Mold and mold growth in guest rooms is a problem at many hotels. Due to the obvious signs of room air and mildew, the hotel reviews are poor. This is a serious problem, and facility managers must strive to avoid complaints and possible requests from guests.

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 moisture in the air, food comes from the many organic substances in the room; Wood, paper, carpet, linen, fabric, foam, etc.

Cleaning crews often leave air-conditioned rooms on, even when unoccupied, to control humidity as well as mold and mildew growth. In addition to consuming a lot of energy, the solution does not solve the problem in a sustainable way. Careful design of the HVAC system during construction or renovation is important to address this issue in a lasting way

2. Mold growth

Mold can be found with water, oxygen, and any food. In winter, they grow on decaying logs and fallen leaves, especially in moist, cool places. In the garden, they can be found in compost piles and in certain grasses and weeds. Even after wetting and drying, mold can grow in moist, warm areas such as bathrooms and closets. Mold can also be grown in places where fresh food is stored, refrigerator drip trays, inside plants, carpets, mattresses, and upholstered furniture. Polyurethane and rubber foams appear to be particularly vulnerable to fungal attack. The various places where mold grows in hotel rooms are inside plaster walls in bathrooms, on ceilings, drywall, wallpaper, mattresses, upholstery, clothing, cabinets, and tile paste.

3, humidity and condensation formation

The South has a mild climate with two seasons: warm and humid from November to April and cool from June to September. May and October are considered transition months. The average relative humidity (RH) in coastal areas exceeds 80% most of the time, with summer temperatures ranging from 30 ° C to 340 ° C and high humidity throughout the year. The summer months are generally wetter. For example, relative humidity was above 80%, compared to 43% in February 2013 and 22% in January of the same year. Winter relative humidity is also high, with a peak of 98% recorded in August 2013 (Source: Mauritius Meteorological Department).

Controlling this level of humidity is a challenge for HVAC engineers. A compromise is often required between passenger comfort and the energy costs associated with dehumidification. In general, interior designers, in consultation with clients, set the internal RH slightly higher than the international recommended standard to reduce initial and running costs. In fact, the need to control humidity in hotels has been a topic of debate. The focus is on achieving internal temperature rather than controlling humidity. Guest comfort is always 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, as hotel rooms are usually not equipped with humidifiers.

Speaking to several maintenance managers about guest habits and HVAC usage, 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, cardboard boxes and even pieces of metal. With this setting, the room is very cold after a while, and so is the interior surface. A few hours later, due to the heat flow, the surface of the partition wall next door also became cold. timg (5).jpg

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, moisture accumulates on internal surfaces such as walls, drywall, picture frames, furniture, foam and polyurethane mattresses. This problem is common in many hotels in Mauritius, especially in those areas where the partition walls are reinforced concrete due to the lower heat flow resistance. The wall surface of an unoccupied adjacent room can reach the temperature of the occupied air-conditioned room more quickly.

Due to high room occupancy during the summer months, condensation does not have time to dry up completely, or to be washed away by air flow in natural ventilation, as the room is constantly occupied and the door is closed. For safety reasons, it is not possible to open the room for a long period of time in order to flush through natural ventilation. And cooling coils don't remove moisture. So the water is trapped

4. Some solutions

4.1 Dehumidification using the HVAC system

One of the most important causes of moisture accumulation in hotel rooms in hot and humid climates is an overemphasis on proper cooling, while choosing a room HVAC system at the expense of potential cooling. (Latent heat load is the energy required to remove water from the air; Sensible cooling load is the energy required to cool the air.) As mentioned earlier, guest satisfaction has always been affected by temperature and humidity to a lesser extent. HVAC equipment is often more effective at cooling air than dehumidifying it. As a result, the outside air entering the room can be cooled to the desired temperature before being properly dehumidified, resulting in higher relative humidity and mold growth inside. In addition, since HVAC equipment is usually controlled by temperature (thermostat) rather than humidity (humidifier), a dehumidifier is needed here.

Since most hotel rooms are cooled by a single unit or end unit (fan coil, VRV, split unit, dehumidifier) rather than a centralized air handling unit, humidity control can be very challenging if the unit is improperly sized and selected. Many times, contractors only need the total cooling load, and there is no information about the potential or sensible heat needs of the room. In this case, the latter may mistakenly order the required cooling coils. This is because standard off-the-shelf cooling coils or standard catalog products usually have sensible heat ratios between 70% and 80%, and in most cases in the South, it is much higher than the actual indoor sensible heat ratio. (Sensible heat ratio is the ratio between sensible heat load and total heat load, and characterizes the dehumidification effect of air conditioning equipment). Let's illustrate this with a real case study.

