I may have touches on this before, but I had my equipment put in a little different than they said it would be done. Probably not a big deal, but I must appeal to the guru's of TFP:
If a three-way valve is open to a stop partially -- lets say I want half of the water to go to the pool returns and half to the spa -- is the valve in this position efficient? If the flor is 100% (lets say) full open to pool, is is 50/50 now? Or is it more like 40/40 with 20% loss?
I have a bypass for my spa jets that misses the filter/heater and re-joins after. I had a physical piped bypass on my schematic and they plumbed just a three-way valve. I understand this can be used for this purpose, but I knew the other way I would have at least 100% flow (as the valve would be 100% toward the spa and the other pipe would always be feeding the spa -- adding additional flow).