Make sure the cooling system is full then warm up engine, with your hand feel the temperature of both heater hoses, if they are both cool the thermostat may be stuck open, if one is hot and the other is cool the heater core may be plugged, if they are both hot the heater blend door actuator may be bad.
It could be the coolant level is low or a faulty thermostat.
Low on coolant, stuck open thermostat, plugged heater core, faulty temperature blend door,
What temp. thermostat did you put in? Try a higher temp. one. or check to see if heater core is clogged
Low on coolant. Stuck open thermostat. Plugged heater core. Faulty temp blend door.
It could be a partially blocked heater core, a faulty heater valve that allows the coolant through the heater core or it could be a faulty water pump.
Because you haven't diagnosed and repaired the issue, and this is something which takes a hands-on diagnosis to assess. A plugged heater core, faulty blend door, bad thermostat, and low or burned out coolant are your common culprits here. If your engine cools normally and where it's supposed to, it's probably not the thermostat.
Could be: Low coolant in the radiator - or - thermostat stuck open - or - heater core plugged.
Plugged heater core. Faulty temp blend door mechanism.
Faulty thermostat. Plugged heater core. Temp blend door malfunction.
Poor circulation due to a faulty thermostat or a plugged heater core. Be sure the coolant reservoir is not empty.
Poor circulation due to a faulty thermostat or a plugged heater core. Be sure the coolant reservoir is not empty.
Year, make and model would help but, it could be a faulty thermostat, a faulty temp blend door actuator, a plugged up heater core or the coolant system is low on fluid (leak).