I have an '04 Nissan Sentra. I know there is a problem with my fuel pump, which can delay the start on my car, but that is not my real problem. My problem is I have no heat. My car starts fine and never moves above or below the middle on the temperature gauge, but it almost never warms up. The only time it seems to is if I am going over 55 mph and that is only the first time I go over that speed. Once I slow down it stops working and does not come back. The air blows out, but the air is colder than my A/C. I have looked at the coolant and that seems to be fine, so I am not sure where to look next. I would appreciate any suggestions as I live in ND and it gets cold here fast. 
Is your check engine light on? When did you lose the heat, and did you notice any other performance changes at the time (rough idle, poor gas mileage)? On most cars, if you swing the cabin temp control all the way from one side to the other, you will hear a soft sound like a thud as the temp control door changes position…do you hear anything like that?
My check engine light is on. It has been going on and off for the last year or so because of my fuel pump problem (esp when my car is hard to start). However, even when I have no issues getting the car to start, the heat still does not work. My gas mileage has been fine and the idle, most of the time, is just fine. Only when it was really hot out did I have issues with the idle, but I never checked the heat at that time.
When I change the cabin temp control, it does sound like it’s shifting from cold to hot, but the air does not change.
Check your coolant level in the radiator, not in the coolant tank when the car is cold. Air in the radiator can do this.
Get the thermostat looked up. My guess is that it could be faulty.
If the blend door (controls the mix of warm and cool air) is working, then the only explanation is that there is no heat from the heater core. Either because the coolant is not hot or because the heater core is plugged (edit: or low coolant or air in the system). If you’ve got a bad thermostat your engine could be running colder than it should. I don’t think your heat would be “colder than A/C” but it would definitely underperform. Low engine temp would set the check engine light, but your light is already on so you would miss that signal.
" I know there is a problem with my fuel pump, which can delay the start on my car, but that is not my real problem. My problem is I have no heat. "
How could you drive for a year with a bad fuel pump and “Check Engine” light on and dismiss it as not a real problem ? It seems this is probably not a well maintained car.
I’d bet that no heat isn’t the real problem, either. You may have cooling system leak, external or internal. I’d follow Oldtimer’s advice. He’s correct in that low coolant can cause no cabin heat (air in heater core and/or poor coolant circulation, except when traveling fast) and even a normal or low coolant temperature indication (air surrounding sensor) and yet be overheating the engine.
So, be sure the cooling system is full of coolant, first (not just the recovery reservoir). Then check other items.
CSA
@ common sense answer: My car was checked for the fuel pump and it is not a problem with the fuel pump itself. I could replace the fuel pump and it would not make a bit of difference. The problem lies in how well the set up is with the fuel pump. It is not keeping the fuel in the engine when the car is not running, so it is going all the way back to the tank. I was told by the mechanic that the problem was common with my car and will delay the start time of my engine, but does not affect the heat or anything else. Therefore, it is not something I think is related to the heat issues I am having.
I will check the coolant levels and radiator and see if that helps. Thank you for the suggestions.
I think there is a check valve in the fuel line that maintains the pressure. It should be pretty easy to replace. I’m having dubious thoughts about your mechanic.
My wife has a 2004 Maxima, same problem with heat. Thermostat is working, radiator is full, have let run with cap off in hope of removing air. Heater works fine if the car is moving, but when at idle blows cold air. Nobody has been able to find a solution.