On Sat, 8 Jul 2017 12:43:46 -0700 (PDT), trader_4 wrote:

On Saturday, July 8, 2017 at 12:43:26 PM UTC-4, wrote:

There will be flow too, in MOST cars still on the road. The vast
majority do not control water flow through the heater core any more -
they just control air flow, either through or around. This is
particularly true of cars with "climate control" or automatic
temperature control / AC

This is shocking misinformation. Why in the world would any climate control or even manual AC be putting heat into the heater core unnecessarily while trying to cool the car? They're fighting for anything that can improve fuel economy and they have the heater core putting extra heat in the cabin? WTF? Why would they do that? And all it takes is to close a valve that's already there.

No it's not misinformation. Anybody who's ever replaced a heater core on GM vehicles that
I know of, know that the core has an inlet and outlet and no valves restricting flow.
It's the job of the mixer door to adjust heat in the cabin. There are some vehicles that
have heater water valves, but I haven't worked on them.
And their is no "extra" heat.
Engine designers can't eliminate waste heat until they design one that's 100% efficient.