Myth―An unproven or false collective belief

By J. David Odom and Richard Scott-AIA, NCARB, LEED AP of Liberty Building Forensics Group and Norm Nelson of CH2M

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If PTAC units were a federally regulated drug, then each equipment sale would include a list of side effects and cautionary notes. Unfortunately, the opposite is true—each PTAC buyer is provided with a list of unproven expectations and myths. It is these unproven PTAC performance myths, enduring for decades, which have contributed to an outsized number of moisture and mold problems.

Over the past 30 years, our building experts have investigated hundreds of hotel moisture problems (involving over 100,000 guest rooms) and in the process, have seen three repetitive problems that appear to be ignored by the hospitality industry:

  1. PTAC units cannot ventilate interior spaces.
  2. PTAC units cannot pressurize hotel guest rooms.
  3. PTAC units are often ineffective in dehumidifying hotel guest rooms, especially when outside conditions are hot and humid.

Even though PTAC units cannot provide these features, the myths are that hotel industry, including owners, operators, developers, contractors, and designers, believe that they can.

For decades the hotel industry has used packaged terminal air conditioning (PTAC) or packaged terminal heat pump (PTHP) units, which are collectively referred to as PTAC units here. Dating back to at least the late 1980s, the industry has been experiencing moisture, mold, and odor problems related to PTAC units. (In the early 1990s, The Marriott Hotel Corporation developed a prototype to address significant moisture and mold problems in their new Courtyard chain. This prototype used PTAC units combined with a ducted desiccant-conditioned makeup air system. At approximately the same time, the American Hotel & Motel Association (AH&MA) developed a manual to address the significant industry-wide moisture and mold problems.)

Problems have not occurred every time these were used, nor have the problems occurred equally in every climate, but they have been relatively pervasive and some have been catastrophic. Inconsistent problem patterns have likely helped perpetuate the unfounded belief (or myth) that PTAC units can achieve what the manufacturer’s literature implies and what designers and owners believe. Belief in these myths results in design decisions that are counter to good building practices, emerging green building codes, and increasingly stringent energy efficiency building codes.


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Figure 1
Figure 1: Example of a mold problem caused by PTAC-related issues in a relatively dry climate after vinyl wall covering was removed from a demising wall between guestrooms. Negative building pressurization resulted from a lack of makeup air caused by the inability of the PTAC unit to offset the toilet exhaust system. This pressure differential drew hot, humid outdoor air inside the wall cavity and condensed when the air from the PTAC unit cooled the wall below the dew point of the cavity air, causing condensation and mold growth. Note: The photo shows the wall sleeve after the PTAC unit was removed.

Low initial cost and simplicity are two of the most appealing features of PTAC units and why they are standard common choice for many hotels and dormitory-type facilities.

Our forensic investigations of moisture and mold problems in PTAC-conditioned hotels during the last three decades found very widespread and severe conditions.

Although many of the most severe problems have been documented in the Southeast U.S., problems are also found as far north as the Mid-Atlantic States and the upper Midwest. In short, these failures are not confined to ASHRAE’s classic definition of warm, humid climates: “Warm, humid cooling climates are defined as climates where one or both of the following conditions occur: (1) a 67⁰F or higher wet-bulb temperature for 3,000 or more hours during the warmest 6 consecutive months of the year, or (2) a 73⁰F or higher wet-bulb temperature for 1,500 or more hours during the warmest 6 consecutive months of the year” (ASHRAE 2005).

It is our experience that the three problems (or myths) identified below often result in moisture and mold problems, especially in climates that experience warm, humid summers. The common approach seems to be that designers or contractors select and install these simple systems and then let the occupants or the building owners decide what temperature they want the rooms to be. If good temperature control and low initial costs were the only expectations for PTAC units, then they are the perfect solution for many facilities. Furthermore, when everything works perfectly, PTAC units provide an adequate solution to an important business challenge for the hospitality industry―how to achieve good guest room temperature control for the lowest initial cost.

