Replace thermostat heater blows cold air like AC is on?

Hello,

There could be multiple reasons why the heat is not functioning.
First of all, check the coolant level to make sure it is correct. As low coolant can lead to no heat due to little to no circulation of coolant through the heater core.
Also, after replacing the thermostat, is the engine warming up properly? It is getting to the operating temperature. Faulty thermostat will not allow the engine to warm up and that will lead to cool air out of the heater.
Another possibility is a faulty blend door actuator. The blend door actuator controls the hot and cold air mix. Faulty actuator can fail on the hot or the cold side, leading to no heat or no A/C.

Here is a helpful guide that goes over the typical issues:

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

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.