HVAC (Heating, Ventilation, and Air Conditioning) systems are a significant part of modern buildings, providing temperature control and air circulation for human comfort. These systems can sometimes create unexpected situations, especially when animals inhabit these spaces.
One interesting example is the case of bats. When bats encounter buildings with HVAC systems, their echolocation abilities can be both helpful and challenging. The air currents produced by HVAC vents and fans can disturb the flight patterns of bats, making it harder for them to fly and hunt efficiently. However, on the flip side, these same air currents can also help to disperse the insects that bats feed on, creating an unintended bonus for the bats. Researchers studying this phenomenon have found that certain HVAC designs can unintentionally create "bat-friendly" buildings by providing suitable conditions for their foraging activities.
ABOVE, . Swarm of the Flying fox bats out hunting for food during dusk in Cairns, in Northern Queensland. Credit: Shutterstock KCEmperor
It can be a variety of factors which makes Rooftops HVAC systems a hotspot for animals, it's not uncommon to find evidence of bird, rodent or even reptile activity. We’ve come across a variety of alarming situations, and a system contaminated with animal matter is a sure way to make building occupants sick.
Debris and faecal matter from animals degrades air circulation provided by a Roof Top Unit (RTU). If left unchecked, particulate matter enters the system and builds up on components. The first line of defence are the filters. Particulates become trapped and will choke a filter bank. This creates a significant resistance to airflow. So, static pressure increases and inhibits efficiency. Contaminants also find gaps in the filter housing to enter the coils and drain pan.
Debris and droppings clog evaporator coils, affecting static pressure and performance. Fouled coils also degrade airflow, Delta-T and cooling capacity. It’s a recipe for indoor air quality issues and mechanical failure.
Breathing dust or water droplets containing contaminated bird droppings can lead to several diseases, including a pulmonary disease called Cryptococcosis.
HVAC units located on a roof are most likely affected by birds. Much like other rooftop equipment they weather the elements year round. RTUs are the perfect place for birds to roost and even nest. A frequent population of birds wreaks havoc on RTUs and leads to health issues for occupants.
HVAC and building hygiene is also jeopardised by bird infestation. Bird faeces and debris pose serious health threats to building occupants. Such detritus is a rich environment for microbial growth. Bacteria and fungi thrive in these conditions. They are easily distributed through a building by the HVAC system. So, breathing dust or water droplets containing contaminated bird droppings can lead to several diseases, including a pulmonary disease called Cryptococcosis.
ABOVE, . Bird Debris and Faecal Contamination, in and around HVAC system. Source: Clean-Air site visit, Feb 2023.
If rats and mice are using a ventilation system as a home or a superhighway across the building, there’s a 100 percent chance that they have voided throughout the ductwork. HVAC blows anything in your system towards your vents and into your occupied space. Urine and droppings from rats carry the germs that cause Hantavirus Pulmonary Syndrome (HPS), a severe, sometimes fatal, respiratory disease in humans. 
ABOVE LEFT, Residents of a home in Gangzhou, China, filmed the spectacle of a snake dangling down from the wall mounted AC unit whilst eating a rat. ABOVE MIDDLE, Rat photographed inside AC Ducting. ABOVE RIGHT, A torn flex creates an entry point for vermin
Usually where there’s rats and birds, there’s snakes. Our team recovered a shedded python skin from ductwork on a job at a building in Bli Bli, on the Sunshine Coast of Queensland, which now serves as office wall art.
There have been various reports of snakes found in HVAC units. This instance in Mississippi, USA, where a technician was startled when he opened an AC unit to find a baby ball python. The homeowner said "I’ve been having problems with rats on the roof. At night, I would hear a thud on the roof, and I think it may have been that python dropping out of the tree and gorging himself on the rats.”
ABOVE LEFT, Clean-Air's employee Linto recovered a shedded python skin from ductwork on the Sunshine Coast, Australia. ABOVE RIGHT, Numerous instances of snakes being found by AC units, like this one in Mississippi, USA
As referenced by Worksafe Queensland, the risk associated with bird and bat droppings are infections and illnesses including:
- Histoplasmosis – a fungal infection caused by Histoplasma capsulatum, found in bird or bat droppings. Inhaling the fungal spores can lead to infection, with the highest risk for young children, the elderly, and immunocompromised individuals. 
- Cryptococcosis – caused by the fungus Cryptococcus neoformans, found in environments with bird droppings. Inhaling the fungal spores leads to infection, with higher risk for immunocompromised individuals. It often causes meningitis, but can affect other body parts too. 
- Psittacosis – caused by Chlamydia [or Chlamydophilia] psittaci bacteria shed in the droppings and respiratory secretions of infected birds. Inhaling the bacteria can lead to mild to moderate respiratory illness with pneumonia, though severe cases can occur. Those at highest risk are young children, the elderly, pregnant women, and immunocompromised individuals. 
- Gastrointestinal illness – Bird and bat droppings can contain germs that cause gastrointestinal illness with symptoms of nausea, vomiting and diarrhoea. People can be infected if they inadvertently swallow the germs, usually via contaminated hands.
Without a doubt, adhering to regular inspections of your HVAC system, inline with AS3666 is the best way to safeguard against animal infestation, eliminating any issues far before a serious health emergency occurs. Where issues do exist, care must be taken to remediate the site and system safely, so always consult with a professional.
 https://www.wlox.com/2019/09/25/python-now-safe-home-after-being-found-homes-ac-unit/  http://conditions.health.qld.gov.au/HealthCondition/condition/14/92/76/Histoplasmosis  http://conditions.health.qld.gov.au/HealthCondition/condition/14/92/32/cryptococcosis  https://www.worksafe.qld.gov.au/safety-and-prevention/hazards/hazardous-exposures/biological-hazards/diseases-from-animals/psittacosis  https://www.cdc.gov/hantavirus/hps/index.html#:~:text=Hantavirus%20Pulmonary%20Syndrome%20(HPS)%20is,primary%20risk%20for%20hantavirus%20exposure.