I'm installing an Intellichem and debating whether I should put the injectors and flow cell outlet before the valve manifold, or only in the pool return line.
There's one valve that controls whether water is returned to the spa, or through one or more of the pool/skimmers/cleaner valves.
I was initially thinking the injectors should be upstream of the valves, so acid injection and chlorination could occur as needed regardless of whether the spa or pool is running. But, I saw a post on TFP that suggested it might make more sense to plumb things so injection only occurs when water is being returned to the pool.
The Intellichem is set up for a specified volume of water. So, maybe the amount of chlorine and acid would be too high if water is circulating through the spa only? The sensors would "see" the effect of acid/chlorine injection, but if the control algorithm is something like PID, it might take too long to get an optimum response to a given "error value" of PH or ORP.
In practice, does it really matter? How is the plumbing typically done for a Pool/Spa setup?
I have a schedule that spills the spa into the pool several times a day. I think this should keep the water in the spa pretty close to the same chemistry as the pool.