Hyundai Repair: Heat, coolant level, temperature cable

Question

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The text above is a follow-up to ...

-----Question-----
The heat in my 2002 elantra is not working correctly.  If I have it on defrost, it blows cold air.  If I have it on face level, it does blow hot hair, but if I'm stopped, the air gets cooler.
-----Answer-----
You should probably start with checking your coolant level (after the engine has cooled).  If your coolant level is low, you may not have sufficient coolant to flow through your heater core.

Next, you might try checking the temperature knob on the heater control assembly.  If it's cracked where it attaches to the stem on the assembly, the knob may turn without adjusting the temperature.

Also, if you have a cable operated system (you'll need to look to see whether there are cables running to the heater box behind the dash), it's possible that the temperature cable is not properly attached to the heater box.

Hello again!
My friend filled it with coolant (he said it was a little low).  It still is not working correctly.  Again, if it's on defrost, it blows cold air!  If I have it on bi-level it blows warm air, but only if the car is moving.  When I stop, it gets cooler.  My friend said one of the hoses from the radiator is hot, but the other is cold and should be hot.  He says it appears it's not circulating correctly or something.  Could this be the thermostat or the radiator itself?  I haven't seen any leaks.  Help!

Answer
The thermostat being stuck shut is the number one cause of coolant not flowing when the engine reaches operating temperature.  Once the thermostat opens, both hoses should be hot, like your friend says.

The radiator being clogged would be a distant second.

You should have your car checked immediately.  If the thermostat is indeed stuck shut, it can cause the engine to overheat and result in severe engine damage.