I replaced my thermostat in my 2008 kia v6 and now heat only blows when engine is under load...need help thanks?

You have air in the cooling system. You need to follow the proper bleeding procedure for you car and fill it up all the way with coolant.

You have an air pocket in the motor. Take the radiator cap off. Set the climate controls to the hottest setting, the air flow to the center vents, and the blower fan on low. Have the vehicle parked with the front higher than the rear. run the motor for 20 minutes topping up the coolant as the air purges from the radiator neck. You may want to rev the motor up and down to 2,000 rpm and squeeze the upper radiator hose to help move a stubborn air pocket out of the motor and heater core.When all the air is out of the system you'll have hot air out of the vent at idle speed.