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I checked out a friend's portable spa after he told me the fumes from the spa were bad, made him cough and his eyes water.
Sure enough, the fumes were very noticeable and I have never smelled fumes like that (made me cough too). It was not what I expected to smell from chloramines. (Maybe it was mono-chloramines or whatever they're called when there are almost zero chloramines). The fumes and water had a very sour, almost plastic type odor.
Free chlorine was 1.2ppm and combined 0.4ppm. pH was 8.0 (so those needed fixing) but the water looked fine.
I'm wondering if there might be some type of algae or fungus growing in the pipes.
I suggested circulating water with a strong chlorine dose and then dumping and re-filling. Any suggestions as to how much liquid chlorine to add to 450 gallons of water for this purpose - or any comments on what may be causing such a strange odor.
Ross Townsend

Ross:

The recommendation is 200 ppm of chlorine. Assuming 12% NaOCl you would need:

10.6 fl. oz. X .045 X 200 = 95.4 fl. oz.

(It takes 10.6 fl. oz. of liquid chlorine to get 1 ppm of FAC in 10,000 gallons.)

There are 128 fl. oz. in a gallon and 32 fl. oz. in a quart. I would probably just use the whole gallon. You must lower the pH before adding the gallon or you will have a pH of 13 in the spa. I would guess about 6-8 oz. of acid. Circulate and then add the gallon of liquid chlorine. Let it run for 24 hours. Drain and refill.

Bob