Mold and mold growth in guest rooms are a problem for many hotels. The hotel reviews were poor due to obvious signs of air and mold in the room. This is a serious problem, and facility managers must work hard to avoid guest complaints and possible requests.

1 Introduction

Mold and mildew require three growth conditions; air (oxygen), water and food. There is plenty of air in the room, moisture comes from the humidity in the air, and food comes from many organic substances in the room; wood, paper, carpet, linen, fabric, foam, etc.

Cleaning staff often leave rooms with air-conditioning turned on. Even when they are not occupied, they can control humidity and the growth of mold and mildew. In addition to consuming a lot of energy, this solution cannot solve the problem in a sustainable way. Careful design of the HVAC system during construction or renovation is important to solve this problem in a lasting way

2. Mold growth

As long as there is moisture, oxygen and any food, mold can be found. In winter, they grow on rotting logs and fallen leaves, especially in humid and shady places. In the garden, they can be found in compost piles and certain grasses and weeds. Even after damp and dry, mold will grow in humid and warm areas, such as bathrooms and closets. Mold can also be grown in places where fresh food is stored, drip trays in refrigerators, inside plants, carpets, mattresses, and upholstered furniture. Polyurethane and rubber foam seem to be particularly susceptible to fungal attack. Various places where mold grows in hotel rooms are in the gypsum walls, ceilings, gypsum board, wallpaper, mattresses, upholstery, clothing, cabinets and tile grout in the bathroom.

3. Humidity and condensation formation

The climate in the south is mild, with two seasons: warm and humid from November to April, and cooler from June to September. May and October are considered transition months. Most of the time, the average relative humidity (RH) in coastal areas exceeds 80%, the summer temperature is between 30°C and 340°C, and the humidity is high throughout the year. The summer months are generally humid. For example, relative to the 43% in February 2013 and 22% in January of the same year, the relative humidity is above 80%. Relative humidity is also high in winter, with a peak of 98% recorded in August 2013 (source: Mauritius Meteorological Department).

Controlling this humidity level is a challenge for HVAC engineers. There is often a compromise between passenger comfort and the energy costs associated with dehumidification. Generally speaking, after consulting with the client, the interior designer sets the internal RH slightly higher than the international recommended standard to reduce the initial cost and operating cost. In fact, the need to control hotel humidity has always been a topic of debate. The focus is on achieving internal temperature rather than controlling humidity. The comfort of guests has always been related to the internal temperature. This is mainly because guests are always aware of the internal temperature of the thermostat, but are less sensitive to humidity, because the hotel room usually does not install a humidistat.

When talking to several maintenance managers about guest habits and use of HVAC systems, I was told that guests tend to set the dehumidifier as low as possible; 160C in most cases. Some rooms leave the room when they go out, often bypassing key tags with other cards, cartons or even metal fragments. With this setting,

The room is very cold after a while, and so are the internal surfaces. A few hours later, due to the heat flow, the surface of the partition wall next door also became cold.

In summer, the outside air is very hot and humid, and the temperature can be very high. This air will condense on surfaces in the room below this temperature. In fact, whenever outdoor air enters an air-conditioned room, or even an unconditional adjacent room, internal surfaces such as walls, gypsum boards, picture frames, furniture, foam and polyurethane mattresses will accumulate moisture. This problem is common in many hotels in Mauritius, especially in areas where reinforced concrete partition walls are used because of the low heat flow resistance. The wall surface of the unoccupied adjacent room can reach the temperature of the occupied air-conditioned room more quickly.

Due to the high room occupancy rate during the summer months, condensation does not have time to completely dry up, or is washed away by air flow in natural ventilation, because the room is constantly occupied and the door is closed. For safety reasons, it is not possible to open the room for a long time in order to flush through natural ventilation. Coupled with the cooling coil, the moisture cannot be removed. So the moisture is retained

4. Some solutions

4.1 Dehumidification via HVAC system

In a hot and humid climate, one of the most important reasons for the accumulation of moisture in a hotel room is to overemphasize reasonable cooling, and the room HVAC system is selected at the cost of potential cooling. (Latent heat load is the energy required to remove moisture from the air; sensible cooling load is the energy required to cool the air.) As mentioned earlier, guest satisfaction is always affected by temperature, and humidity is less affected. HVAC equipment is generally more effective at cooling air than dehumidifying it. Therefore, the outside air entering the room can be cooled to the required temperature before proper dehumidification, resulting in higher relative humidity and mold growth inside. In addition, since HVAC equipment is usually controlled by temperature (thermostat) rather than humidity (hygrostat), a dehumidifier is needed here.

Since most hotel rooms are cooled by single equipment or terminal equipment (fan coil, VRV, split equipment, dehumidification equipment) instead of centralized air handling equipment, humidity control may be very challenging if the equipment size and selection are not appropriate Sex. Many times, the contractor only needs the total cooling load, and there is no information about the room's potential or sensible heat demand. In this case, the latter may incorrectly order the required cooling coils. This is because standard off-the-shelf cooling coils or standard catalog products usually have a sensible heat ratio between 70% and 80%, and in most cases in the south, it is much higher than the actual indoor sensible heat ratio. (The sensible heat ratio is the ratio between the sensible heat load and the total heat load, and characterizes the dehumidification effect of air conditioning equipment). We use a practical case study to illustrate this problem.

