1998 Chevy Blazer with 267K miles. It has been a great car needing little work except routine maintenance. The water pump went out and the car over heated. I replaced the water pump and thermostat, the car runs fine and temp stays normal. However, when I turn the temp control up for heat, there is no warm air, but the force of air coming out the vents inside is cut down to a fraction of what comes out when the temp control is turned down. I disconnected the hose going into the fire wall for the heater core and ran a garden hose to it, and water flowed through heater core and water pump fine, so there is no clog. Why is there no warm air, and little air when heat is turned up? Possibly electric or electronic component not sending , or door stuck in position somewhere maybe??
The reduction in air flow makes me thing the temperature control door is not moving properly, and is not sending air through the heater core.
As I believe MC intended, I suspect someone knocked a vacuum hose off when doing the repair and that hose either supplied the heating system or is not draining the vacuum system so it can’t open the heat blend door.