However, low cost and good temperature controls are not the only expectations of PTAC units. PTAC units are usually expected by the hospitality industry to accomplish the following three additional important functions that go beyond temperature control—which we have labeled as widespread industry myths:

  • Myth 1―The Myth of Adequate Outside Air (OA) Ventilation
  • Myth 2―The Myth of Positive Building Pressurization
  • Myth 3―The Myth of Good Dehumidification

The inability of PTAC units to meet these additional expectations has been the cause of many moisture and mold problems over the past several decades. Such problems are typically more severe in warm, humid climates, but our experience has shown they can occur in temperate climates (Figure 1). Additionally, Myths 1 and 2 cannot be met in any climate but often go unnoticed because they do not always result in moisture and mold problems.

Unfortunately, there is an absence of specific PTAC performance data, despite large numbers of hotels with PTAC units that should be provided with the required information. In the absence of specific proof (i.e., empirical evidence and direct performance data), designers, contractors, developers, and owners rely on unproven statements in the manufacturer’s literature or on whatever equipment has been previously specified.

Often, the problems experienced by believing and applying these myths end up in court, where any “lessons learned” are not widely distributed to the hospitality design and construction community. Furthermore, it is our belief that many participants in the PTAC industry perpetuate these myths by not clearly identifying what these units will and (more importantly) will not do.

Myth 1 – The Myth of Adequate Ventilation Air


The Myth: PTAC units are widely believed to provide code-required ventilation air to the occupied space.

The Reality: PTAC units do not provide code-required ventilation, even though this myth is an almost universal belief within the hotel design and construction industry.

This myth is the reason that the majority of PTAC-equipped hotels do not have an alternate way to properly ventilate the individual guest rooms. Corridor makeup air systems that use the guest room doors’ undercut as their method of air ingress are an inadequate means to ventilate a guest room and also do not comply with NFPA (2012).

Most building codes reference ANSI/ASHRAE Standard 62.1 (2013) for the required ventilation rates. For a hotel guest room, this typically requires a minimum of 30 cubic feet per minute (cfm) of ventilation air. In the absence of a dedicated OA ventilation system, the PTAC unit must be relied on for the proper introduction of fresh air. When a PTAC unit is expected to perform this function, the following is required in an occupied guest room:

  • The PTAC unit indoor supply air fan must continuously introduce the required air quantity.
  • The PTAC vent door must be open.
  • The PTAC indoor fan must run continuously.
  • The outdoor air quantity must be measurable and verifiable (ANSI/ASHRAE Standard 62.1).

PTAC manufacturers describe the ability of their product to introduce outdoor air through the operable door in the bulkhead wall separating indoor from outdoor sections of the chassis. Some equipment manufacturers state that their equipment will introduce up to 70 cfm of outside air when the operable door is open and the PTAC fan is on high speed. The implied intent of this design is to provide code-required ventilation air quantities without using a separate dedicated fresh air supply.

The outdoor air ventilation rates included in the manufacturer’s literature may be achievable in factory bench tests, but our experience is that these rates are not achievable in actual field conditions. (Note: The single exception to the previous statement may be when OA is blown into these units by windy conditions. Obviously, wind-induced fresh air supplies are temporary and do not comply with either the letter or the intent of good design practices.)

