Check coolant level? Heater core plugged or restricted? Heater core airbound? Bad thermostat? Water pump not circulating coolant? Heater control cable not adjusted properly?
A car heater taps off engine heat. If you aren't driving, then the engine is cold and there's no heat for the car heater to use.
Try your thermostat
I assume your car heater is blowing cold air, when the heater is on. It seems that, you have air on your coolant. that air pockets blocks the cooland to travel through your heater. it may be a sign of a bigger problem such as blown head gasket in the old cars, such acura legend
it could be the heater core and ac housing unit that allows hot air to be mixed with the cold air or it could just cancel out the cold due to the motion of the car
Only blows cold air with the a/c on while driving, your cooling fan or fan clutch is bad depending on the year, make and model.......... Only blows cold air with the heater on while driving, you will have a bad or broken internal a/c - heater control door.....
An automobile's heater that blows only cold air could either be a faulty thermostat or, more likely, a faulty heater core. A trip to a mechanic or shop that specializes in radiator and cooling system repair should be able to diagnose and repair any problems.
You may be low on coolant, or the thermostat is stuck open, or the heater core is plugged up, or the temp blend door is broken.
Although my car is not an Eclipse, I experienced the same issue. A mechanic determined that my radiator was partially blocked and recommended replacement. My heater now blows heat whether I'm stopped at a light or moving.
Well the main reason why your car blows out cold then hot is because the air conditioner may not be working properly. You should go to the car shop and get it fixed.
The Air Conditioner is on
check heater core could be cloggedCheck the engine coolant level also.*no, the heater relies on the engine for heat and when you drive the engine warms generating heat:)*
Maybe thermostat is stuck open With my car, it was Heater matrix clogged with radiator sealant. The workshop cleaned it and it's start working fine!