here's what i think i'm doing; what have i missed?
tub >> drain tee valve >> fill tee >> pump >> thermo/mano meters >> filter
>> heater >> thermo/mano meters >> chemical injection tee >> flow
switch >> water meter >> check valve >> aeration tee >> tub
in case it matters, the tub is _above_ the rest of the parts. to lock the system for maintenance, i will rely on the check valve and the neutral position of the drain tee valve. fill (at mains pressure) will use both lines, reversing flow on one of them.