The PTAC unit will not provide the code-required ventilation for the following reasons:

  • The pressure drop through the vent door/air filter is higher than the pressure drop through the evaporator coil/filter, especially as the vent door filter becomes dirty. Air through the vent door and the indoor coil are controlled by the path of least resistance, which does not favor the vent opening. The reason is because the vent door size is a fraction of the cooling coil size, and the vent door fine-mesh screen filter adds considerable pressure drop when compared to the washable cooling coil filter (see Figure 2).
  • The guest controls the PTAC fan and can choose any speed available, including OFF, which eliminates any possibility of fan-assisted ventilation air. If the design relies on the equipment to meet the ventilation code, then the occupant or guest cannot be allowed to change fan speeds or turn the unit off.
Figure 2
Figure 2: View of PTAC unit removed from wall sleeve-room side is to right. Inset in upper left shows front view of unit in sleeve with cover removed. Supply fan draws unconditioned outdoor air (red arrows) into supply fan plenum. Fan also draws room return air (yellow arrows) through evaporator coil cooling room air (blue arrows). Unconditioned outside air mixes with conditioned room air and discharges to room (green arrows) through supply air louvers.

The majority of PTAC manufacturers describe how the vent door may be OPEN or CLOSED but provide little, if any, guidance about the impact of an OPEN or CLOSED vent door, other than the possible energy implications. What minimal guidance that is provided is often confusing and without clear direction.

The photographs in Figure 2 were taken during the investigation of moisture problems in a location where only the summer months are humid (northern West Virginia), and where the PTAC unit design was intended to provide code-required ventilation to the guest room.

As shown in Figure 2, the vent damper is open, and the path for ventilation (outdoor) air is past the outdoor coil and through a washable filter. The ventilation air in many PTAC units does not pass through the cooling coil before being mixed with the return air. This can result in increased room humidity.

Even if PTAC units could supply sufficient ventilation air, there are other problems inherent with the general PTAC design. These problems can exist even with ventilation supply that is less than the code-required ventilation rates. Unconditioned ventilation air being provided through the PTAC unit often results in elevated moisture levels in the space (see Myth 3). The elevated moisture levels are especially problematic in hot, humid climates, but also occur in more temperate climates during the summer season, often resulting in mold growth.

Reviewing manufacturers’ literature for guidance on these ventilation-related problems, there are at least the following that are not conveyed to the application engineer:

  1. Recommendations or more specific directions from the manufacturers on leaving these ventilation doors open or closed.
  2. A discussion of the moisture and mold impact of leaving the ventilation doors open due to the entry of unconditioned air.
  3. A cautionary note on the potential problem of wind-induced infiltration and lack of cooling and dehumidification caused by an open vent door.

The air tightness of buildings is becoming an increasingly important goal to reduce energy usage. Open PTAC vent doors work against this goal. Whether open PTAC vent doors will even continue to be allowed under increasingly stringent energy performance codes or green building practices is questionable.

Myth 2―The Myth of Positive Building Pressurization


The Myth: PTAC units can positively pressurize guest rooms.

The Reality: PTAC units cannot pressurize building spaces, because to positively pressurize a room using a PTAC unit, the following would have to occur:

    • The ventilation air pulled through the vent door would have to exceed the toilet exhaust and room air leakage volumes. This does not happen.
    • The PTAC fan would have to be able to provide pressurization regardless of wind effects. However, wind effects can overpower PTAC fans and will actually draw air out through the vent door on the leeward (negative pressure) side of the building.

      Figure 3
      Figure 3: This graph of pressure measurements compares guest rooms to outdoor air (i.e., the reference condition). Test 1 (provides results of pressure tests with the system “as designed.” Test 2 (blue column) readings were made with all toilet exhaust fans turned off. Note: +5 Pascal represents the recommended level of positive pressure for hot, humid climates.
    • The PTAC fan must operate at all times when the room is occupied (for ventilation) or when the toilet exhaust is operating (for pressurization), regardless of what the guest desires.

However, guest control of the PTAC can result in intermittent fan operation, which will cause periods without positive pressurization even if the above two conditions did occur.

Myth 2 is closely associated with Myth 1 because room pressurization directly depends on the volume of ventilation air provided to the space via the PTAC unit in excess of the toilet exhaust air. This is true of intermittent and continuous exhaust systems, but problems tend to be more severe when the exhaust fans are continuously operated. However, in either circumstance, PTACs do not provide enough ventilation air to compensate for the toilet exhaust, and depressurization occurs.

