Many of the components of structural drying are straight out of the physics books. 

From the foundation, drying is merely understanding how to manipulate energy to cause a desired reaction: Evaporation.

However, the application of the physics is far from a science. It is an art form.

Here is a reality check. Each and every project is highly unique, bloated with obstacles both obvious and disguised, with each presenting both challenges and opportunity to the professional restorer.


The art? Finding the most practical means to the common end — returning the structure to a condition where the amount of moisture in every component within every assembly no longer presents a risk to building appearance, function or health.

The ability to identify the unique characteristics within a project that can support or hinder this goal is a critical skillset. As important as it is, however, mastery of this skill is rare. 

Let’s take, for example, one major category of such characteristics: Building HVAC systems.

When present, centralized heating, ventilation and air conditioning (HVAC) systems are the respiratory system of a structure.  Engineered to ensure full structural air circulation at rates of four times per hour or more, these systems can be both highly advantageous and disastrous to the restorer.


A primary objective within an affected space is to utilize air flow to circulate air from drying systems (dehumidifiers) throughout each space where wet materials are present. In instances where the majority of a structure is affected, centralized air can dramatically assist with this goal. 

Further, many centralized HVAC systems can supply a constant stream of drier outdoor air if they are equipped with fresh air returns. This can substantially aid in humidity control within the drying space. 

Lastly, when equipped with cooling capability, centralized HVAC can be used to help prevent high temperature buildup and remove moisture during the drying effort. 

Understanding these benefits, and evaluating their impact, requires some inspection and monitoring.

First, identifying the presence and general design of the system is necessary to understand what the HVAC system can offer for both risk and benefit. 

Here are some of the most important elements to check:

  • Does the system incorporate a fresh air return? Look for this near the central air handling unit if there is safe access. If a fresh air return is present, there will be a relatively large plenum (larger than a flue pipe) running from the outdoor air to the intake side of the main plenum. Verify that the electrical and mechanical components are not at risk to have been damaged by the water intrusion. If there’s concern, have it checked out by a qualified HVAC firm. To get benefit from a HVAC system with a fresh air return, you’ll also need to verify the outdoor air is dry compared to the indoor, affected areas.
  • Is there a refrigerated air conditioning system incorporated? Look for an evaporator coil assembly near the central unit indoors, and a condenser coil unit outdoors, either on the ground or roof mounted. The presence of either will indicate the system may have capability to assist in dehumidification.  While you’re there, ensure there is no sign of leakage and water damage from the evaporator coil drainage system.
  • Does the system appear to be clean and dry, especially in the ductwork? In many cases, HVAC systems will utilize the space within flooring assemblies to contain ducts. If water damage has occurred along these same floors, the ducts themselves may be wet and compromised. In such cases, further inspection by a qualified HVAC firm is needed.


Even when there seems to be a benefit in utilizing a centralized HVAC system during drying, there are several disadvantages that must be considered. 

Often, even when there is a direct benefit, these disadvantages outweigh the value, and the system is not used. First and foremost, before using an HVAC system, the restorer must consider whether or not they are qualified to assess the system and deem it operational.

Depending upon the scope of the water damage and complexity of the system, the risk of causing damage to expensive components — such as refrigeration sealed systems, controls and motors — can be high. This alone may cause the restorer to ensure the system is not used during drying. 

In addition to this, here are several items to consider that may prevent the system from being used:

  • Humid outdoor conditions: In many cases, the outdoor air will not aid in drying. To the contrary, if humidity is high, it may slow or even reverse the process. If a fresh air return is present, then using the system would require isolating the fresh air return. This can be risky, and should be performed by a qualified person.
  • Secondary damage: When the structure contains areas that are unaffected, centralized air handling systems will cause humidity to be distributed to areas where it can cause damage. Consider the scope of the migration, and the sensitivity of items in unaffected areas. If allowing your wet (or even dry!) air to spread throughout the entire space raises concern, the system should not be used.
  • Stress to the HVAC system: Most HVAC systems are designed for intermittent operation. During a drying process, the added heat and moisture will cause the HVAC system to run for longer durations. This additional stress on an undersized or aging system can cause mechanical failure. Depending on the size of the system, the cost associated with this risk can be quite high. When concerned, again the system should not be used.
  • Freezing HVAC systems: One common problem experienced while using an existing HVAC system during drying is causing the main cooling coils to freeze over. This happens because the system is running for longer durations, especially with abnormally dry air in the structure (late in the project). This can cause substantial ice to form on the cooling (evaporator) coils. If not properly equipped, the ice can cause damage to the system, or a secondary water issue. If the system is being used, inspect this frequently to verify it is not a problem.

Other considerations

In addition to the advantages and disadvantages presented by HVAC systems, it is also necessary to understand the health and safety hazards they may present.

This is especially true when the system incorporates a combustion fired (propane, natural gas, etc.) element, as pressurization and airflow must be carefully evaluated.

Combustion of any fossil fuel will result in byproducts, such as water vapor and carbon monoxide. These byproducts are normally exhausted through a flue to the outdoor environment.  If air pressure is reduced in the structure or the area of the mechanical system, however, these gasses may be drawn into the structure.

The moisture will likely be of only moderate concern. The carbon monoxide (CO), however, can be deadly. When a combustion appliance of any type is present in a structure behind dried, the restorer should utilize carbon monoxide sensors and alarms, as a minimum. Further, air pressures and airflow near the appliance should be very carefully evaluated to ensure this backdraft does not occur.

In some cases, the decision to utilize the HVAC system is out of the restorer’s control. Especially when drying only portions of an otherwise occupied structure, it may be required to run the system. In these cases, it is important to still consider all of the items mentioned. 

Where risk of slowing drying, damage or health and safety are identified, appropriate controls, containment and expert opinions need to be employed.

Remember, as a restorer, you aren’t called upon to be the expert in every discussion. It is far more important to recognize and admit when you are not the expert. Ensure that the right expert becomes involved in the discussion.


Utilizing an HVAC system can allow the restorer to gain significant ground while drying. 

However, using the system comes with some risk, and can also be disastrous in the wrong circumstance. 

It is important to consider an expert opinion from a qualified HVAC firm when deciding whether to use the system, if it has suffered damage from the loss and which manipulation can be performed — if any — to ensure maximum benefit. 

Like all other aspects of restoration, the ultimate decision for each project will be unique, based upon the project at hand. Ensuring the right decision requires the restorer to diligently consider both the value and risk.

Brandon Burton is the technical education manager for the Restoration Sciences Academy (RSA), a part of Legend Brands. He teaches IICRC-approved classes in the categories of Applied Structural Drying (ASD) and Water Damage Restoration (WRT). Burton has served the restoration community for more than 15 years as an IICRC-approved instructor, ANSI/IICRC S500 chair, RIA restoration council member, and many other industry roles. You can contact him at [email protected].