There is a valve type thing under the hood where the heater hoses are connected What's that called and what's it do do you have to have one to make the heat work.
Looking at a car that doesn't have one.Does it keep the heat out of the heater box when your not using heat like in the summer?

I don't know what its called but it cuts off the flow of hot water to the heater core whenever you don't want hot water in the heater core. I don't think you need one so long as water flows through the heater core

heater valve... was used on A/C cars only as far I know. the total temp inside heater/AC box is graduated by the quantity of hot water running inside

dunno how it was graduated on just heaters cars

( living in a tropical contry we got AC cars and not only heater cars )

AC cars only. Temperature is regulated by the amount of ambiant air let in in heater-only cars.

in my Dakota, hot water flowed through the core all the time, and inside temps were controlled by flappers that directed airflow. don't want hot air? a flapper would close off airflow through the heater core.

I did not know that A/C only cars got that firewall control valve. something was wrong with my set-up on my Cuda, I had a brand new cable that would freely slide in the shield, the lever on the dash was free, the valve would move freely on it's own. hook it all up and something got into a bind and I could never operate it from inside the car, without fear of breaking off the lever on the dash controls!

I always just set it from under the hood, and then controlled heat inside the car with the blower motor. until the resistors blew out, and then it wouldn't work! so I had to bypass all that with a single off on switch for full blower!

71-74 non-a/c B-bodies with the stratovent got this valve as well. I know my sig car has the valve and is not a factory a/c car.

Is this what you guys are discussing?

Basically, yes. It looks a bit different depending on the year and it's got a vacuum can because operation of it is controlled by a vacuum hose comming from the heater controls.