In 2014, we renovated a five-star resort hotel where we conducted this study using rooms for standard guests. All dynamic thermal simulations were performed using the hourly weather database of the weather station, with the port being the closest station to the hotel studied. Shadow analysis is performed using Suncast to consider shadow effects, local shadows, and self-shading features of adjacent buildings. A cooling water fan coil system was selected, with fresh air fed directly into the coil without any precooling. This is typical of many hotels in Mauritius.

4.2 Fresh air supply tempering

An outside supply of fresh air can also cause humidity. The supply of fresh air in hotel rooms is a regulatory requirement that requires the removal of carbon dioxide and other pollutants. However, traffic for hotel operators is different. Given that ASHARE design guidelines recommend that each room needs 30 l/s, some operators can specify a maximum of 40 l/s of fresh air in the room, which can be a significant source of humidity. Usually fresh outside air is blown directly into the room or through a room cooling coil. When supplying through cooling coils, it is important to ensure that the potential and perceived heat load from this airflow is also taken into account in the cooling coil selection.

The best way to control the humidity of an outside fresh air supply is to pre-cool it below the wet bulb temperature to remove moisture. Depending on the wet bulb temperature and the RH set internally, reheating may or may not be required, and dehumidification can also be performed using a dehumidifier. Many hotels now have a dehumidifier under a table.

4.3 Air penetration control

The infiltration of outside air into the room is another issue that must be addressed to avoid high humidity and therefore mold growth. It can also avoid high energy consumption. Designers often assume their cooling load is calculated at 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 permeates the building through vents and grates, through cracks, 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 external atmospheric pressure, the air will permeate the room instead of permeating. This pressure difference is largely determined by the amount of air introduced into the room, the amount of air inside the toilet, and the wind pressure on the facade, which in turn depends on terrain, direction, wind speed, and several other factors. Calculating this pressure difference is complicated because it is inherently dynamic as wind speeds vary 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, toilet air extract consists of an indoor supply of fresh air. Typically, the fan coil unit blows fresh air into the room, and the toilet extractor replenishes the air by taking it out of the transfer grille located on the ensuite door very close to the entrance. This creates local negative pressure in the entrance hall and draws in moist outside air around the door seam and bottom cut. To avoid this, the toilet make-up air grille must be 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 bottom cuts or gaps infiltration.

4.4 Thermal Barrier

Condensation forms rapidly on surfaces below the air dew point temperature. As explained earlier, the floor and walls of an air-conditioned room "sweat" when the door is opened. Since heat flows through walls and thermal Bridges such as gaps, cracks, beam edges and expansion joints, condensation can also form on the walls and floors of free rooms adjacent to air-conditioned rooms. Sweating on the floor above an air-conditioned room can be avoided by directing the return air to the fan coil instead of using the ceiling gap as the return air room. This prevents the lower surface of the slab from getting cold and therefore avoids the floor surface of the upper room.

Isolating the partition wall and eliminating all possible thermal Bridges prevents condensation from forming on the walls of adjacent rooms. There are many low cost and acceptable insulation materials on the market today, and architects should consider them in their designs with the assistance of HVAC engineers. This will reduce the amount of heat passing through the partition, so condensation will not be triggered.

4.5 Setting the Return Temperature

Another possible solution to reduce moisture buildup in guest rooms is to set its temperature above the air dew point. For a given climatic condition, the dew point is easily determined from a psychometric map. For example, the dew point of the outside air can be determined based on the current external relative humidity and a room temperature set above one degree. This will avoid condensation forming inside and in adjacent rooms. Ideally, it's best to turn off the air conditioning, but many hotels must keep the room temperature down to avoid overheating and complaints when guests return to their rooms. 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 that comes with the dehumidifier HVAC system. External sensors can be installed to monitor temperature and humidity and send this information to a central controller, which calculates the dew point and automatically adjusts the return temperature in the room.

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.

NAISIDA, a brand of Guangdong Aoyu Electrical Appliance Co., LTD., mainly develops and sells "household dehumidifier, commercial dehumidifier, industrial dehumidifier, fresh air purification dehumidifier, cooling dehumidifier, temperature control dehumidifier, constant temperature and humidity machine, rotary dehumidifier, ultrasonic humidifier, wet film humidifier and non-standard product development".