[oldtamiyaphile]

Most cars seem to have heater cores that constantly circulate coolant -whether cabin heat is on or off.

All that separates that heat from the HVAC system is a thin plastic blend door - and the heater box itself must bleed a fair amount of heat into the cabin. Certainly after an hour or more of driving, this must be adding to in cabin heat soak.

So why not fit a valve to the top hose of the heater? Doesn't have to be anything fancy - a manually operated valve that you turn twice a year would be fine. As a temporary test you could just use some wood blocks and a G clamp.

It would also be a great mod for those of us with ACC, as I'm forever trying to guess the temperature at which it will neither use A/C nor bring in unwanted heat.

I've just disconnected my Proton's heater core which got me thinking about this - unfortunately that means I can't run a test to see how much heat gets through a heater core that's 'off'.