Jump to content

Cedar Hot Tub is 2 months old - Tannins, Chlorine Demand, and Ionizer


Recommended Posts

Greetings everyone,


I've been researching on how to best take care of my new 400 gallon Cedar Hot Tub. It's just over two months old. I've been using chlorine for sanitization, trying to keep it within the recommended 1-2 ppm, and never greater than 3 ppm.  Too low I have a sanitation problem - Too high I risk wood damage.  It has an ionizer installed which is not being used yet until the tannins are no longer leaching into the water.**1

The tannins are now vastly reduced.  The water looks clear to me now.  Even the filter canister water looks clear.  It used to be a very brown tea.  I've done 8 drains and this is my 9th fill.  We've been very cautious with the amount of chlorine. Perhaps a little too cautious?  I'm using 6% bleach (though I had been using dichlor before).  I got a new filter and start fresh - other than cleaning out my pipes.


So here's my question: When do I know I can switch from chlorine as my main sanitizer to using the ionizer (plus MPS)? When my chlorine demand is low? 

Well, I figured after the tannins were no longer visible that I'd have a lower chlorine demand.  But it's still at 100% over a day's time.  I'm currently doing testing every half hour, and I'm getting a chlorine demand of around 0.5-0.6 ppm of free chlorine per hour (using the Taylor K-2006). 

I had an idea to segregate some of the hot-tub water from the wood by placing it in a clean plastic 5 gallon bucket.  I'm comparing thdse two results, and I get nearly the same chlorine demand. That's whether I have a fresh fill and new filter, or now after four days and a bather or two.

So, my conclusion is there's something in the water itself that the chlorine is eating up. It's not the wood itself.

So are those still just the tannins, even though I really can't see them? Except the scum ball is slightly stained.
Or could I have something growing in there, and do I need to worry about how to resolve that?


Second question: With a 450 gallon wooden tub, I am unable to both honor the 5 fluid ounces of 6% bleach per person-hour and keep the free-chlorine level under 3 ppm for that volume.  Half that, just 2.5 fl oz, gives me 3 ppm.   5 fl oz would bring me to 6 ppm from zero.

Am I be risking wood damage by the temporary spike in free chlorine well above the recommended 2 ppm before (or after?) soaking?

In no way am I currently able to retain a free chlorine level above 1 ppm and below 2 ppm unless I live the rest of my hours by the tub.

(I asked my wife but she thinks I should keep my day job ;))


Final question: How long can I let the tub go to zero free chlorine, and not worry?

I test and dose it up as often as I can, but if I leave it with 3 ppm at the start of the work day, it goes to zero before I can get home.


I am looking forward to getting the ionizer started up.  I don't mind using chlorine, but at least that system works automatically.


**1 "For the first few months of the life of a tub it will leach tannins into the water, and these wreak havoc with your water chemistry.  They will essentially absorb any type of sanitizer that you put in the water, whether it's chlorine ions or metal ions, and early in the life of the tub the tannins can literally overwhelm the latter."

Link to comment
Share on other sites

  • 3 years later...

Thanks for this post! It’s well thought out and it’s exactly what I’m going through now with my new cedar hot tub that’s a month old. I would love to hear what you figured out and how you resolved the chlorine demand issues you were having. I’m also going to the dichlor/bleach regimen but seeing chlorine disappear entirely each day and concerned about it attacking and damaging the wood. I would hate to bleach out the inside of the tub by adding too much.

Link to comment
Share on other sites

  • 1 month later...

I’m thinking the leeching of tannins is what’s causing our chlorine demand. Once the tannins slow down and the water stays clear, I’m expecting standard CD going forward - I guess time will tell but I’ve been adding 4oz or so each morning bringing up my FC to roughly 2-3ppm and I add the same amount after each use.

Because bleach has been hard to find recently, I’ve switched to Clorox bleach with cloromax - regular unscented 6%. Are there any concerns with bleach with cloromax or should I try and find Clorox bleach regular (it seems like everything has cloromax in it now)? The other bleach is Clorox germicidal bleach unscented 6%. Is there a difference?

Link to comment
Share on other sites

  • 1 month later...

Update: After doing a fair bit of research on bleach composition, I found and switched to Chlorox Germicidal Bleach 30966, as it contains no poly’s and has 8.25% concentration. Seems to be the best on the market for what we’re trying to accomplish but limited to where you can purchase it. Couldn’t find it at a standard hardware store. I’m adding just under 4oz daily to bring my chlorine levels to 3% after adding dichlor for a week and getting my CYA to just over 30ppm.

My CD is still high (nearly zero 24 hours later) and unsure as to why but assume it’s because of the cedar. Would love to hear why if anyone knows for sure as I would love to maintain a chlorine level daily as to add less bleach.


I am also still using the tub about 3x per week and notice the tannin leaching but it does appear to be tapering off. 2 weeks after the recent water change, I can still see the bottom of the tub but have noticed it to be more cloudy. I have replaced water on average every month but was close to 2 months on prior fill. 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Unfortunately, your content contains terms that we do not allow. Please edit your content to remove the highlighted words below.
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...