Seen online if got air in system it would cause this.
Heater blowing cold air! Changed termostates. No help on 2003 Nissan Altima
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If there is air in the system the waterpump will not pump the coolant throughout. You may have air in the system and need to bleed it out, could be a bad temp./blend door actuator which moves a plastic flap inside HVAC box when you adjust the temp. at the controls, could possibly be a clogged heater core, or your new thermostate could be bad. Especially if it's aftermarket. I've seen a couple new aftermarket thermostats go bad in a week or so after installing them.