Hyundai Repair Questions? Ask a Mechanic for Answers ASAP
The two things that will keep the heat from working are a stuck open thermostat (it doesn't let the engine heat up-doesn't sound like your problem) or an air pocket in the heater core. Getting the air to bleed out of a heater core on the elantra can be tricky.
I assume you did some work to the cooling system and introduced air into it?
How long have you driven it since you refilled the system?
The only effective way that I have found the bleed this system is to raise the front of the car as far as you safely can and then start the engine. Put a funnel in the coolant fill neck that fits snug at the base. Fill the funnel partway with coolant and then rev the engine to about 2000 rpm and hold it there for about a minute. Then let it idle, check the fluid level in the funnel and try it again. It should burp air out and suck fluid back in as you do this. The design of the car makes the radiator much lower than the engine and the heater core. Since air always goes the highest point, it will be stuck in the heater core unless you lift the front end up.
You could also drive it some more to see if it will clear on it's own, but it will probably take a few days.
Yes. An air pocket will keep the heat from working. The coolant can't circulate through the heater core if it is full of air, and the heater can't use air to provide heat.
If you don't have the means to lift the front of the vehicle safely, you could also find a steep incline and park on it to perform the bleed procedure.