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How Often Should You Change Hot Tub Water? (And Why)

Hot tubs are not like swimming pools.

You can’t just endlessly throw chemicals into the water and expect to stay sanitary, especially under certain conditions.

This article will cover why the water in your hot tub needs changing, what happens if you don’t do it, and how often this change should take place.

Why Do You Need to Change Hot Tub Water?

It doesn’t matter how clean you think you are, every time you use your hot tub, you introduce new contaminants to the water

This includes substances like sweat, skin cells, hair, body lotions, as well as care products like body lotions, shampoo, and deodorants. And this is multiplied for every person that uses the tub.

While sanitizer like chlorine and bromine does an excellent job at neutralizing these contaminants, they can only do so much.

Over time, once the water becomes too saturated with these dissolved substances (known as total dissolved solids, or TDS), your sanitizer is no longer able to dissolve and work effectively.

What happens if you don’t change the water?

Well, for one, the water will continuously become more cloudy and murky, until you can no longer see the bottom of the pool. You may also see a layer of biofilm on the surface of the water, or worse, the presence of algae.

If that isn’t enough to convince you, the undissolved sanitizer will leave the water feeling gritty, resulting in a sandpaper-like feel to the surfaces of your hot tub.

More importantly, however, the lack of dissolved, active sanitizer in the water will eventually cause it to become unsanitary, smelly, and potentially hazardous to your health.

How Often to Change Your Hot Tub Water

According to most hot tub professionals, a general rule for changing hot tub water is every 3 to 4 months.

If you’re looking for a blanket answer, there you have it. You can close this page, stick to a quarterly-ish schedule, and you’ll probably never have an issue with your hot tub water. Probably.

If you want a more accurate answer, however, taking a closer look at the chemistry can actually reveal the optimal frequency for your hot tub, specifically.

Three things determine the optimal frequency:

  1. How much water your hot tub holds
  2. How many people use your hot tub daily
  3. The method you use to sanitize your hot tub

Let’s break it down.

1. How Much Water Does Your Tub Hold?

This is also referred to as volume.

The more water you have in your tub, the more substances it can hold before it becomes too saturated and needs to be replaced. In other words, a bigger hot tub buys you more time.

If you don’t know how many gallons your hot tub holds, there are a few different methods you can use to calculate this number, including using outside dimensions and timing the fill.

The best way, though, is to check the manual that came with your hot tub or spa, as that will always tell you the exact volume. Alternatively, you can online for the official documentation on the manufacturer’s website.

If none of those options work for whatever reason, you can use the following as a rough guide:

  • 200 gallons for a 2-seater hot tub
  • 250 gallons for a 3-seater hot tub
  • 300 gallons for a 4-seater hot tub
  • 350 gallons for a 5-seater hot tub
  • 400 gallons for a 6-seater hot tub
  • 600 gallons for a 8-seater hot tub

2. How Many People Use Your Tub?

This is also called bather load and it’s important for the same reasons we talked about above.

The more bodies you have in the hot tub on the regular basis, the more contaminants you introduce to the water, and the faster you drive up your total dissolved solids (TDS) level.

For example, assuming it takes a single person 6 months of daily use to reach the TDS threshold, here’s what happens when you factor in more people:

  • 2 people reduces it to 3 months
  • 3 people reduces it to 2 months
  • 4 people reduces it to 1.5 months

*These aren’t hard numbers, we’re just trying to illustrate a point.

So then, how do you work out your bather load?

Since you might not always have the same number of people using the hot tub on a regular basis, this number is best calculated as a daily average.

If most days only 2 people use the tub, then it doesn’t matter if you occasionally have more or fewer people using it, as long it roughly averages out.

If you don’t use your hot tub every day, you can also work out a weekly average and simply divide that number by 7.

3. How Do You Sanitize Your Tub?

We won’t overcomplicate this.

Most people use chlorine or bromine to sanitize their hot tubs, and this comes in various forms such as granules, tablets, and even liquid (often just referred to as bleach).

Chlorine also comes in different types, with the most common type for hot tubs being dichlor granules (a stabilized chlorine).

A sanitation method known as “dichlor then bleach” uses a combination of dichlor granular chlorine and liquid chlorine. It’s a very efficient approach but not a necessary one.

The only thing you need to answer here is: do you use the dichlor/bleach method?

If you answered yes, you’ll have a bit more work to do in the next section, otherwise, you don’t need to worry about it.

Running the Math

Once you’ve drilled down your numbers above, you’ll need to put them into a simple formula. Don’t worry, it’s just a few presses on a calculator. We’ll walk you through it.

This will give you what’s commonly referred to as the Water Replacement Interval, or simply WRI — a number that denotes the optimal frequency of hot tub water changes in days.

The formula is this: Volume / Bather Load / 3 = WRI

Or, put another way: Gallons / People / 3 = Days per water change

For example:

  • Let’s assume your hot tub water volume is 330 gallons and an average of 2 people use the hot tub on a daily basis.
  • The input on your calculator would be: 330, divided by 2, divided by 3, equals 55.
  • In other words, you would need to change your hot tub water every 55 days.

Now then, if you were paying attention, you’ll know we’ve yet to include the third and final factor into this formula: how you sanitize your hot tub.

Fortunately, this part couldn’t be easier. If you use the ‘dichlor then bleach’ method, simply double the number of days from the result above.

When Should You Change the Water Sooner?

The Water Replacement Interval (WRI) is a good guideline for how often to change the water in your tub, but it’s still only that… a guideline.

Sometimes, things happen in hot tubs that require a drain and refill, things that chemistry alone isn’t always able to fix.

This might include:

  • Biofilm. Biofilm is also known as hot tub scum or slime, and it can be caused by a myriad of issues from filtration problems to high copper content or even just a consistent lack of sanitizing agent.
  • A little accident. You know what type of accident I’m talking about. And not just the pee kind, either. If anything like that happens, don’t even think about not changing the water.
  • Spillages. Eating or drinking in a hot tub comes with risk, and while it’s often worth the gamble, you should be prepared to change your water in the event of serious spillages.

Note: These issues don’t always require a change of water. Depending on the severity of the issue, it might be worth trying to fix the issue with proper testing and the right chemicals before committing to a drain, as this will be cheaper and faster.

It’s Worth the Effort!

Changing your hot tub water is like changing your bed sheets; it’s not the most exciting job in the world, but it’s always worth the payoff when you finally slip into bed.

Whether you need to go through this process every month or every six months, it’s important maintain the health of your tub to keep things working efficiently, and for the benefit of everyone who uses it.

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