Heater supply shut off valve for 1991 Mercruiser 5.7

[drodkey]

We have a 1991 Trophy 2352WA with a 5.7 Mercruiser closed cooling system. Problem I’m running into is when cruising with the cabin top hatch or side portholes open, the wind blows into the back side of the cockpit walls causing hot air to blow through the red dot heater. I’ve tried sealing the opening in the wall between the cabin and cockpit area but still get the hot air when cruising. I’m wondering if installing a shutoff valve to the coolant line running from the engine to the red dot heater is an option? Would I need shutoff valves on both the supply and return? Do I need to creat a bypass to keep coolant flowing back to the block? Am I over thinking it and can simply put a shutoff at the supply line at the block? Obviously I need to be make this easily accessible for when the first mate wants the heater on.

Thanks,
Dave

 

[Bondo]

Am I over thinking it and can simply put a shutoff at the supply line at the block?

Ayuh,..... This would be correct,.....^^^^

 

[Scott Danforth]

Only one valve needed

 

[drodkey]

Ayuh,..... This would be correct,.....^^^^

That’s my typical approach. Thanks