You may have a loose vacuum hose either under your dash, or in the engine compartment.
the blower motor has gone bad
There are a few possible reasonsÊthat an air would only blow cold when accelerating. The top two reasons are that the compressor fan might be malfunctioning or that the compression clutch isn't engaging.
Because you set it that way
The flap is stuck open that controls when and where the heat goes
why does ac blow warm air? why does ac blow warm air?
You need to first check that you do not have any vacuum leaks and then check the a/c / heater door operations, if all are working then its possible that you have a slightly clogged heater core.........
Bad thermostat
Your heater will blow cold air only.
The first thing to have checked is the amount of R134A that the system has and to service it to specifications as the amount is very critical. If that is okay then then system may need to be reprogrammed.
check your coolant level
Thermostat and/or heater core may be defective