2004 Sonata, heater and defroster no longer blow hot air - have replaced the thermostat and flushed the hoses, still not heating, living in New York we are at the time of needing the defrosters to work in the early morning hours of going to work,

Get the engine and coolant to operating temperature. The upper hose to the radiator should NOT get warm over time, but should get HOT when the thermostat opens. If it warms, then it is leaking and allowing coolant to bypass, and you won't get full heat.
Once warm, check BOTH hoses to the heater core, on the passenger side, the 5/8-3/4" diameter pair. BOTH should get too hot to keep touching. If only one is hot or warm, your heater core is likely clogged or plugged, and restricting coolant flow. Flushing may help, and if not, replacement should cure that problem. tom

If the fans are blowing air out the right places then that part of the system is ok. Lets focus on the no heat. With the system warmed up, check both the heater hoses to see if they are hot. If they are not, then there may be a restriction in the hoses or the heater core its self and the system will need to be flushed. Also watch this video https://www.youtube.com/watch?v=9orWU9B5ZQ0

check the controller an vacuum lines

is there a chance that one of your heater hoses are clogged? or where they connect to the engine are clogged still? sounds like a clogged system. Cracked heater housing? Outside air bypassing the heater coil while driving?

disconnect the heat hoses going into fire wall , put hose into one with pressure and flush as it sounds like it could be bloked in the heater core,the other thing that could be the problem be the hot ,cold switch as it restricts the flow to makes the heater hoter or colder , it might need tlc or replacing as it is manaul not elec

The brown looking chiffon could be oil in the water. However, if you are sure the system is ok then I would check the actuator door motor on the heater and make sure it is allowing the heat into the car. Seems as if you have done as much as you can.

Thermostat.? if you haven't? Do it first then try it. While the car is running for a while touch the heater hoses that lead to the firewall. if one is cold and one is hot? You have a blocked Heater core. Follow the hoses back to the engine. Unconnect them both. By using a garden hose flush out the heater core do one side and then reverse by doing the other hose Lots of junk will come out so be care ful. Mistake people make is "Flushing the system" That doesn't mean your flushing the heater core.
Seeing that you have done everything else it could be the thermostat which you didn't list or the heater core being clogged. When you finished installing the thermostat and flushing the heater core out. Replenish the coolant system . If you have a bleeder on the thermostat housing? use it . loosen it and top off the fluid until you see fluid comeout of the bleeder. The system being air bound would cause no heat..

Sounds like you have air trapped in the heater core when you flushed the heater core. loosen the upper most heater core hose and run the engine untill coolant seeps out. Wait until the coolant has reached running temperature.

feel the two hoses going into and out of the heater core. they both should be hot. if one is cold the heater core is likely plugged. you can try flushing it out to see if you can restore flow to the heater core. if they are both hot the thermostat may be sticking open tho you should have a limited amount of heat. also confirm that the vents are operating as they should be.

robert

Had similar problem in my 2000 Jimmy. No hot air from heater. Had the shop (Midas) disconnect heater hoses at the fire wall and blow out the heater core with air hose. Got rust and junk blown out. Hooked up and flushed system with new anti freeze. Got lots of heat now.