[JK McDonald]

I agree with the others - this vacuum line should probably be connected to the heater (hot water shut off valve) control valve located in the center of the fire wall behind the lower half of the air filter assembly. If the heater control valve is disconnected from it's vacuum source - it falls to the full open position which runs hot water through the interior heater core.

Sometimes an owner will plug the vacuum source and zip tie the plastic arm of the heater control valve to the closed position (hot water can no longer flow into the interior heater core) because the vacuum diaphragm inside the valve has failed. As a temporary fix this will disable the heater during a hot summer. Normally the needed parts are put on order and replaced before winter arrives.

Perhaps the zip tie came off the heater valve control arm leaving you the lack of temp control.

Good Luck, Michael