How Often Should I Drain My Hot Tub?
If you are asking "how often should I drain my hot tub?" then I am very concerned that you are going to find some information online that will not cover what you really need to know. Most often you will find static timelines like every three months, or every six months, and you might also find some hot tub chemical additive manufacturers claiming that you only need to drain and refill your hot tub once per year...assuming that you use their magic elixir that is. As a pool and spa professional it concerns me greatly that hot tub owners might read an article that says to drain and refill your spa once every six months and then simply follow this without question. In the vast majority of cases if you were to follow this advice with your hot tub you would end up with a deplorable soup of chemicals, body oils and biofilm of which you soak in to relax. Sorry Jack, that is just straight up fowl.
If you were to follow a general timeline for how long you can go (should go) between water changes in your hot tub the best number would be around four times per year, or every three months. The problem with advice like this, good as it might be, is this only covers an ideal situation. Another way to look at this might be to ask how often should I drain and fill my hot tub if nobody ever uses it? If your hot tub has lost the novelty and now sits unused then you should drain and refill it every three months. If you actually use your hot tub you might need to drain an refill more often than this. It depends on how you use your tub, how you manage the chemistry, how often you change the filters...there are a lot of ways that you can compromise how long your water will last before it starts to look murky or becomes chemically unmanageable.
Adding scents and oils to your hot tub - If you like to add scents or oils to your hot tub as part of your relaxing soak then you should be aware that doing so will increase the frequency that you need to drain and refill. Both oils and salts will build up in the water over time. This is especially true if you use your hot tub frequently, and choose to add these products to the water often.
Spa bather load - Are you the only person to use your hot tub or do you operate hot tub party central? Four people in a small hot tub is just about on par with gathering up a truck full of stray dogs from your neighborhood and letting them play in your pool, in terms of filtration and effects on water chemistry. Maybe not that bad, but do you enforce a shower first rule with your hot tub? Bather load in a hot tub is one of the main factors that will affect how long you can go between water changes. More bathers means more debris, oils, bacteria, hair and scum. All of this requires more filter changes and more chemical adjustment. If more than two people use your hot tub, and it gets used a few times per week or more, then you can consider this fairly heavy use and bather load in terms of residential hot tubs and three months is probably stretching it.
Changing your hot tub filters - One of the most chronically undervalued hot tub maintenance tasks is to change your filters frequently. If you have a permanent hot tub like a concrete spa then you likely have filtration equipment more like a pool pump and filter, however the rest of the hot tubs out there will all pretty much universally have a paper filter, or series of paper filters for cleaning the water.
Unlike a swimming pool a hot tub is a very small body of water in comparison to the bather load it experiences. Add to this the fact you are sweating the entire time you are in the spa and you can start to appreciate just how much oil can be in the average hot tub. These oils build and produce both scum and foam. If you use your hot tub often then you need to change your filters often. Replacing your filter cartridge weekly with a freshly degreased and cleaned filter is optimal and the commonly referenced 30 day filter change cycle applies to hot tubs that barely get used and are always chemically balanced. If you almost never change your hot tub filter then the idea of going three months between water changes is a borderline biohazard.
Chemical management - Would you estimate that your hot tub water is perfectly clear and perfectly balanced, and it has been this way for the entirety of the three months the water has been in the spa? If so, then three months is definitely possible between water changes. However if you struggle to maintain your water chemistry balance and find that you are constantly fighting with the pH and total alkalinity to be in the right range, or even worse, do you often find your sanitizer (bromine of chlorine) has accidentally dropped to zero? Even worse, have you ever opened the lid to discover that the water in your hot tub has turned murky, brown or even green? If so then you should not getting three months, or longer, with your spa water. Poor water chemistry, over correction of the chemicals and the addition of "things you actually did not need to add" can all contribute to shortening the expected life from the water in your hot tub. Something like green water really should be enough cause to go ahead and begin the draining and refilling process for your spa...but definitely not before you do the MOST important part.
How To Drain A Hot Tub
Before you drain your hot tub it is critically important that you do a pipe flush procedure while the old water is still in the tub. This is easily the number one thing that hot tub owners are not doing that they absolutely, positively should be doing. Probably half or more hot tub owners reading this now are wondering if this applies to their hot tub since they have never heard of pipe flushing before, and have never done this to their spa. Every hot tub and spa of every size, make, model and type needs regular pipe flushing in order to strip biofilm from growing inside the plumbing lines of your spa. The pipe flush product, when added to the old (and still hot) water will work to strip away bio film which looks like a sticky gray sludge.
Biofilm is where almost all of the bacteria in your hot tub is located. Surprisingly the bacteria is not in the water, since you have a sanitizer in your water to take care of that. Biolfim however provides its own layer of mechanical protection such that regular circulation of sanitized water can not fully remove or contact the bacteria that is growing. When you fail to pipe flush a hot tub regularly this bacteria will grow to disturbing levels...and then you turn on your hot tub jets. The force of the water moving through the pipes will work to dislodge some of this persistent bacteria growth and drive it straight into the hot and fully open pores on your back. This is exactly how bacterial infections happen in hot tubs even though the water actually has sanitizer in it.
When it is time to drain your hot tub simply remove your filters, add a pipe flush chemical, and then run all of your jets for a total of about an hour. If you have air line controls on your spa you should close them during the pipe flush to prevent foam from filling up the air lines. Even a regularly flushed spa will develop foam and scum during the pipe flush procedure.
Older spas or ones that have never been pipe flushed can end up with a foot or more of black foam growing off of the water. Once you see this happen you will never forget to pipe flush your spa again, I promise you. You might need to drain, rinse, refill, heat and pipe flush a second time if your spa is particularly bad, or has not been pipe flushed in a very long time (or ever).
Hot Tub Pipe Flush
Directions: Remove filter and add to old spa water. Circulate jets for one hour. Drain.
By removing the biofilm you will make your water cleaner, safer, and you will notice that your chemical maintenance also becomes easier since you do not have bacteria living in the pipes that is constantly eroding your sanitizer residual in the water. If you regularly pipe flush your spa, change your filters every week, always manage your chemicals perfectly and never let the water in your spa turn green, then you can start talking about going 90 days or more between water changes. If you are still learning how to manage your spa, use it frequently (more than a few times per week), rarely pipe flush or often forget to change your filter for a month or more, then you should definitely change your water more often than this.
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