If the amount of ventilation air to compensate for exhaust air could be supplied by the PTAC unit supply fan, then guest rooms could be pressurized. However, hundreds of investigations we’ve made of “failed” buildings show that this virtually never happens. Figure 3 summarizes multiple pressure readings completed on a typical guest room at a Texas hotel. These readings are typical for many hotels with PTAC units that we’ve investigated. The original system design included continuous toilet exhaust coupled with ventilation air only provided directly through the PTAC unit. As the chart below shows, the obvious result in any mode (fan ON, fan cycling, or fan OFF) is that the ventilation air source would not be continuous or robust enough, resulting in room depressurization in all operating modes. The graphic results indicate that the guest room pressure is never sufficiently positive, even with the vent door OPEN, toilet exhaust fans OFF, and fan speed set to HIGH.

Designers often assume that PTAC units can provide the volume of OA air required for pressurization. The problem with this assumption is that it cannot be proven using normal test and balance procedures. Furthermore, when we have measured direct pressurization (using digital micromanometers) inside guest rooms under a variety of operating HVAC conditions, we have found consistently negative guest rooms. (Note: This finding is not an isolated incidence. We have conducted thousands of direct pressure measurements in hundreds of hotel guest rooms over the past 30 years with similar results.)

Myth 3―The Myth of Good Dehumidification


The Myth: PTAC units provide good dehumidification under the majority of circumstances and in virtually all climates if operated correctly.

The Reality: PTAC units do not provide good dehumidification. In order to provide good dehumidification, the PTAC unit would need to meet the following conditions:

  • Be properly sized for full load and partial load conditions so that there is adequate cooling to remove the latent load during all operating conditions. However, PTAC units are often oversized because of conservative load analyses and a tradition in the industry that temperature control “trumps” anything else for guest comfort. This oversizing often results in insufficient run times to achieve proper dehumidification.
  • Be able to condition the intended outside ventilation air through the vent door during occupied modes. Air pathways to the occupied space in most PTAC models do not encourage outside air to pass through the indoor coil (if any air is even introduced) before it is supplied to the room. (See Figure 2 and discussion in Myth 1.)
  • Be able to control guest temperature expectations while still accomplishing the first two requirements. However, guest control for comfort often prevents dehumidification because the thermostats controlling PTACs do not have the ability to control humidity. When a guest (or hotel operator) turns off the PTAC unit, then no dehumidification occurs in spite of the possible need due to wind infiltration through the open vents or because of negative building pressures that draw in OA.

PTAC units come in a limited range of sizes and, in our experience, are often oversized for the typical guest room cooling load. Current energy codes provide building designs that have very limited conduction and solar loads through the building envelope, resulting in relatively small heating and cooling requirements. These heating and cooling requirements are often a fraction of the capacity available in the smallest PTAC size available from manufacturers. Selecting PTAC units that are oversized results in an insufficient amount of cooling run time to properly dehumidify the room. Lack of proper dehumidification can lead to occupant discomfort as well as moisture and mold damage.

Figure 4
Figure 4: The spreadsheet uses typical meteorological weather data available and calculated room cooling loads for each weather “bin.” (Annual hourly weather data are combined into 5°F weather bins and the average temperature is used for each bin combined data.) The room calculated sensible load divided by the equipment sensible capacity determines the run-time fraction. This value is used to estimate how much latent removal is available to meet the room needs.

PTAC units do not control space humidity because they are controlled with a thermostat that senses indoor dry-bulb temperature only. This means that temperatures may be controlled, but humidity levels are not unless the unit operates enough hours to result in “passive” dehumidification of the space due to the amount of actual compressor operation. Every air conditioning apparatus conditions the space by changing (lowering) the temperature of the air (referred to as “sensible cooling”) and removing some amount of moisture (referred to as “latent cooling”).

