Household heating and cooling devices: no Legionella risk from reverse-cycle AC, but CO risk from older gas heaters

A Melbourne reader asked whether reverse-cycle air conditioners could spread Legionnaires’ disease. Health professionals say the domestic risk from these systems is low when the equipment is properly maintained, and they emphasize addressing other indoor air quality factors instead.

Legionella bacteria cause Legionnaires’ disease, a severe form of pneumonia. It thrives in stagnant warm water and is spread via mist from contaminated sources such as cooling towers, spas and fountains. A typical home reverse-cycle split system, which cools and heats using refrigerant rather than stored water, does not store water and thus is not considered a breeding ground for Legionella. The chief domestic concern is dust, mold and other allergens in dirty filters, which can aggravate asthma or hay fever. Regular cleaning and filter maintenance help keep indoor air cleaner and reduce respiratory irritants.

Gas heaters and indoor air quality present a different set of risks. Unflued or poorly maintained heaters can release carbon monoxide, a colorless, odorless gas that can be deadly at high levels and harmful at lower exposures over time. Nitrogen dioxide, another combustion byproduct, has been linked to asthma flares in children and lung irritation in adults. Health authorities and peer‑reviewed studies point to these risks as measurable concerns. The good news is that a properly flued and serviced heater in a well‑ventilated room is much safer. If a gas appliance is old, unflued, or hasn’t been checked in years, it should be inspected, and replacing it may be wise. Installing a carbon monoxide alarm is a prudent safety step.

How to stay safe involves practical steps: continue using reverse-cycle air conditioning for cooling and heating when it’s serviced, retire unflued or very old gas heaters, install a carbon monoxide alarm, ensure adequate ventilation when running gas appliances, and seal gaps to improve insulation and reduce how hard your systems must work. Regular cleaning reduces dust and mold buildup that can aggravate allergies and asthma. In Melbourne’s variable climate, that approach helps you stay comfortable while minimizing health risks indoors.

Context matters: the guidance aligns with public health messaging that Legionella risk from typical home air conditioning is low, whereas gas-heater safety remains a clearer area of concern. Homeowners should prioritize professional servicing of heating and cooling equipment, routine filter cleaning, and safe operation of gas appliances. For households uncertain about their equipment, a licensed technician can assess the system’s flue status, venting, and overall energy efficiency.

Ultimately, households like Carla’s—who asked the question—can keep using reverse-cycle systems without increasing Legionella risk, provided regular maintenance is kept up. The emphasis is on correctly operating and maintaining gas heaters, using CO alarms, and ensuring proper ventilation when gas appliances are in use. With these precautions, homes can stay warm in winter and cool in summer without compromising health.