The heat control (cold-to-warm) on my 1999 Civic operates a valve near the firewall, controlling coolant flow from the block to the heater core. Yours may be similar. With the ignition on, you can have someone operate the cold-warm knob while you watch the action at the valve near the firewall.
It’s one reason I bought the car. I did not like the radiant heat that is always present in cars that have the heater core always getting full flow, and use doors and the fan to reduce or increase the air flow over the hot heater core and into the cabin.