Can You Fill a Hot Tub With Hard Water?

If you live in a hard-water area, you’ll be all too familiar with scaling. The damaging effects of hard water probably make you wonder whether it’s safe for use in your expensive hot tub.

You should not fill a hot tub with hard water because it contains excess levels of calcium. High calcium levels are damaging to spas, resulting in cloudy water and scaling that can cause components to fail. Excess calcium also prevents sanitizers from working properly, exposing you to life-threatening bacteria.

If you live in a hard-water area, you might not have any choice but to use hard water. Fortunately, there are ways to treat high calcium levels, which make it safe for both you and your hot tub.

What should the water hardness be in a hot tub?

Calcium hardness is a measure of how much calcium (lime) is dissolved in water. Hard water contains high levels of calcium, as well as other minerals such as magnesium.

When you total up the combined level of calcium and magnesium dissolved in water, you get a reading of water’s total hardness.

When balancing your water chemistry, only the calcium component is important. Because the magnesium levels aren’t important, it’s perfectly fine to follow guidelines for both calcium hardness or total hardness.

Calcium hardness and total hardness is measure in parts per million (ppm). The total hardness of hot tub water should be between 175 and 250 ppm.

Although it’s safe to allow the total hardness to reach 400 ppm, it’s better to err on the side of caution, not allowing it to get so high. That way, you have some breathing room should the levels rise without you realizing it.

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

When calcium levels get too high, it causes all sorts of problems that affect both you and your hot tub.

If you live in a hard-water area, you’ll be used to scaling. You’ll have noticed it on sinks, faucets, showerheads, and inside kettles.

Kettles are particularly bad because hot water exacerbates scaling. As the temperature increases, the calcium is no longer able to stay dissolved, resulting in unsightly, white flakes.

Because the water inside your hot tub is so warm, it has the same effect as inside your kettle.

Left untreated for long enough, the scale builds up to the point where the pump and heater can no longer operate properly, causing them to fail prematurely.

But the problems don’t just stop there. Here are some other issues caused by high levels of calcium:

  • Sanitizing issues: Chemicals aren’t able to work properly when the water has too much calcium, exposing you to dangerous bacteria.
  • Scale: Scaling builds up in the plumbing, reducing circulation and increasing pressure. The scale clogs filters and deposits along the water line and around less frequently used jets.
  • Higher running costs: Scale on the pipes and coils acts as an insulator, which slows heat transfer and makes it more expensive to heat the water.
  • Cloudy water: The build up leaves residue in your hot tub, causing the water to become cloudy.
  • Skin irritation: Excess levels of calcium can clog your skin’s pores, which leads to dry, flaky, and itchy skin. The excess calcium can also worsen pre-existing skin conditions such as rosacea and dermatitis.
  • Eye irritation: The pH can become unbalanced when calcium levels are too high, causing irritation to the eyes.
  • Discomfort: High calcium hardness can snag swimwear and cause abrasions for bathers. The scale deposits feel like sand paper on your skin.

The most alarming issue on the list is that high calcium hardness prevents sanitizing chemicals from working effectively because they’re not able to dissolve and disperse properly in the water.

If the chemicals aren’t able to sanitize your spa, it increases your chances of being exposed to nasty bacteria that cause Legionnaires’ disease, E. coli, and hot tub folliculitis.

If your hot tub is fitted with a saltwater system, then maintaining the correct calcium hardness is even more important.

The reason for this being that it stabilizes pH levels and helps your cell or cartridge last for longer. For salt systems, the calcium hardness should be between 25 to 75 ppm.

Unfortunately, lowering the calcium hardness in a hot tub is a lot more work than raising it.

But one thing you can do to immediately lower the calcium levels is to buy this pre-filterOpens in a new tab. on Amazon that attaches directly to your hose.

Pre-filters are a good idea for most homeowners, but particularly those like me that live in a hard-water area. They work by filtering out calcium and other damaging minerals from the water.

Pre-filters are inexpensive and last about a year (or 3 full fills). The filtration protects my spa’s expensive components and even saves me money on chemicals by needing to use less.

How do you treat hard 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.

For those living in areas with particularly hard water, it’s better to test using a calcium liquid drop-count kitOpens in a new tab. as they provide a more precise reading of the calcium content.

