The way the monovalve works has a giant design flaw IMO. When you shut off the ignition, you lose power to the valve, and it goes to open, allowing hot water into the core on the inlet side.

And on the outlet side, I don't think there is anything to stop hot water from getting into the core by convection.

So I think your idea of valves to allow for summer and winter mode makes a lot of sense.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe