There are a number of misconceptions regarding the use of a building's air conditioning system as a replacement for dehumidification. Air conditioners do dehumidify, but are designed to help keep the indoor environment comfortable.

The purpose of this article is to point out the differences in the design and performance of air conditioners and dehumidifiers. With some of today's more aggressive drying methods that require significantly increased amounts of drying equipment, heat can be a problem and we need the air conditioner to keep the building at a reasonably comfortable level.

When you see the word air conditioner in this article, I am referring to refrigerant-based central forced-air systems that are designed to condition the air in the indoor environment. In these systems air is forced through ducting and distributed throughout the building. Some of the information will also apply to window and portable air conditioners only on a more limited scale.

Why Use the Air Conditioner?

Comfort
The air conditioning system will help maintain a comfortable environment for the building occupants. Generally, warm and humid climatic conditions make people very uncomfortable. When refrigerant-based air conditioning systems remove heat from the air, they also remove moisture from the air just like a refrigerant dehumidifier. The air conditioner usually places the condensing coil outside of the building resulting in cool air being released by the system, whereas a refrigerant dehumidifier places the condensing coil in the airstream resulting in warm dry air. While every person has their own definition of a comfortable temperature and humidity, most people find warm and humid conditions uncomfortable. Air conditioners keep the environment comfortable by removing both heat and humidity from the air.

Electricity
Properly designed air conditioning systems will have a dedicated electrical circuit in the building. Using this power to assist your dehumidification needs can give you a big advantage. As is the case with larger drying jobs, you may need to get the full potential from the electricity the building has to offer.

Dehumidification
The largest refrigerant dehumidifiers used in our industry will have a compressor rated at somewhere between 13,000 and 15,000 BTU. A residential central air conditioning system will have a compressor rated at 30,000 BTU and up. Most commercial units are even larger. At least 50% of the energy used by air conditioners is expended to remove humidity from the air. There is a great deal of dehumidification power available to us if we use it correctly and the conditions are right. Therefore, restorers should use the dehumidification power of the air conditioner whenever possible. However, there are additional considerations.

The Difference
The evaporator coil on refrigerant dehumidifiers will, depending on ambient conditions, generally be at a temperature of 20° to 30° F colder than the dew point temperature of the ambient air down to a low of about 28° F. An air conditioner's evaporator coils will only be 10° - 14° F colder than the ambient air. The colder the coils, the more moisture vapor the device will remove from the air. The much colder coils on the dehumidifier allow for dehumidification after approximately 24 – 36 hours. Air Conditioning systems generally will not dehumidify after this time.

Important Considerations

Evaporative Coolers
In many dry climates such as the desert and mountain regions of the western United States, many buildings use evaporative or "swamp" coolers. This type of equipment cools the air by the process of evaporation, which adds humidity to the air. Obviously, running an evaporative cooler when performing restorative drying services would be counter-productive to the drying process.

Thermostat
Never set the thermostat so the temperature drops too low, either for the comfort of occupants, or to retard the rate of evaporation. Our goal when using the air conditioner for restorative drying is to have the unit in operation as much as possible. If the logistics are right, setting the thermostat a little lower than normal will help. Running dehumidifiers will help keep the temperature elevated. Also, placing a dehumidifier near the thermostat will help keep that area warm which in turn will "fool" the system into thinking that the building is warmer than it actually is. The result is dryer air in less time because you have the advantage of running both the air conditioner and dehumidifiers at the same time.

Drain Pan
A refrigerant air conditioning system will have a drain to remove the water that is created from the condensation that takes place on the coils. Since the humidity created from a restorative drying project will create an abnormal load on the drain system, it would be a good idea to check that the system is draining well. We should look at the drain pan under the evaporator coil, and see that the water is flowing freely to the drain and is not collecting in the pan. We should probably locate the exit point of the drain on the exterior of the building and see that the water is flowing freely. If something looks suspicious, we should consider advising our client to have the matter investigated by an expert. We certainly don't want to have any additional damage from an overflowing drain pan.

Fan on "Auto"
Many HVAC experts agree that the fan on an air conditioner should shut off when the compressor is not running. Here's why. When the compressor shuts off, there can still be water on the evaporator coils. If the blower is still running, this moisture can be blown downstream into the ductwork and distribution box. Since the ductwork can collect organic substrate, and is usually in an area where the temperature is not controlled, the combination of dampness, warmth, darkness, and no air movement can contribute to the growth of microorganisms. So, we should never switch the fan to "on" to make it run continuously.

Additional Considerations

Fresh Air Inlet
Many well-designed air conditioners will intentionally have 25-30% of the conditioned air enter the system from outside the building for two reasons. 1) With tighter construction methods, this allows the building to have a source of fresh air to reduce the chances of indoor air quality problems. 2) Bringing in outdoor air will result in a positive air pressure in the building, which reduces the possibility of the infiltration of humidity when the moisture load from the outdoor air is high during hot and humid conditions. Both of these features also enhance the restorative drying process.

Filter
It would also be a good idea to check the filter used by the system to make sure it is clean. In fact, some restorers actually install a high quality pleated filter on jobs just to help reduce the particulate load that occurs when air movers are used.

Conclusion

There can be significant advantages to utilizing an air conditioner to assist in dehumidification needs when performing restorative drying services. If the building has one, we should make efforts to use the system when conditions are right and comfort levels are maintained. The air conditioner will seldom do the job alone. However, when the HVAC system is used in combination with restoration dehumidifiers, we will get the added benefit of drying faster.

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