Why Your Hot Tub Smells Like Mildew (and What to Do About It)

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When your hot tub smells like mildew, it's not a pleasant experience. Unfortunately, this is a common issue with hot tubs, and one that many people don't know how to fix.

Generally, hot tubs get a mildew smell due to excessive mold and algal growth in the water. These growths can come from a lack of sanitizer, plumbing issues, or your hot tub cover. Removing mildew from your hot tub will typically require you to drain, carefully clean, and refill it.

In this blog post, we'll discuss what causes a hot tub to smell like mildew in more detail, and what you can do to get rid of the smell once and for all. So, if your hot tub has been giving off an unpleasant odor, we'll help you solve the problem in no time.

What causes hot tub mildew?

There are several possible sources of moldy growth in your hot tub. And the more of it there is, the more pungent the stench will be.

Here are the most likely causes:

  • Lack of sanitizer: Warm water is the ideal home for many types of mold and bacteria. As a result, you should be regularly adding spa sanitizer. The most common option is chlorine, but bromine systems are also popular (see the links for a full guide on how to use each).
  • Filter: Worn-out hot tub filters won't properly cleanse your water of contaminants. So ensure that you rinse yours every 2 weeks, deep clean it every 1-3 months, and replace it annually—even if it doesn't look dirty or moldy.
  • Hot tub cover: Believe it or not, your spa's cover can spread mold to your water. If you notice any slime or mold on your hard cover, thoroughly clean and disinfect it. Meanwhile, a mildewy cloth cover may need airing out or replacing (most removable hot tub covers eventually become waterlogged and need to be replaced).
  • Plumbing: Mold in your tub's plumbing may spread to the water, usually in the form of white flakes. Solving this issue typically requires hyperchlorination, a line flush, and draining and refilling your spa.

Signs of mildew and mold in hot tubs

If you want to get rid of your hot tub's mildewy smell, it's important to know that what you're dealing with is actually mildew. That way, you can identify and address the issue as soon as possible.

Below are some indications that your hot tub is growing mold or algae:

  • Musty smell: As you may well know, a musty smell can indicate that there is mildew in your tub. This scent may also come across as sickly-sweet or acrid.
  • Cloudy water: Your hot tub water should be crystal clear besides bubbles. So if you notice cloudiness, you may have algae or mildew growing.
  • Dark spots: Black mold tends to grow along the waterline of hot tubs. And since it's at eye level, the evidence may be particularly noticeable.
  • White flakes: Have you seen pale little flakes on the surface of your tub's water? If so, your spa's plumbing likely has a white mold build-up.
  • Discoloration: Green and brownish water indicate that your hot tub is full of algae or mold. This issue is also often the result of a pH imbalance since mildew thrives in acidic environments.

Removing the mildew smell from your hot tub

If you've already narrowed down the cause of your mildew smell to mold, you may need to deep clean your tub to remove its musty stench.

To help, here are the basic steps to washing your hot tub of mildew:

  1. Remove any debris on or in the water. Otherwise, it may get in the way of draining your tub.
  2. Use a hot tub plumbing cleaner like Ahh-Some Hot Tub/Jetted Bath Plumbing & Jet Cleaner. Allow it to circulate according to instructions to ensure the solution gets into the jets.
  3. Turn off your hot tub.
  4. Drain your hot tub entirely. Direct the water safely into a drain or recycle it in another way, as the chemicals inside of it can damage your plants.
  5. Use a shop vac on the hot tub jets. Doing so helps suck out any old water trapped inside of them.
  6. Spray the tub shell with a hot tub cleaner or mild bleach solution. Wipe the surface using cloths or paper towels.
  7. Scrub at any mold spots until they clear. The less mold you leave, the less likely it is to grow back.
  8. Refill your spa, from the filter area if possible. That way, the new water is as purified as possible.

How to prevent mildew in your hot tub coming back

Mildew can cause all sorts of issues—such as that nasty, moldy smell. However, there are ways to prevent it from growing in your hot tub in future.

Here is a list of steps you should take to reduce the chance of mildew:

  • Keep your water balanced: Keep the pH, total alkalinity and calcium hardness balanced. Check these every 1-3 days ideally.
  • Maintain correct sanitizer levels: For most hot tubs, you to shock your water weekly and after every use. If you aren't already, that might be all you need to do to solve your mildew problem.
  • Change your filters regularly: Remove your spa's filter once a month and hose it down. Additionally, try to replace it yearly at a minimum.
  • Remove any built-up gunk: Use a net or spa wand to remove any foam or debris floating in your hot tub. In addition to looking gross, it could breed bacteria if left in the water.

Final thoughts

Your hot tub is a quiet escape where you and your family can unwind and relax. So naturally, you want to keep it as clean and stench-free as possible. Thankfully, you have several options for removing mildew—and preventing it from returning.

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