THIS PROBABLY ISN'T GOING TO BE A POPULAR ANSWER BUT YOU CAN BUY A MANUAL CUT OFF VALVE AND INSTALL IT IN THE HOSE YOU CLAMPED OFF. THEN IF YOU EVER NEED HEAT WHERE YOU LIVE THEN JUST TURN VALVE ON! OR YOU CAN REPAIR THE PROBLEM FOR MORE$$$$$$. JUST A THOUGHT.
I had a thought once, too , but then I got a headache and the thought went away.-------Anyhow .....possible blend door($$$$$$), or heater control valve(cable controlled according to my info), so also check ALL controls are working properly before replacing parts. OW , my brain hurts. Fortunately , small brain , so it doesn't hurt much.
IF THE HEATER CONT. VALVE IS CABLE ACTUATED THEN MAY NOT BE TO MUCH TO FIX IT. BUT THE CUT OFF VALVE WORKS ALSO JUST A BIT OF TROUBLE TO TURN ON & OFF. Take 1/2 aspirin and post results so we can adv. WHAT UP ZIP.