Cutting off Heater Core

[JustPassinThru]

Not sure where this goes. If the mods want it elsewhere, feel free to move it.

I'm thinking of putting a valve on the heater hoses - to cut off the flow to them in hot weather. Why? Some residual heat comes out the fresh-air vent system, even when it's on full-cold. Any temperature over 60, the air is too warm - even at night, no UV rays through the acre of glass.

I have air conditioning in the car, but it's silly to use it when it's jacket-weather outside. It uses gasoline; and gasoline costs.

The problem is this: SOME engines, some brands, NEED the heater-hose flow channel to circulate warming coolant - else the coolant in the water jackets boils while the coolant at the thermostat is still too cool to open it up. Some are designed expecting that flow; and some are not.

Which one is the Yaris engine?

If I do this, would a simple watercock (as Nissan/Datsun used to use with its heater on early trucks) do; or would I need to put on a Y-gate valve? Or re-route the heater hoses? I'd rather not get too-much into it; since it's a 2012 and still under warranty.