It’s no wonder that mold development within HVAC systems is so common, as the manner in which a conventional heating, ventilation and air conditioning system operates creates a great deal of moisture within a confined area. In order to understand how mold has the potential to develop, we need to understand how an HVAC system functions.

A conventional split AC system is composed of two main components – an air handler unit and a condenser unit, which make up the cold and hot sides of the system. Aside from electrical connections, these two primary components are linked by way of two refrigerant lines – one line supplying the air handler with cold refrigerant and the other line moving hot refrigerant back to the condenser unit. Air from within a property is brought into an air handler unit by way of a return grille, or simply the bottom of an air handler unit. This air is drawn in by a fan motor, which exhausts toward the opposite end of the unit – commonly referred to as the supply side.

Now that we understand how air is brought into an air handler, let’s discuss how this air is cooled – providing comfort to the environment in which air is supplied. Air being drawn in through the air handler must pass through a coil, which is chilled by refrigerant from the condenser unit. As this room temperature air passes through the coil, the heat from the air causes condensation on the coil, creating moisture that is directed out of the property via a condensate line. The refrigerant passing through the coil absorbs this excess heat and transfers this to the condenser unit, where the heat can be expelled outdoors by a fan.

With me so far? Excellent! The HVAC system sounds like a well oil machine, so why is it so at-risk for mold development? In terms of a math equation; Excess Moisture + Darkness +Dirt/Debris = Potential Mold. Is this an absolute for mold growth? Shucks no – but odds are pretty high that something will be developing within those environmental conditions. So, what are some common reasons in which you’d have excess moisture within an air conditioning system? Let’s first discuss the elephant in the room. The most common cause for microbial growth within an AC system, and even in the surrounding areas, is an incorrect setting on a thermostat. By placing your thermostat fan in the “ON” setting, you are exponentially increasing the chances of a humidity bloom – the growth of mold within an environment due to prolonged relative humidity levels above 60%.

Another infrequent cause of excess moisture within an HVAC system is when the system is low on refrigerant. When this happens, a system cannot chill the refrigerant to the level needed – which results in less condensation at the coil and more pass-through of the existing conditions within the indoor air. These situations can be diagnosed by a qualified HVAC professional – sometimes just by comparing the intake and supply temperatures. A good rule of thumb is that the temperature of air being supplied is 14-20° colder than the air returning to the air handler. 

While it may be a no-brainer for some, others may need to hear this: Keep your HVAC components clean and free from as much debris as possible. Given that mold requires not only moisture, but a good source, it’s good practice to have your AC system cleaned regularly. How often is “regularly”? Well, that depends on a number of factors – pets, household dust and outdoor conditions, just to name a few. Based on our recommendations, your entire HVAC system should be inspected annually – likely having your condensate line cleared out more frequently. This is especially important in the Summer months, where higher temperatures are expected. Having exceptional indoor air quality starts with you, so apply this new or refresher knowledge to greatly assist in promoting your healthy home!