Top Signs Your House Has Mold in the Water System

Mold isn’t something we usually associate with water coming from a faucet. We expect it in bathrooms, basements, and sometimes inside HVAC systems—but the water system itself? Most people are surprised to learn that mold can develop anywhere moisture lingers, especially inside parts of a plumbing system that don’t move water very often. When mold creeps into the water system, the signs may be subtle at first, but once you know what to look for, they become hard to miss.

One of the earliest clues is an earthy or musty smell that appears when you run hot water. Mold releases compounds that create a very distinct scent, and those compounds can travel through steam. If your shower suddenly smells like an old, damp basement, there’s a chance mold or bacteria are growing somewhere in your plumbing.

Another sign is slimy residue inside faucets or on the underside of aerator screens. When mold and bacteria form a biofilm, it becomes sticky and coats the surfaces where water slows down or sits between uses. Many homeowners discover this accidentally when cleaning their fixtures. The residue often appears slightly discolored, ranging from white to brown or even black.

Strange-tasting water can also signal mold activity. Mold itself doesn’t always mix into the water, but the organic compounds produced by biofilm can influence taste, especially if the water line feeding your refrigerator or sink hasn’t been flushed in months. If your water suddenly tastes earthy, stale, or unusual in a way you can’t describe, it’s worth investigating the plumbing around the affected tap.

Hot water systems are especially vulnerable. Water heaters contain warm, dark, sometimes stagnant water. This is the perfect environment for mold-like microbial growth. If the smell or taste issue only occurs with hot water, the water heater is the first place to look. Flushing it can help, but if you’ve never tested for bacteria, this is a good time to do so. Even though mold itself doesn’t typically grow inside flowing water, the presence of mold-like odors often means bacteria are present, and a screening test can provide clarity before you use the water again.

Finally, if mold appears in places like toilet tanks, refrigerator drip trays, or humidifiers, this is a sign that conditions in your water system may be inviting microbial growth more broadly. If moisture is lingering in multiple areas, it raises questions about what’s happening inside the plumbing as well.

If you suspect mold in your water system, testing for bacteria is the smartest first step. Mold and bacteria often coexist in the same conditions, and while you can see mold, you can’t see microbial contamination. A simple screening test gives you answers without guesswork. Once you know what’s happening inside the system, you can flush, sanitize, and repair the areas that need attention.

By Chris M., PhD