Borate application, redux

[waterbear]

If you have a pH of 7.8, a TA of 50 ppm, a borates of 50 ppm and a CYA of 40ppm(?), I'm assuming you'll have a very low CA? And thus, you'll have little pH drift from CO2 off-gassing during periods of aeration?

At PoolSpaForum we tend to recommend (with the Dichlor/Bleach method in an acrylic portable spa or one unattached to the pool with either dual or shared pump/filter so they are two separate bodies of water) TA of 50-60 ppm, borate of 50 ppm, CH about 130-150 ppm or higher (depending on how hard the fill water is, if the fill water is soft we don't recommend raising it above about 130-150 ppm since that amount of hardness is enough to deter foaming), and achieve a CYA level of 20-30 ppm by using dichlor and then switching to bleach for chlorination and shocking until time for the next drain and refill with spas (every 3-6 months,depending on usage). With a plaster inground spa with it's own plumbing, pump, and filter (either stand along or single/dual equipment with an unattached pool we basically keep the same recommendation but raise the CH to keep the SI in proper range. With a shared pump filter system with a spillover spa you essentially have a single body of water and one pump and filter. There might be a secondary filter or blower for the jets much like water features in a pool might ave a secondary filter but the main circulation is a single system and the spa is only isolated when being brought up to temperature and then used. This tends to be the most common configuration with both plaster and fiberglass pools and either plaster, fiberglass, or acrylic spas. This is essentially a pool with high aeration, not that much dfferent than a negative edge pool or a pool with water feature and running a low TA and maintaining the pH at a higher level and also adding 50 ppm borate is an effective way to minimize and slow pH rise and obtain pah stability for a longer period of time. It also requres a bit more attention to water parameters, testing and dosing. Adding a SWCG to the mix adds its own unique set of problems to minimize cell on time (by keepign CYA at the max) to lessen the aeration effect caused by hydrogen bubble generation and thereby also slowing the outgassing of CO2.

Are you / they adding dichlor (CYA) to spas because they are uncovered outdoor spas, or to reduce DPB formation? I get the "don't let CYA build up" thing.

Most portable spas are kept covered to maintain the heat and keep energy bills down,particularly since many are used in northern climates when pools are normally closed (We tend to be busiest at the forum in the dead of winter and have a LOT of Canadian and northern US members!). The idea of using a small amout of CYA is because of, as Richard likes to describe it to the members there when explaining the dichlor/bleach method), its 'buffering' effect on chlorine (making it less aggressive). IF the spa is heavy usage then the addition of a decent ozone unit or the use of MPS is recommended to help oxidize organics.
This has turned out to be a huge improvement in the sole use of dichlor, which we have foundcan lead to outbreaks of pseudomonas and other water born illnesses as the CYA level gets higher and higher.

So, I can see strategy of high pH + low CA + elevated non-CA TA + CYA control as a reasonable effort.

Can those results be improved by using that strategy, except switching to a very high CYA plus very high Cl residual? Something like 200ppm CYA plus 25 ppm Cl as a normal level? That might overcome the problem of insufficient total chlorine residual (in 300 gal water) to react with the bather goop of a couple of spa users.

Interesting idea but then you have to get over the hurdle of convincing users that the high FC/CYA combination is not going to kill them of make them sick and there is the real concern that it could void the warranty on their spa.
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I've also realized I don't have any idea of what the 'normal' spa configuration is. In my area, even today, most spas are detached. But, I gather that attached spas may be more the norm in California and Florida.

- Are most of these in a shared equipment / unshared water configuration?

There are basically three configuations:
1. unattached dual equipment--the pool and spa are two separate bodies of water and each have their own pump, filter, heater, etc. Not that common because of equipment costs and now usually seen when there is an existng pool and a stand along portable spa is added to the mix.

2. unattached shared equipment (normally only found with automation--there is only one pump,filter, heater, etc. and the are valve actuators on timers that switch between pool and spa so each can have a filter cycle during they day but they are two separate bodies of water that are treated individually. Spas in this configuration are usualy kept at temperature and covered to maintain the heat.

3. attached pool/spa and shared equipment (spillover spa)--this is becoming the most common configuration since it minimizes equipment costs and simplifies water maintenance since it is essentially one body of water. Normal configuration is water enters spa, spills into pool, and to filter from pool skimmer and drain. The spa drains are off. There are often also returns in the pool that are active. When the spa is desired there are valves or actuators that shut off the pool skimmer, drain, returns and open the spa drains (shutting off the pool, stopping the spillover) and making the spa the only thing in the loop. The heater is turned on to bring the spa up to temerature, which normally does not take very long. In places like Florida heat pumps and solar are often all that is needed but in more northern climates gas or electric heat (either main or as a supplementary spa heater) is often needed.

Normal operation is as a single body of water at pool temperature and after the spa is used the water is essentially 'dumped and changed". The main problem is the higher aeration, which can be dealt with by lower TA, tighter pH control, and borate.

- Spill-over spas have to be in at least partially shared water configuration. Is it typically 100% shared, or partially shared?

100% shared. not sure how you could even have a partially shared configuration.

I'm going to need to see if I can find some piping and control layout diagrams for these spas, to see have they've been design and should operate.

Try this Hayward Aqualogic Prologic manual pages 9-11. It has some diagrams that might help. Dual systems and shared water (spillover) sihared systems do not have to be automated but many are. Systems that are unattached but share equipment are not really practical without some form of automation to control run times for the pool and spa.