Does anyone know why there are 3 #21 shut off valves

[Rinny]

Does anyone know why there are 3 #21 shut off valves listed for the rear heat?
http://www.wholesalegmpartsonline.com/p ... 70012.html
click the second diagram down.

[MI_Ghost]

Easy enough.
1. 4 port valve for standard front heater. No rear heat. Delco 15-5557 (Should be #20)
2. 2 port valve for rear heat only. Delco 15-5586 (Vacuum operated)
3. 2 port valve for rear heat w/ AC shut off. Delco 15-5877 (Shuts off AC when rear heat is used)(Vacuum/Electric)
4. 6 port valve for front and rear heat. Delco 15-5551 (#20)

Numbers 2&4 or 3&4 are used together with supply going from engine to 6 port valve to 2 port valve the to rear heater core. Return is routed directly back to 6 port valve. This allows the rear heat to be shut off when not needed.

All 4 valves are still readily available. Shop around for the best price.

And for reference.
#12 Delco #15-30578 is the vacuum hose that controls the vent location selection (No Dash vents issue)

[Rinny]

Thank you. Very helpful post!!