Help balancing a holiday let hot tub (new water every 3-7 days!)

benseb

Member
Mar 23, 2021
21
UK
Hi there

We have a fibreglass lined wood-burning hot tub (Approx 1000L), and it will be used in a holiday rental. As such, our laws (UK) say it must be emptied and refilled after each rental, which is usually 3-7 days. We opted for a wood-burner as they heat up in 2 hours, and we can leave the guests in control of when they want it warmed up.

So my challenge is getting the water chemistry right. Traditionally with wood-burning tubs, you don't need chemicals as the water only stays in 3 days. However, by law we must add Chlorine due to it being used by other guests.

I've been reading the guides and advice, but my problem is a lot of them require you to do things quite slowly. We will be filling up the tub in the morning, and have until mid-afternoon when guests arrive, so not a huge amount of time to get things right. I will then be checking the water levels each morning and adjusting where possible. I'll be using Chlorine granules, but was advised a floating tablet might be good to leave in overnight after it's been used to keep the levels a bit more consistent, as we can't ask the guests to add Chlorine when they get out and want to avoid it being zero for an extended period.

Now my thoughts were that the chemicals needed should be fairly consistent each time we fill it up from empty, so after a while we'll know what to dose up, then test and fine-tune if needs be... However I'm not quite at that point yet!

So here's my first attempt, would be amazing for some feedback (I'm using AquaChek digital meter which reads their test strips):




22/3 9am:

2.0 Cl, 6.9 pH, 040 TA

Added 50g Total Alkalinity+. Added 2x20g tablets in a floating dispenser, open partially.

18:00 :

4.2 Cl, 6.7 pH, 090 TA

Happy with the TA, so added 5g of pH+. Chlorine a bit higher than expected, so removed floating dispenser

Hot tub was used by 1 person that evening. Floating dispenser returned to water overnight

23/3 9:20am:

6.0 Cl, 7.4 pH, 000 TA

Alkalinity has dropped right down. Chlorine too high. Removed Cl dispenser completely from water. Added 50g TA+

23/3 12:00: 4.3 Cl, 7.7 pH, 040 TA

TA still low, so added 25g TA+. Dispenser still removed from water.

23/3 19:05 7.7 Cl, 7.4 pH 040 TA.

Planning to add more TA+, then presumably will need to pH- a little bit once TA is correct. Why is Chlorine so high? Why did TA not rise?


Things were following my plan yesterday roughly, apart from the Chlorine dispensing a bit too quickly. I was surprised the TA dropped right down this morning, then didn't rise after adding 25g TA+ but I'm thinking that it's been buffering the pH and so more is needed? Also confused why the Chlorine jumped from 4.3 to 7.7 even when there had been no tablet in since 9am - what could cause this? The hot tub has bubbles (turned off) but no filter, the water gets circulated by the wood burning stove, but we've not heated it since last night (Temp gone from approx 40c to 27c) during the day.



Any tips or advice would be very welcome!
 
Last edited:

benseb

Member
Mar 23, 2021
21
UK
I just retested as Mrs Seb wanted to use the hot tub,

This was about 2 hours after the last reading above (which was 7.7 Cl, 7.4pH, 40 TA).

I've not added anything since the last test, lid has been on, bubbles off. The temperature has increased about 10c though,

Readings are now 5.6 Cl, 7.8 pH, 120 TA.

Is it possible for the readings to change so much, just from a 10c increase in water temp, or have I got a dodgy test meter/strips?
 

cowboycasey

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Jul 3, 2013
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Welcome to TFP :)

You have discovered exactly why we say Do not use guess strips... they are so far off most of the time you will be chasing your tail on an hourly basis...

We have some awesome people here and they know where to get a kit that will work :)

How to Buy LaMotte 7002 FAS-DPD Commercial Series Kits (FAS-DPD Commercial 7 Kit):
UK: poolandspacentre.co.uk or sword-scientific.com or pro-swim.co.uk

Can you use a salt water generator? It would be the easiest way to sanitize and not worry about anything...

Using chlorine granules or tabs will drop your TA and PH fast in a hot tub so you will be fighting a daily battle, with a SWG you set it and its good for the week, empty and repeat.. :)
 
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benseb

Member
Mar 23, 2021
21
UK
Ah good, I’m quite relieved it could be random test strip readings!

Yes any recommendations on test kits, the easier to read the better. I’ll check out the one above, thanks.

I’d not heard about SWG before yesterday and saw it mentioned. Maybe a stupid question but do they make the water feel salty? Just thinking of what will be acceptable to our guests. But yes if it makes the process easier I’ll certainly research. They just have to be as “guest proof” as possible!
 

cowboycasey

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I had a salt water hot tub and LOVED it, it makes the water feel silky smooth and when you get out it feels like you just took a shower.. It just works as our skin likes it... You can taste it at first but after a minute it is nothing... The one below says 3500 to 4500 salt, start at a lower 3000 ppm and see if that works, it will create a little less chlorine but that may be a good thing...