Test stripsOpens in a new tab. are fine for people whose water is neither extremely hard nor soft, but they lack the precision necessary to make small adjustments.

The colored blocks on test strips indicate total hardness readings of 0, 100, 250, and 1000 ppm. Drop test kits are accurate to within 10 ppm, enabling you to make precise adjustments to the calcium hardness to ensure the water is within the safe range.

For example, if you were faced with having to decrease the calcium hardness from 280 ppm to 200 ppm, a test strip would be useless. A drop test is the only way to decrease the calcium hardness precisely by 80 ppm.

How to lower water hardness in your hot tub

There are a few different methods you can use to reduce the calcium hardness in your hot tub’s water.

1. Drain and refill

Oftentimes, the easiest way to deal with high water hardness is to perform either a partial or full water drain. Given the amount of effort needed, it’s usually easier to perform a full drain.

A full drain is also preferred as you can clean out the hot tub shell and filters, which have been affected by the excess calcium levels.

How to drain your hot tub:

  1. Turn your spa’s power off and unplug from the electrical source.
  2. Drain the water into the sewer system using a garden hose or submersible pumpOpens in a new tab.. Never drain into a storm drain.
  3. Once fully drained, wash down the hot tub with a soft cloth (to avoid scratches) and rinse thoroughly. Drain the rinse water and remove any remaining standing water by hand or using a wet/dry vacOpens in a new tab..
  4. Remove the filters from their housing and clean with this special filter cleaning solutionOpens in a new tab..
  5. Refill the hot tub with fresh, cold water using a garden hose that has a pre-filter attached.
  6. Test and balance the water with the drop test kit.

Once you have the water hardness at the correct levels, it’s fine to perform weekly tests using test strips. But make sure to use the water drop-count test kit once a month to get an accurate reading that enables you to make precise adjustments.

If the water in your home is softened by a filtration system, it might be a good idea to partially fill the hot tub with the softened water after performing a full drain.

Begin by filling your spa using hard water from the outside faucet, and top up with soft water. Never fill your entire hot tub with soft water as the low calcium hardness levels will cause a whole lot of damage.

An extra bonus for those with filtered water is that the water should be soft enough for you to be able to perform a partial drain of your spa when the water hardness becomes too high.

2. Use a flocculant or water clarifier

Performing a full drain takes quite a bit of effort and time. Another way to deal with the issue of excess calcium is to use something known as a flocculant or floc.

Flocculants bind the calcium and other debris together into large clumps, which then sink to the bottom of the tub, enabling you to remove them with a cordless spa vacuumOpens in a new tab..

An alternative to flocculants is this water clarifierOpens in a new tab., which works by coagulating the calcium. The liquid formula attracts calcium and other contaminants that are too small for the filter’s fibers. By clumping the small particles together, they become large enough for the filters to trap and remove.

Flocculant method:

  1. Turn off the power to your hot tub and unplug.
  2. Balance the pH in your spa. It needs to be between 7.4 and 7.6.
  3. Read the instructions and pour the proper dosage of flocculant into the hot tub.
  4. Turn the hot tub power back on and run for one hour to let the flocculant circulate.
  5. Remove the clumps that have sunk to the bottom. Be careful not to whip the clumps up as you’ll have to wait for them to settle again.
  6. Open up the filter housing to check for any smaller clumps that have accumulated in the filter fibers.
  7. Rinse the filters clean using your garden hose.
  8. Retest and balance the water.

Clarifier method:

  1. Turn off the power to your hot tub and unplug.
  2. Balance the pH in your spa. It needs to be between 7.4 and 7.6.
  3. Read the instructions and pour the proper dosage of clarifer into the hot tub.
  4. Turn the hot tub power back on and run for 24 hours to allow the the clarifier circulate.
  5. Open up the filter housing to remove the clumps that have accumulated in the filter fibers.
  6. Rinse the filters clean using your garden hose.
  7. Retest and balance the water.

Spa water clarifier is effective and requires a lot less effort than flocculant, but it works more slowly. It can be used anytime, which is great for keeping your water sparkling if it’s become a little cloudy.

The water clarifier I linked to also has the added benefit of not affecting your hot tub’s pH level, making it easier for you to keep your water balanced.

Flocculant works a lot faster but requires more work. The large clumps that sink to the bottom of the spa have to be removed manually with a spa vacuum.

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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