A renter who peeked inside a window air-conditioning unit with a phone camera found an alarming reddish sludge coating the interior.

The poster said the unit also left a blanket damp after running overnight, while their girlfriend "gets a stuffy nose whenever she's over."

Here's what to know

In the Reddit post, the original poster asked: "Is this mold? Should I refrain from using it until my landlord can fix it? How bad is it?"

As users in the thread noted, the buildup couldn't be identified with certainty from a photo, but multiple commenters agreed it didn't look good.

"That's definitely not normal. It could be mold or other microbial growth, but you can't tell for sure from a photo," one commenter wrote.

Another user added: "That's almost certainly mold or at least bacterial slime, the red/pink stuff is usually serratia marcescens or similar."

A third commenter said: "That evaporator coil looks pretty badly packed with grime."

Dirty filters, damp coils, and poor maintenance can worsen indoor air quality and reduce efficiency, often leading to higher energy bills on top of potential health concerns.

For homeowners looking into better long-term options, Palmetto can help explain efficient heating and cooling choices — including heat pumps — and connect people with vetted installers through its Comfort Plan network. Upgrading heating and cooling is one of the best ways to save money on utility bills and protect yourself from rising energy prices.

More background

Window and central AC systems naturally collect moisture as they cool the air. When dust and grime build up on wet surfaces, they can create an environment where mold or bacterial slime can thrive.

In this case, one of the Reddit commenters said: "Covering the vents with a blanket definitely isn't helping, traps moisture in there."

If an AC unit is blowing air over dirty, wet components, it can also struggle to cool efficiently, forcing the system to work harder and use more electricity.

What can be done?

Renters should start by making a clear record of the problem. People in the Reddit thread urged the poster to contact the landlord in writing and ask for a professional inspection and cleaning. That paper trail can help document neglect and may prompt faster repairs if the unit is unsafe.

Homeowners have more flexibility, and replacing an aging AC with a high-efficiency HVAC system or heat pump can reduce wasted energy while improving comfort.

Palmetto's Comfort Plan includes $0-down options that can lower heating and cooling costs by up to 50%, plus 12 years of free maintenance, which can help prevent exactly this kind of buildup from returning.

Homeowners can pair solar panels with electric appliances, including efficient HVACs, to drive utility costs even lower.

EnergySage makes it easy to find the best solar system and installer for your home and budget, potentially saving you up to $10,000 on installations.

"Not worth gambling with your girlfriend's breathing," one commenter added. "I'd run it as little as possible until the landlord gets someone out."

Where can I learn more?

These stories cover more situations involving moisture, grime, and neglected AC units.

• One tenant shared photos of a disgusting air-conditioning unit that commenters compared to black mold.

• In Missouri, a renter said an outdated AC system left them distraught and desperate.

• A homesteader found a clogged air filter behind a blinking red warning light.

• Homeowners say your AC's dry mode can curb moisture and help stop mold.

• In the Southeast, a sick toddler remained in a home that still climbed to 94 degrees.

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