You just got your first hot tub. Congrats! It’s a very exciting time, and I know you’re ready to jump in and enjoy it. I wanted to write up my experience filling a hot tub to hopefully save you some time and frustration.
This guide assumes you’re planning to run a hot tub with chlorine.
What do you need for success?
Tools:
- Reliable test kit, such as a drop test kit from Taylor.
- Five gallon bucket (optional).
- Pre-filter for your garden hose (optional).
- Vanishing Act Pillow (optional).
- App or calculator to determine correct dose of chemicals.
- I really like the PoolMath app, but you can also use the free web version.
Chemicals:
- Granular chlorine (Dichlor)
- pH down (Dry Acid) OR Muriatic Acid
- Alkalinity Increaser (Baking Soda)
- Ahhsome or some other purge product (optional; highly recommended).
Let’s make a plan!
You’ll want to start out by determining the capacity of your hot tub. Usually this is listed on the manufacturers website. We’ll assume that we’re working with a hot tub that is 400 gallons in this post.
Let’s do some upfront work to make our life easier later. If you bought a pre filter, go ahead and attach it to your garden hose. Next, take the five gallon bucket and time how long it takes to fill up the bucket.
Let’s say it took 2 minutes and 15 seconds to fill the bucket. We know that it takes 2 minutes and 15 seconds to get 5 gallons of water. Let’s make the math a bit easier to work with by dividing 2 minutes and 15 seconds by 5 to get 27 seconds. It takes 27 seconds to produce a gallon of water. Let’s multiply that by 400 and convert those seconds to minutes and learn that it will take 180 minutes to fill the hot tub.
Next up, let’s test our fill water. Take a sample from the 5 gallon bucket to your Taylor test kit and record the following parameters:
Let’s say that our results were the following:
- Total Alkalinity: 220
- pH: 7.5
- Calcium Hardness: 250
If you suspect you might have metal in your water, take your sample to a pool store and have it tested for iron and copper. If there’s detectable levels, purchase a product called Metal Gon (or similar) to sequester the metal and prevent it from causing problems in your hot tub.
In our case, we need to make a plan to deal with the Total Alkalinity. It’s way too high and will cause a lot of issues keeping pH in check. Your first big decision is if you’ll use Dry Acid or Muriatic Acid to lower the Total Alkalinity.
Dry Acid (pH down) is easy to handle but causes sulfates to build up in the water. If you’re only trying to move Total Alkalinity a little bit, Dry Acid might be an okay choice. In our case, we’d need over a pound of Dry Acid (added in rounds not all at once) to lower it into range. I would be concerned at that point about the sulfates and the damage it might have on a heater.
Muriatic Acid is harder to handle, smells terrible (avoid the fumes, they are not good for your health) and needs to be added carefully to avoid damage to the hot tub surface. If you choose this route (which I would in this example), always pour the Muriatic Acid away from surfaces near a running jet to ensure it mixes and doesn’t touch the surface of the hot tub.
PoolMath, or some other calculator, can give specifics about how much of a specific chemical you’ll need given your starting parameters.
If your calcium hardness is above 150, you might choose to lower it. You don’t have to, but it can be helpful in maintaining balanced water. Balanced water is affected by a combination of Total Alkalinity, pH and Calcium Hardness. If your Calcium Hardness is lower, your TA and pH will need to be higher to have balance. If your Calcium Hardness is higher, your TA and pH will need to be lower to have balance.
To Purge Or Not To Purge
You might be thinking, “this is a brand new hot tub, why would I consider purging it?” All hot tubs are wet tested before they are delivered. Unless the manufacturer completely drains the hot tub and sucks every bit of water from the lines, biofilm and other nasties begin to grow in the plumbing during transit.
If you decide to purge, and I think you should, go ahead and fill the hot tub (in our example, we’d fill it for 180 minutes). Afterwards, heat the water to about 80 degrees and then add the Ahhsome product per instructions.
Some tips:
- Fill through the filter compartment (or as recommended per your manual) to avoid airlocks.
