water treatment for cold plunge

[fortunate]

I'm in the design phase of a cold plunge. It'll be < 100 gallons. It will have an insulated cover and the chiller+pump will run all the time, keeping the water at ~40F. The chiller loop has a filter, but nothing else (no ozone). It's indoors, though not an air conditioned space and the climate is tropical/coastal/humid. It gets used about once/day.

Given that, what is the best way to treat the water? It seems a lot easier than a pool: cold may retard growth, no rain, no UV, small amount of water.

One option is a spoon of chlorine after every plunge (monitoring chlorine levels of course), then change the water every 2-3 months. However, I'm curious if there are other systems that don't use chlorine?

I came across Clear Comfort, discussed here before. Everyone is skeptical and probably rightfully so, but that discussion was in the context of a pool. Would it work better for my tiny cold plunge?

I've also come across silver/copper ionization. It seems even less likely to be effective, but I don't really know. Plus, again, few discussions are considering it for a cold plunge.

Are there other solutions that could work for a cold plunge?

 

[ajw22]

Treat it like a spa. You still have a large bather load of one person in a small amount of water.

You want around 20ppm of CYA to buffer the chlorine. Otherwise even a few ppm of chlorine can be harsh.

Check the pH and keep it in the 7s.

What is the pH and TA of the fill water?

Chlorine is the only practical sanitizer.

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[fortunate]

I suppose that makes sense, thanks!

I'm not sure on the fill water (house is being built). It'll be through a carbon filter and softener though.

 

[fortunate]

Here's a thought: if I use a regular bath tub for the cold plunge, the chlorine might damage the tub surface.

I'm having trouble finding a cold plunge tub. There are a few units with a built-in or attached compressor, but I need to place the compressor on the other side of the wall, outside but covered. I don't have the space for the compressor, but also the indoor space is not air conditioned. The compressor would be inefficient in such a warm space and would work better outside.

A built-in compressor is a no-go. I may be able to extend pipes on some units, but it's hard to tell.

I looked at steel and copper tubs, but they have special needs for sanitation. Using a regular tub would be ideal, as it's comfortable and easy to clean (unlike a standing plunge), but there may be interactions with chlorine. In particular I have been looking at Aquatica which uses some sort of bauxite / acrylic resin combination. I'll email them about chlorine. I'm open to other tub brands.

 

[ajw22]

Here's a thought: if I use a regular bath tub for the cold plunge, the chlorine might damage the tub surface.

Not with 20ppm of CYA in addition to the chlorine. With 20ppm of CYA you can have up to 8ppm of chlorine to bath in with no problems.

I don't have the space for the compressor, but also the indoor space is not air conditioned.

I think you will have problems with evaporation, humidity, and mold if the indoor space does not have some climate controls.

 

[fortunate]

Interesting! How confident are you that the tub surface would be safe? For example, would you buy a $5k tub and put chlorinated water in it?

The cold plunge tub will have an insulated cover, so I hope it is a closed system when not in use. At 40F it's like a bowl of water in the refrigerator.

If I go with a regular bath tub I'll have to figure out how to make my own cover, but it can't be that hard. I'm considering tubs that are rectangular to make that easier.

The indoor space does have some ventilation: there is an exhaust fan that runs briefly twice a day. It will likely get warm in there from the sun.

 

[ajw22]

Interesting! How confident are you that the tub surface would be safe? For example, would you buy a $5k tub and put chlorinated water in it?

What is the tub surface? Porcelain? Acrylic?

Any typical bathroom fixture is chlorine safe, especially with CYA.

 

[fortunate]

It's "AquateX", marketing junk for some sort of bauxite / acrylic resin combo. I've emailed asking if it can withstand chlorine pool water in the tub 24/7.

They say:

AquateX™ is an inert, hypoallergenic, non-toxic, 100% recyclable and fire-resistant stone composite material that combines two opposites - a warm, silky soft exterior and a tough, durable interior. It has a strong resistance to UV radiation, is dense/non-porous, hygienic, durable and is pleasant to touch due to its velvety texture.

 

[ajw22]

What color are you getting?

The only side effect of chlorine would be bleaching out of color if they did not use a colorfast pigment.

 

[fortunate]

I was thinking white. They also have a black material, but it might show bleaching, as you mentioned.

 

[ajw22]

White and chlorine should be very compatible.

 

[fortunate]

OK, cool! Thanks for your help!

 

[fortunate]

I had a phone call with the bath tub people. They say chlorine is "too strong" and suggest bromine instead. I guess I need to read up on bromine.

My only experience with bromine was a hotel pool that had weird brown foam around the edges of their bromine pool. It was gross, but I'm guessing they were doing something wrong.

 

[ajw22]

The bath tub people don’t understand about chlorine with CYA.

You can see what works for you.

 

[Bperry]

I had a phone call with the bath tub people. They say chlorine is "too strong" and suggest bromine instead. I guess I need to read up on bromine.

My only experience with bromine was a hotel pool that had weird brown foam around the edges of their bromine pool. It was gross, but I'm guessing they were doing something wrong.

The tub will probably be fine if it can handle tap water with normal amounts of chlorine in it. Unless PR has abnormal amounts of chlorine.

 

[fortunate]

PR water is normal, safe to drink.

I'm sure I'm overworrying, as I do, but these things are expensive and I'm trying to do it well.

It appears that enamel is very resistant to chemicals. That's probably my safest bet out of any tub material. Cast iron seems to be nicer than steel, so I'm off to find a cast iron enamel built-in tub with a shape that I like! This one seems pretty good:

Kaldera 60" X 32" Undermount Or Drop-In Bathtub with Internal Overflow Hole - Standard White | Model Number: KAL2-N-SW-IO

KAL2-N-SW-IO Gloss White Modern Kaldera 60 Inch X 32 Inch Undermount Or Drop-In Bathtub in Volcanic Limestone with Internal Overflow

houseofrohl.com

60" long and can be 30", 32", or 36".

I'll have to cut the holes for an outlet and inlet, but that seems doable. Even if the edges of the holes aren't great (eg the enamel chips), I'll find some sort of jet cover to seal over it.

 

[Mdragger88]

How do I use Chlorine in my Spa (or pool)?

If you have a question about any information within this Sticky, please start a new thread for yourself where you will receive very quick responses to your specific questions. Thank you! Introduction There are two main things to consider when maintaining your hot tub water, Water Balance and...

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