In 2014, we refurbished a five-star resort hotel and we conducted this study using standard guest rooms. We performed all dynamic thermal simulations using the hourly weather database of the weather station, and the port is the station closest to the hotel under study. Through the use of Suncast for shadow analysis, the shadow effect, local shadow and self-shadow characteristics of adjacent buildings are considered. A cooling water fan coil system is selected, and fresh air is directly sent to the coil without any pre-cooling. This is typical of many hotels in Mauritius.

4.2 Tempering of fresh air supply

The external fresh air supply can also cause humidity. The supply of fresh air to hotel rooms is a legal requirement that requires the removal of carbon dioxide and other pollutants. However, the traffic of hotel operators is different. Given that the ASHARE design guide recommends 30 l/s per room, some operators can specify up to 40 l/s of fresh air in the room, which may be an important source of humidity. Usually fresh outside air is blown directly into the room or through the room cooling coil. When supplying through the cooling coil, it must be ensured that the potential and perceivable heat load from the air flow is also considered in the selection of the cooling coil.

The best way to control the humidity of the external fresh air supply is to pre-cool it below the wet bulb temperature to remove moisture. Depending on the wet bulb temperature and the internally set RH, reheating may or may not be required, and a dehumidifier can also be used for dehumidification. A dehumidifier is now placed under the tables in many hotels.

4.3 Air penetration control

The penetration of outside air into the room is another problem that must be addressed to avoid high humidity and therefore mold growth. It can also avoid high energy consumption. Designers usually assume that their cooling load is calculated to be 0.5 air changes per hour, but if the actual penetration is higher, the humidity in the room will be higher. This is proved in simulation case 2.

Outside air penetrates the building through vents and grilles, through cracks, and around doors and windows that are not properly sealed. Infiltration can be controlled by actively pressurizing the room. If the indoor pressure is higher than the outside atmospheric pressure, the air will penetrate the room instead of penetrating. This pressure difference depends largely on the amount of air introduced into the room, the amount of air in the toilet and the wind pressure on the door, which in turn depends on the terrain, direction, wind speed and several other factors. Calculating this pressure difference is very complicated because it is inherently dynamic as the wind speed changes throughout the day and year. Computational fluid dynamics software can be used to predict the permeability of buildings, but it is rarely used by designers.

In a typical hotel, the toilet air extract is composed of fresh indoor air supply. Normally, the fan coil unit blows fresh air into the room, and the toilet extract supplemental air is taken out of the conveying grille located on the apartment door very close to the entrance. This creates a local negative pressure in the entrance hall and draws in moist outside air around the door gap and undercut. To avoid this, the toilet touch-up air grille must be far away from the entrance, for example, on the wall facing the room if possible. In addition, the entrance door should be properly sealed to avoid undercuts or infiltration of gaps.

4.4 Thermal barrier

Condensate quickly forms on surfaces below the dew point temperature of the air. As explained earlier, when the door is opened, the floor and walls of the air-conditioned room will “sweat”. Since heat flows through walls and thermal bridges, such as gaps, cracks, beam edges and expansion joints, condensation will also form on the walls and floors of free rooms adjacent to air-conditioned rooms. Sweating on the floor above the air-conditioned room can be avoided by directing the return air to the fan coil and avoiding the use of ceiling voids as the return air chamber. This prevents the lower surface of the slab from getting cold and therefore avoids the floor surface of the room above.

Isolating the partition wall and eliminating all possible thermal bridges can prevent condensation on the walls of adjacent rooms from forming. There are many low-cost and acceptable insulating materials on the market. Architects should consider them in their design with the assistance of HVAC engineers. This will reduce the amount of heat passing through the partition wall and therefore will not cause condensation.

4.5 Set back temperature

Another possible solution to reduce the accumulation of moisture in the guest room is to set its temperature above the dew point of the air. For a given climatic condition, the dew point can easily be determined from a psychometric chart. For example, the dew point of the outside air may be determined based on the current external relative humidity and the room temperature set to be higher than one degree. This will avoid the formation of condensation inside and adjacent rooms. Ideally, it is best to turn off the air conditioner, but many hotels must keep the room temperature down to avoid overheating and complaints when guests return to the room. The retraction temperature should be higher than the dew point to avoid condensation and moisture accumulation. This can be easily achieved by the maintenance team through the centralized control system attached to the dehumidifier HVAC system. External sensors can be installed to monitor temperature and humidity, and send this information to the central controller, which can calculate the dew point and automatically adjust the room's return temperature.

5 Conclusion

The problem of mold and mildew in the hotel can be controlled by maintaining humidity. Careful design of the HVAC system is necessary to keep the humidity level below 70% of the mold growth threshold. Designers should understand this problem at an early stage in order to approach the solution in a comprehensive way during the design and construction of the hotel.

The brand NAISIDA of Guangdong Aoyu Electrical Appliance Co., Ltd. mainly develops and sells "household dehumidifiers, commercial dehumidifiers, industrial dehumidifiers, fresh air purifying dehumidifiers, cooling dehumidifiers, temperature regulating dehumidifiers," with constant temperature and humidity products. Constant temperature and humidity machine, rotary dehumidifier, ultrasonic humidifier, wet film humidifier and non-standard product development".