- Wipe down the gunk from the waterline as soon as the purge is complete. Do not let it dry.
- Drain the hot tub through the drain valve in addition to any sump pump you might be running. Draining through the drain valve helps flush nasties from that part of the system.
- Consider a quick wipe down of the interior after draining.
Fill, for real.
Your hot tub is free of biofilm and you’ve got your initial water parameters — let’s fill the tub for real this time. Let the tub fill for 180 minutes (checking periodically to make sure it’s progressing as expected). After filling it up, begin heating the water. Wait until it’s 80 degrees before adding any chemicals.
Total Alkalinity
Let’s start by adjusting TA. If you don’t start here, you’ll be chasing pH for a while. Recommendations for TA range from 80 to 150, but the folks at Trouble Free Pool recommend going even a little bit lower, down to 50 if needed. If I had a TA of 220, I’d start by adjusting it down to around 80. I’d fine tune from there over the next few days or weeks.
We know that our fill TA is 220. Our initial goal is 80. We need to lower Total Alkalinity by a whopping 140ppm. If we’re using Dry Acid, we’ll probably end up needing about 19.2oz. If we’re using Muriatic Acid (31.45%), we’ll probably end up needing about 14.3oz. I used the PoolMath app to calculate how much of these two products it would take to lower TA by 140ppm.
Importantly, the PoolMath app also says that adding this much Dry Acid or Muriatic Acid would lower our pH by 8 points so we can’t add it all at once if we care about our hot tub at all. We need to add it in rounds, where we are careful to not drop our pH below 7.2.
Using PoolMath or some other app, calculate how much acid it will take to lower pH to 7.2. In our case, we’re currently at 7.5 and need to drop to 7.2. It would take 1.3oz of Muriatic Acid (31.45%) to do it. Run the jets with the air valves open, slowly pour 1.3oz into the hot tub away from shallow parts where it could land on surfaces.
Why with the air valves open? We’re aerating the water which will raise the pH without raising the TA. If you were to lower the pH with acid and then use a pH up product to raise it, you’d be increase TA again. We need a method to raise pH that has no impact on TA, and that’s just what running the jets with air valves open does for us. It won’t take very long in a hot tub, especially at a high TA.
Check the pH after running the jets for 15 minutes. If the pH is 7.8 or higher, repeat the process by using PoolMath to calculate the required acid needed to lower back to 7.2. Run the jets again and raise to 7.8. Rinse and repeat until you’ve used the 14.3oz calculated to lower TA by 140ppm. As you get closer to the last round, feel free to check TA and see if you’re close. Calculations are not an exact science, so err on the side of testing more often than not.
pH
When TA is at 80, make one final adjustment to pH to land it somewhere between 7.4 and 7.6. You’ll find as you use the hot tub, running the jets will cause the pH to rise. Simply lower it back in range when it exceeds 7.8. You’ll eventually fine tune the TA by doing this to where it doesn’t rise much beyond 7.8.
Calcium Hardness
At a level of 250, this step is optional. I’d personally go ahead and lower CH to 150 using the Vanishing Act Pillow. You can place the pillow in the bottom of the hot tub near a return or, if you have a waterfall feature, I find it easy to just place the pillow under the water fall to force water through it.
Recheck the CH every few hours if you’re using the waterfall method. I find that it does not take long to lower the hardness that way. I’ve personally never used it as instructed in a return at the bottom so I can’t say how long that would take.
Final steps
Your water is balanced and now all that’s left is to add some chlorine. You have many options for keeping your tub chlorinated moving forward. I would recommend the Dichlor / bleach method, but you need to find what works for you.
Use PoolMath to calculate how much Dichlor (granular chlorine) to add to reach 6ppm of FC. If you’re going with the Dichlor / bleach method, use Dichlor until your CYA levels reach 30 and then switch to liquid chlorine. I’ve found the best success by buying higher concentrated liquid chlorine from pool stores (12.5%).
Once your hot tub is at temperature it’s time to soak!