The amount of sensible cooling divided by the sum of sensible-plus-latent cooling is referred to as the sensible heat ratio and typically varies between 0.6 and 1.0. In general, PTAC units have a very high sensible heat ratio (0.90 to 0.95), which means that, regardless of the size, the amount of latent heat removal (dehumidification) capacity is only 10% or less of the total cooling capacity. Thus, the ability to passively dehumidify requires significant compressor operating time for proper moisture removal.

Experience has shown that, to maintain good indoor relative humidity levels, the PTAC run-time (i.e compressor run time) must exceed 50% during cooling season. This amount of run time rarely occurs. Longer run time is particularly important in warm, humid climates where some amount of unconditioned OA often finds its way into the occupied spaces or wall cavities. A simplified method of estimating the amount of PTAC run-time is shown in Figure 4. The calculation procedure is based on the typical weather “bin” data and the calculated space cooling load for each weather bin. Using this information, the anticipated operational time for the air conditioner can be estimated.

In the actual case study shown below, the location is Charleston, West Virginia, and the equipment cooling capacity (12,000 BTUH) is considerably larger than the calculated cooling load (< 4,000 BTUH), resulting in a peak operating fraction of 34% of the time. (This particular size of PTAC unit is a very common over-sized unit for guest rooms. Most new hotel rooms can be adequately conditioned with the smallest size PTAC available, 7,000 BTUH.) The calculations estimate that 855 gallons of water would be added to the space annually to the guest room with 60 cfm of outdoor air because of the short run time. In a 100-guest-room hotel, this would be the equivalent of more than eight residential swimming pools of water.

The Figure 4 analysis assumes a constant rate of infiltration/ventilation through the vent door as if it was a controlled air flow amount. In reality, the open vent door will not provide constant air flow because it will vary depending on the wind and toilet exhaust quantity. However, using the simplification of constant outside air flow rate, a typical PTAC unit cannot dehumidify the guest room with significantly less outside air than the manufacturers indicate they could provide through the open vent. This problem is further complicated because any air through the open vent does not go through the indoor coil, meaning that none of the outside air would be dehumidified before entering the room. This will result in increased interior relative humidity levels and increased mold problems.


Not all PTAC hotels have moisture and mold problems, but some that do have problems that are significant and even catastrophic. Equally important is that these problems are predictable and avoidable. However, avoidance requires that the myths around what these units can, and cannot, do must be recognized. These units cannot provide outside ventilation air, cannot pressurize the interior space, and cannot always dehumidify the space. Designers often justify not installing a separate make-up air system by using mythical PTAC unit performance numbers to show their buildings meet overall air balance targets.

What is surprising is that so few professionals over the past several decades have demanded proof that these simple HVAC units can accomplish all of the functions promised, especially because some of the functions are contradictory. Even in instances where severe moisture and mold problems occur due to the PTAC limitations, many of the parties often continue to deny the evidence.

Whatever the reason for not addressing the basic limitations of PTACs, it is certain that many of the industry parties (designers, manufacturers, and developers) all have a strong incentive to continue to believe the myths and fail to seek proof of their performance. If the PTAC units actually performed all the functions that were claimed (or implied), then they would be the least expensive and least complicated HVAC solution ever invented. The alternatives to using PTAC units are all more expensive, require more installation space, more equipment, and additional maintenance expense, but they work!

Even PTAC-equipped hotels without significant problems are almost always under-ventilated and under-pressurized according to code or good practice, unless there is a separate, dedicated fresh air ventilation system ducted to each guest room. Door undercuts have never proven to be an effective way to transfer corridor air into the guest rooms, and most jurisdictions no longer allow this practice.

There is a single, unavoidable solution to the limitations of PTAC units—use a fully ducted makeup air system in combination with the PTAC units, and don’t expect door undercuts to provide transfer air from corridors or adjacent spaces.

