Car heater puts out cold air?

Hello,

A heater can blow cold air for few reasons.
It can happened is the coolant does not flow thru the heater core properly due to any type of restriction in the pipes or a clog in the heater core.
Sometimes flushing the system can improve the heater performance.

https://www.2carpros.com/articles/how-a-car-heater-works
https://www.2carpros.com/articles/car-heater-not-working

Also you might get cold air if the thermostat is stuck open. When this happens the engine does not reach its operating temperature and the air for the heater comes out cool.
Check to see if the engine runs cooler than usual. That could be a sign of a stuck open thermostat.

https://www.2carpros.com/articles/replace-thermostat

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.