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1: not enough liquid in the system (air in the heater core) 2: inoperative heater valve not allowing hot liquid into the core 3: hot/cold blend door in the heater box stuck on cold 4: clogged heater core A clogged heater core will have one hot hose and one significantly cooler hose at the firewall connection; easily tested by hand. a good core will have both hoses very hot to the touch when the heater is on full blast. Make sure that the system is full and at operating temperature, and the heater controls set on full heat. Feel the hoses at the firewall; if they are both too hot to touch then the problem is in the heater box or controls.
I like that you were knowledgeable to check both coolant lines tells me you are getting hot coolant through the heater core next thing to check will be the blend door on the inside of your vehicle. The blend door is a door that switches from cold air duct to heater duct depending on what you are using at the time. If it is stuck on the cold air duct you will not get any heat sometimes these doors are vacuum actuated and sometimes they are electrically actuated most the time you are able to see the movement of the motor from inside if you don't see any movement but you hear it move chances are it is the door blend I hope this helps.
The AC compressor will come on when you turn on the defroster. If you're not getting heat it's most likely that your heater core is going bad. You can flush it out and see if it's getting clogged. Also check your water pump to make sure you have enough flow through the core. The reasoning behind the ac blowing when the defroster is on is that it blows dryer air on the windshield.
There are a few things that can cause this problem. Reason #1 a low coolant level can cause the heater core to receive less coolant flow hence causing the heat to come less quickly. reason #2 sticky heater control valve, if this control valve is stuck closed coolant cant enter the heater core hense wont heat up the cabin, reason #3 clogged heater core, over time some heater cores will clog up even sometimes start to leak in cabin. All these are normal circumstances and should be checked. Good luck
Had the same problem with my Mazda6 awhile back. Took it to the dealer and they found the radiator wasn't holding pressure. They replaced the whole radiator under warranty (WHEW!). Not sure how to check for that kind of thing, but it's a start.