There are some measures that can be taken to reduce the likelihood of moisture and mold problems in PTAC hotels where installing a ducted makeup air system is not practical (e.g., existing facilities) or where the owner is not willing to accept the additional costs of a makeup air unit. These measures include: 1) reduce toilet exhaust rates to the lowest extent possible to reduce negative building pressures; 2) replace oversized PTAC units with smaller units to reduce overcooling; and 3) eliminate vinyl wall coverings on all interior wall surfaces.

In the absence of change in the current design approach, our belief is that new PTAC hotels will continue to be at increased risk for moisture and mold problems. The presence, or absence, of future mold problems in any individual hotel will largely be a matter of luck. (After completing a 2-year, $500,000 study in 1996, the Florida Solar Energy Center (FSEC) stated that “…whether a building will avoid serious or even catastrophic problems due to uncontrolled air flow is primarily a matter of luck.” Our experience suggests that the state of the industry has not dramatically advanced since FSEC wrote their report 18 years ago.) The basis of “luck” between Hotel A (without problems) and Hotel B (with problems) is usually the cumulative effect of seemingly small and relatively imperceptible decisions made during the design and construction process, often without any knowledge of their cumulative impacts.

In those instances of success, hotels have managed to achieve the delicate balance between PTAC equipment sizing and OA vent position, toilet exhaust air flows, correct exterior wall construction, and a variety of other factors. Conversely, in the hotels that do experience moisture and mold problems, there is usually a massive amount of finger-pointing and an absence of clarity as to the true cause, often under the threat of litigation.

Notwithstanding the limitations and risks of using PTAC units, the industries that have traditionally relied on them will probably continue to use this equipment. Quite simply, for many people the lower initial cost of using PTAC units without ducted fresh air systems probably offsets the increased risks of future moisture, mold, and ventilation problems. The single factor that may cause an HVAC marketplace transformation is increasingly stringent green building standards and energy codes that may no longer allow uncontrolled and unconditioned air flows through open holes (i.e., open PTAC vents) in every guest room.


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American National Standards Institute/American Society of Heating Refrigeration and Air Conditioning Engineers (ANSI/ASHRAE). 2012. Standard 62.1, Ventilation for Acceptable Indoor Air Quality.

American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE). 2005. Handbook of Fundamentals, Chapter 24, pages 7-8.

American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE). 1997. Handbook of Fundamentals, Chapter 15, pages 7-8.

National Fire Protection Association (NFPA). 2012. 90A, Standard for the Installation Air Conditioning and Ventilating Systems. NFPA 101 Life Safety Code.

About the Authors

J. David Odom and Richard Scott-AIA, NCARB, LEED AP are both building experts at LBFG. Norm Nelson is a Senior Technologist at CH2M.

Mr. Odom was a Senior Building Forensics Consultant with Liberty Building Forensics Group. He has managed some of the largest and most complex mold and moisture problems in the country, including the $60M construction defect claim at the Hilton Hawaiian Village in Honolulu and the $20M claim at the Martin County Courthouse. He has also managed over 500 projects for the Walt Disney Corporation dating back to 1982 that have included technical issues related to corrosion, moisture, and design & construction defect-related problems.

Mr. Scott, a Senior Forensic Architect at LBFG with more than 35 years’ architectural experience and an expert in building envelopes, has conducted more than 500 forensic investigations and has helped solve some of the most complicated mold and moisture failures in the world.

Mr. Nelson, a forensic mechanical engineer who investigates indoor air quality and moisture problems within buildings, specializes in energy efficiency retrofit of existing buildings, as well as commissioning and retro-commissioning of building mechanical systems. He designs mechanical systems for commercial, institutional, industrial, and residential projects.

LBFG has provided mold and moisture diagnoses and solutions for buildings to owners, contractors, and developers worldwide. The firm has project experience in the U.S., Canada, Mexico, the Caribbean, Central America, the Middle East, Southeast Asia, and Europe. Contact us at or by phone at 407/467-5518.