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Pex is to rigid, more expensive and not needed. As you can tell I don't have any crimp rings on the 90 degree turns because I don't want the metal in any way touching the tub because it is and inflatable spa. Remember this system is all low pressure if running wide open. I didn't even need to have put the crimp rings on any of the fittings but did for the sake of it. The only way you will have the fittings come off is if pressure builds for some reason and pops them off.
noooooo....you need to treat it as a pool. Look on amazon:Leisure Time 22337A Spa 56 Chlorinating Granules for Spas and Hot Tubs (1 Pack)andPoolmaster 22211 Smart Test 4-Way Swimming Pool and Spa Water Chemistry Test Strips, 50 countThis would be the minimum for you to buy in order to keep the chlorine in the "safe" zone and measure with the strips. Then you would be good to go and you wouldn't have to worry about bacteria or getting super sick.
Yes the timer is to not make the heater from shutting down. It is a built in safety thing. Are you using this with a hot tub? If you are why are you not using any chemicals? If you don't use chemicals then you need to treat that tub as a bath tub and drain the water after each use. You are correct if you don't use chemicals then bacteria will bloom and you do not want to go down that road. A UV sterilizer won't be enough.