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How Rain & Flooding Affects Your HVAC System

March 17, 2019

Heavy rains in Middle Tennessee serve as a reminder of the dangers posed by excessive water.  Flooding of roadways and fallen trees come to mind, but have you considered how excessive rain can affect your heating and air conditioning system? The typical central heating & air-conditioning system has 3 main components:

  • an outdoor part – which sits beside or behind your home,
  • an indoor part – usually in a basement, attic, or utility room, and
  • ductwork – which runs all around various spaces inside your home.

Let’s look at how heavy rains can affect each of these.


The outdoor parts – the compressor and condenser coil – are built to withstand all kinds of weather.  The sensitive electrical components are sealed and the connections are insulated and made impervious to moisture. The outer-facing parts are comprised of aluminum, copper, and other metals, which are made to handle heavy rains.  But too much rain can impact the outdoor unit in the following ways:

  • Erosion, gullies, and mudslicks can cause the pad the outdoor unit is sitting on to shift as the heavy rain washes away the earth beneath and beside it. The result is that the unit is no longer level, or in extreme cases, the unit has washed partly or completely off the pad. If you encounter either of these situations, turn the unit off and do not operate it until the unit is made level again, on a firm footing.
  • Debris from the yard or blowing in the wind can wash up near the unit or become lodged in the unit. Branches, leaves, and other debris can fall inside the unit (where the fan blades are), or can damage the fins on the sides of the unit. After a storm, always check the outdoor unit. If the fins have been damaged or you see gashes or holes, turn the unit off and call a professional. Operating the system with debris in or against the unit will cause damage to the unit!
  • Standing water or flooding around the outdoor unit can be quite serious, as it may have damaged the internal components. Turn the unit off and have it professionally inspected before you restart it. In the meantime, check the source of the water: do you have a broken or blocked gutter or a downspout that needs to be re-directed? If the source of the standing water is due to the unit being located in a low spot in yard, or in an area with poor drainage due to vegetation or slopes, consider relocating the unit or raising it up several feet to prevent future flooding.
  • Tip: Never use a tarp or plastic bag to cover the outdoor unit. It traps moisture inside, leading to rust, mold, and mildew, which weakens the components and will cause them to break down sooner. It also creates an attractive nesting ground for insects and vermin. Covering the outdoor unit while running the air-conditioner will damage the compressor.


The indoor part – typically the furnace, heat exchanger, evaporator, and air handler – is usually located in a basement, attic, or utility room. Because it is designed to be indoors, its electrical components and construction are not built to withstand rain or extreme moisture. In a heavy rain, or even during a water heater mishap, your basement or utility room may experience flooding. If your unit is located in the attic, be mindful that a roof leak as a result of the heavy rain could develop. Any time your indoor unit is wet or encounters standing water, a short in the electrical circuits can damage the wiring, motors, and electronics, and make the system hazardous. The system should be turned off from the main circuit breaker (not just at thermostat). Do not attempt to start it again without having it thoroughly inspected by an HVAC technician.


Ductwork, which is often run in crawlspaces and attics, can be impacted by heavy rains. If your crawlspace floods or your roof leaks, the ductwork will get wet. Ductwork is typically not waterproof. Water entering the ducts may remain inside the duct long after the flood itself has receded, and will cause the ductwork to deteriorate. Mold and mildew will begin growing inside your ductwork, giving your whole house a musty smell, and could lead to the perfect environment for toxic mold growth. Roof leaks can often go unnoticed in the attic for some time, and moisture or even standing water in a crawlspace can go unnoticed if it doesn’t raise to the level of impacting the floors. This ongoing moisture will saturate the insulation around the ducts, in the attic, and under the floor joists. Wet insulation no longer does its job, and the increased heat transfer causes your heat and air conditioner to run longer cycles to compensate, or forces you to continually adjust the thermostat to compensate for the reduced efficiency.  But the biggest danger to you and your family from wet ductwork is the reduced air quality and health hazards of mold.

If you are concerned that your system is not working properly due to recent storms in the area, give Interstate AC Service a call at (615) 832-8500. We’ll determine if your system is safe to operate or is in need of repairs.