If you’ve ever read a summer headline about Legionnaires’ disease in Philadelphia, you know it can sound alarming. This type of pneumonia, caused by inhaling tiny droplets of water containing Legionella bacteria, is more commonly associated with large cooling towers or commercial building water systems. But what about your own home HVAC system?
The truth: while residential HVAC systems are generally a low-risk environment, there are certain conditions — and certain types of equipment — that can create the right breeding ground for Legionella. Knowing what to watch for, especially in Philly’s climate, can help you keep your system safe and your air healthy.
Where Legionella Could Hide in a Home HVAC System
In a typical Philadelphia rowhome or twin, your heating and cooling setup probably doesn’t have a cooling tower like a big office building does. Still, some equipment can store warm, stagnant water — and that’s where Legionella thrives.
Possible culprits include:
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Central Humidifiers – Especially drum-style or reservoir types attached to furnaces, if not cleaned regularly.
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Evaporative Coolers – Less common in Philly, but sometimes used in older houses without ductwork.
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Hot Water Heating Systems – Hydronic systems or indirect water heaters connected to air handlers can harbor bacteria if the water isn’t kept hot enough.
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Clogged Condensate Pans – If your AC’s drain pan isn’t sloped or cleaned, water can stagnate and collect dust and debris — a perfect growth environment.
Why Philly Homes May Need Extra Attention
Our city’s weather can add to the challenge:
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Humid summers mean more condensation in AC systems.
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Cold winters often lead homeowners to use humidifiers heavily, which require regular cleaning to stay safe.
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Older housing stock sometimes has retrofitted HVAC where drainage and water storage areas aren’t designed as efficiently as newer systems.
DIY Steps to Reduce Risk
You don’t need a hazmat suit — just some routine maintenance habits:
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Drain & Clean Humidifiers – Follow the manufacturer’s cleaning instructions at least once a month during use.
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Flush & Disinfect Condensate Pans – Make sure your AC drain pan is clear and drains freely.
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Keep Water Heaters Hot Enough – At least 120°F (but take care with scald risks).
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Run Systems Regularly – Avoid long periods of stagnant water in any part of your system.
When to Call a Pro
If you notice persistent musty smells, standing water in HVAC components, or visible biofilm (slimy buildup), it’s time for a professional cleaning. A trained HVAC technician can disinfect your system safely without damaging parts.
For Philadelphia homeowners, local HVAC pros like McCorry Comfort know how to work with the city’s older systems and humid climate to keep your air clean and your home comfortable.
The Bottom Line
Legionnaires’ disease makes the news for good reason — it’s serious — but the risk in residential HVAC is usually low with proper care. In Philly’s climate, regular maintenance isn’t just about efficiency; it’s also about safety.
A little attention to water-holding parts of your system now can mean peace of mind all year round.