Decades of Experience Reveal Risk in Utilizing PTAC Units in Hotel Rooms Nationwide
In the mid-1980’s, certain hotels began launching products designed for the traveler that focused around a brand standard requiring the use of PTAC units. PTAC units were chosen because they were less expensive, easier to install, and made economic sense in those types of hotels. While they were successfully used in many areas of the country, they began to result in widespread moisture and mold problems when installed in warm and humid climates. Unfortunately, we are still seeing these same mistakes being made today, thirty years later.
PTAC Units and Moisture Problems: Understanding the Risk
PTAC units are designed to be stand-alone units used to heat and cool small spaces like hotel guest rooms, hospitals, and student housing. PTACs are popular and will continue to be popular, even in hot and humid climates, due to their low cost of installation and their simplicity. When they wear out, they are easy to replace. There is no reason why they should not be used for heating and cooling. It's only when they are expected to function like make-up air units or dehumidification units that buildings end up with problems. If those two expectations are eliminated and they are used only for temperature control, they should be successful.
The problem rests in the fact that several of these large hotel developers have been trying to significantly convert PTAC units in an attempt to make them simultaneously perform three other necessary functions: ventilate, pressurize, and dehumidify. When PTAC units are solely relied upon to accomplish all three of these tasks in a hot and humid climate, mold and moisture problems will result.
The primary reason that PTACs are not able to provide code-required ventilation, pressurization, and dehumidification is because guests control PTAC unit fans. There is no way of limiting this effect unless occupants are prevented from controlling the units, which can be quite difficult to do. The other issue at play is that the pressure drop through the vent door air filter is higher than the pressure drop through the evaporator coil and filter. While many PTAC manufacturers describe how the vent door may be open or closed, the majority provide little to no guidance about the moisture impact of an open vent door. In short, cautionary notes about the increased risk of moisture problems that can result from open vent doors are usually absent.
The diagram above shows the PTAC unit vent door open, and the natural path or infiltration of outdoor air as it passes through the outdoor coil (red arrows) and then through a washable filter at the vent. The ventilation air in this PTAC unit does not pass through the cooling coil before being mixed with the return air. The return air (yellow arrows) goes through the evaporator coil and is cooled (blue arrows), then mixed with the outside air (red arrows) and discharged into the room (green arrows). In hot and humid climates, the result of this situation is increased room humidity, because the outdoor air has not been properly dehumidified as it passes through the PTAC unit and is not even touched by the cooling coil.
One way to solve this humidity problem in a hot and humid climate would be to keep the vent door closed to prevent the outside air from coming in. But if this PTAC unit - in the absence of any other make-up air system into the room from another source - is expected to not only provide code-required ventilation but also to positively pressurize the room, the vent door would have to stay open to allow outside air in to ventilate and pressurize. Even keeping the vent door open, however, will not result in sufficient ventilation and pressurization to adequately prevent a room from experiencing mold problems.
Insights from Mold and Moisture Experts
We recognize that not all PTAC-equipped buildings and hotels have moisture and mold problems. However, enough do experience significant and even occasionally catastrophic problems to justify exercising caution. Those buildings that have successfully avoided mold or moisture problems have managed to achieve the delicate balance between correct PTAC equipment sizing, proper outside air vent position, minimal toilet exhaust air flows, correct exterior wall construction, and a variety of other factors, such as being in a forgiving climate.
Over the years of our work, we have found that these conditions and situations are predictable as early on as the design phase. In fact, they are so predictable that we have developed a mold tree demonstrating where one might expect to have mold occurring in a building depending on several factors, such as continuous versus non-continuous toilet exhaust, make-up air or no make-up air, etc.
Despite the limitations and risks of using PTAC units, industries that have traditionally relied upon them will continue to use them. In most cases, their lower cost and simplicity outweigh any potential risk, and often make their usage a wise business decision. That being said, we strongly believe that the decision to use PTAC units should be made with a full awareness of the risks involved and a proper understanding of what can be done to reduce those risks. Our 30 years of PTAC experience should not be ignored.