Coils with wider fin spacing allow the condensed water to drain off more quickly.

For humid applications, or where a high level of humidity control assurance is needed, designers and contractors also should consider independently controlling temperature and humidity:

  • 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 arise due to the introduction of warm, moist air for ventilation requirements. This permits outdoor air to be separated from indoor return air.

Both approaches knock out a major part of the moisture load and allow the primary coil to do a better job of sensible cooling.

The correct application of equipment and operational strategies can handle challenging comfort conditioning requirements and provide for proper moisture control. However, many practices, procedures, and approaches used in the past are inadequate for providing adequate moisture control in geographic areas of moderate-to-high outdoor relative humidity.
The proper sizing and selection of HVAC equipment are key to controlling humidity levels. This requires that the HVAC system meets both sensible and 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 Hourahan

Posted In: Building Performance, Technical Tips

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