Found one for you, just plumb it into your system.. It has an adjustment on it for percentage, hopefully it will be easy to set it and get a 7ppm chlorine level and call it good :)

CHICIRIS 100‑240V 8g/h Pool Saltwater Salt Chlorine Generators​

 

cowboycasey

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I just converted your hot tub to gallons, at 250 gallons that is a small tub, mine had 700 gallons... You can add 6 pounds ( 3000 ppm, use pool math) of salt into your tub now to see how it tastes and feels... Get a salt test kit to test the salt level, I doubt you have any but you want it anyway to make sure you have the correct salt level.. :)

Order from these guys LaMotte Pool and Spa Testing product

As much as you will be emptying the hot tub this should work perfect
 

benseb

Member
Mar 23, 2021
21
UK
Awesome, thanks @cowboycasey - definately something to look into, I quite like the idea but will see how easy they are to integrate (and guestproof!)

Any tips on getting the initial water chemistry to where it should be as quickly as possible? I'm guessing once we have reliable testing, the chemicals we'll need each fill should be fairly consistent, but what I need to avoid is the "Add x then wait an hour then add y" as we have people checkout at 10am and have to have things ready for 3pm for next guests.

Certainly a challenge but it's obviously possible as a lot of rental places have hot tubs. I guess I just need to make sure I'm not overly complicating things.
 

cowboycasey

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With a hot tub and a small amount of water I waited about 5 minutes with the pumps on.... The PH and TA will take longer to get right.. You can bring the PH up by running the tub/jets.. I can not believe your PH comes out if the tap at 6.4 with alk at 0 ... is this test strips?
 

benseb

Member
Mar 23, 2021
21
UK
I can not believe your PH comes out if the tap at 6.4 with alk at 0 ... is this test strips?

Yes correct, I guess I'll have to wait and see what it reads with a proper kit. But as it stands those readings are outside of the UK acceptable limits anyway for tap water so it does look as though the strips + digital meter are way off.
 

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cowboycasey

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Not sure those inline SWGs will work, our hot tub is a 'dry' system and has no wet pumps, so there's nothing to plumb those into. The water movement is via the boiler and that's just stainless steel pipes which circulate the heated water due to the heat difference.

That is AWESOME... I like it... You need a drop in SWG like this...


I do believe Controlomatic will ship to you, send them an email and see :)

 

benseb

Member
Mar 23, 2021
21
UK
So just checked out the use of the SWG with the Hot Tub manufacturer. Unfortunately, because the wood burning stove is 304 stainless steel, it's not advisable for it to be in contact with salt water for prolonged periods, as it will lead to corrosion.

So back to the drawing board on that one :/
 

jseyfert3

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Oct 20, 2017
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Pool Size
15000
Surface
Vinyl
Chlorine
Liquid Chlorine
A lot of pool manufacture's say not to have a salt water chlorine generator, but they do not use enough salt to be very harmful to metals. However, since you have to dump the tub every week, a SWCG may not be the most practical solution here?
 

benseb

Member
Mar 23, 2021
21
UK
What would you recommend @jseyfert3 ?

ive been researching what other places do and from what I can see a lot of them just balance pH and Chlorine at the start then either asks guests to top up or leave it until changeover

The UK H&S laws have recently changed and stipulated you must test twice daily etc.

I’m hoping I can get into a routine where it takes me 5-10 mins AM/PM ideally.
 

cowboycasey

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They are correct, salt water is 35000 ppm, that's 35k not 3500, so 10 times less salt than salt water...

With that said it would have been better if they used 316 SS... I tried :)

Good old liquid chlorine is the way I would go... You have to check twice a day anyway just add the chlorine needed..
 
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benseb

Member
Mar 23, 2021
21
UK
Been calling around today to get a test kit. Seems a Taylor kit in the UK is impossible to find.
I called some of the links above and they mentioned that the labelling on these mid range kits is not EU compliant so they’ve had to stop stocking them.

I can get one off eBay perhaps but don’t want to buy something that I can’t get reagents for easily.

The recommendations I’ve had from my calls are:

Lovibond Scuba II with DPD Chlorine test
Lovibond MD100 6-in-1 with DPD (££££!)

Palintest Balanced Water

Pool Lab 1 (Cl DPD up to 8ppm)

The stockists are either poolandspacentre.co.uk. Or. Testallwater.co.uk

there seems to be quite a heavy stock of the digital kits in the UK. If they use the DPD reagents are they ok?
 

cowboycasey

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While we usually do not recommend digital ones but I really like that Pool lab 1.0 It uses the same way we do it it just reads it for you and you can get reference standard kit to make sure it is reading correct..

And it uploads and saves on your phone so you can prove to whoever you have been testing :)

 

benseb

Member
Mar 23, 2021
21
UK
Purchased the Poollab. Arrived today

Very very very impressed. Rock solid bit of kit, very well made. The results are consistent and much more believable. I also like the way the timer is built in so no need to look at a watch then test.

My water alakalinity from tap is 19 ( we do live in a very soft water area). I adjusted this up to 50 and checked pH and it was 7.45. Easy!

So hopefully that has made things much easier.
A quick question. If our calcium hardness is very low, do you think it’s worth increasing this, bearing in mind we’ll be emptying and refilling every 3-7 days?

Just trying to avoid the chemicals adding up to too much as we’ll be doing it a lot.
 

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