Can You Fill a Hot Tub With Soft Water?

Soft water is great for bathing in and makes excellent drinking water, but is it ok to use in a hot tub?

You should not fill a hot tub with soft water because the calcium hardness level is too low. Low calcium in a hot tub causes foaming and damage to the spa shell, as well as pitting in surrounding surfaces. Low calcium can cause the pH to become unbalanced, resulting in corrosion to the jets, plumbing, and heating.

If you live in a soft-water area, you might not have any choice but to fill with soft water. Fortunately, there are ways to treat low calcium hardness levels, which make it safe for your hot tub.

What should the water hardness be in a hot tub?

Calcium hardness is the measure of how much calcium (lime) is dissolved in water. Soft water has low levels of calcium and other minerals such as magnesium.

While the combined level of calcium and magnesium give us a reading of water’s total hardness, only the calcium component is relevant in the water balance calculation for hot tubs.

You’ll see resources online providing guidelines for both calcium hardness and total hardness. Because the magnesium level isn’t important, it’s fine to follow guidelines for calcium hardness or total hardness.

Water’s total hardness is measured in parts per million (ppm). The total hardness of your hot tub water should be between 175 and 250 ppm.

While a hot tub can tolerate a total water hardness up to 400 ppm, do your best to keep it within the optimum range.

What happens if water is too soft in a hot tub?

While soft water might feel great on your skin, it will hurt your hot tub. When the calcium levels fall below 175 ppm, the water becomes aggressive in its attempt to obtain the calcium it lacks.

It starts to dissolve calcium from any surface that it comes into contact with, resulting in the following problems:

  • Pitting of hot tub flooring and walls
  • Delaminating of plaster surfaces
  • Etching or pitting of hot tub stairs and decks
  • Etching or pitting of stone and concrete surfaces surrounding your hot tub
  • Dissolving tile grouting
  • Foaming

While foaming can be treated and got rid of for up to 24 hours using chemicals, there’s no way to completely undo the damage caused by soft water.

The problems are even worse if the pH levels stay low for an extended period of time, causing corrosion to the following components inside your spa:

  • Jets
  • Pipes
  • Pump seals
  • Heating elements

Left for too long, it can get to the point where repair is no longer possible, with the only option being an expensive replacement.

Avoid filling your hot tub from your home’s faucet (tap) if the water has been softened. It’s better to use a garden hose fitted to an outside faucet as the water probably hasn’t been treated.

If you live in an area with naturally soft water or your home’s entire water system is filtered, you’ll need to add calcium hardness increaserOpens in a new tab. when the hardness level falls below 175 ppm.

If your hot tub is fitted with a saltwater system, then maintaining the correct calcium hardness is even more important. For salt systems, the calcium hardness should be between 25 to 75 ppm. The reason for this being that it stabilizes pH levels and helps your cell or cartridge last for longer.

How do you treat soft water in a hot tub?

To avoid damage to your spa and the surrounding areas, a calcium hardness test should be performed at least once per month.

If you live in an area with very soft water, it’s best to test using a calcium liquid drop-count kitOpens in a new tab. to obtain the most accurate readings so that you can make the needed adjustments.

Unfortunately, test stripsOpens in a new tab. are only able to test for total hardness and lack the precision of liquid drop tests. Strips are fine for people whose water is neither extremely soft nor hard.

The color blocks on test strips indicate total water hardness readings of 0, 100, 250, 500, and 1000 ppm. This is problematic when you need to make small adjustments.

Drop test kits allow you how to harden soft water in your spa by adjusting the true concentration of calcium hardness in increments of 10 ppm.

For example, if you were faced with having to increase the calcium hardness from 130 ppm to 200 ppm, a test strip would be useless. A drop test is the only way to increase the calcium hardness precisely by 70 ppm.

How to increase water hardness in hot tub

Increasing the calcium levels in your hot tub water is very simple to do using the chemical calcium chloride, otherwise known as calcium hardness increaser.

Calcium hardness levels can wreak havoc with the alkalinity and pH levels, throwing the entire hot tub chemistry out of whack.

So before you go testing for the amount of increaser needed, you first need to ensure that the alkalinity and pH are balanced.

By the way, alkalinity and pH are related but not the same, so take the time to understand the difference to avoid mixing them up.

1. Test and balance the alkalinity

The first thing to balance is the total alkalinity levels, which is a measure of all carbonates, including calcium, in the water. Alkalinity is the capacity of water to resist acidification. Let me explain in plain English.

Every little thing that comes into contact with your hot tub water can have huge effects on the pH. Alkalinity works to protect the pH level.

When properly balanced, alkalinity takes the hit before affecting the pH. This stops the pH in your water from flying around all over the place.

Adjust the total alkalinity levels to between 80 to 120 ppm before moving onto the pH. Depending on the reading, you’ll need to either use an alkalinity increaser or alkalinity decreaser. This bundle has all the chemicalsOpens in a new tab. you’ll need for the alkalinity and pH.

Add whichever of the two chemicals you need and leave the cover off. Allow to circulate for 15 minutes before testing again.

2. Test and balance the pH

The final step before tackling the calcium levels is to make sure that the pH is within the correct range. The pH is a scale of how acidic or alkaline (basic) your water is.

When spa water is overly acidic or alkaline, it makes it more difficult for the sanitizing chemicals to work properly.

This increases your chances of being exposed to nasty bacteria which cause hot tub folliculitis, Legionnaires’ disease, and E. coli.

An acceptable pH level for hot tub water is between 7.2 and 7.8. However, do your very best to get the water into the optimum pH range of 7.4 to 7.6.

Not only does this allow the sanitizers to work at their best, but it also gives you some breathing room should the pH change without you realizing.

Take a pH reading using either a test kit or test strips and adjust using the pH increaser or pH decreaser from the bundle of chemicals in the previous link.

Again, add whichever chemical is needed and make sure to leave the cover off while it circulates. Test again after 15 minutes.

3. Test and balance the calcium hardness

Phew! With those two perfectly balanced, you now have the perfect environment for correctly adjusting the calcium hardness levels.

Hopefully, you’ll have got yourself the liquid drop test kit I mentioned earlier.

Using a clean cup, collect a sample about elbow-deep beneath the surface of the water from the middle of your hot tub. Never take water samples from near the jets.

For the most precise reading, allow the water to cool to room temperature. Seeing as your hot tub is probably outside, it won’t take long to cool down.

Now add the appropriate amount of calcium hardness increaser and (you guessed it!) allow the water to circulate with the cover off for at least 15 minutes before retesting.

Now that you’ve got the calcium hardness where it needs to be, you can use strips for quick and easy, weekly tests to ensure the color block indicates a hardness of 250 ppm.

Anything other than a reading of 250 ppm using a test strip, and you need to perform a drop test to know exactly how much calcium is in your water. Always perform a liquid drop test once per month as a precaution.

Joshua Milton

Joshua Milton is a seasoned hot tub enthusiast. With many years of experience in the industry, he offers valuable insights on hot tub maintenance, health benefits, and relaxation techniques.

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