Heat in car taking awhile to hear up?

I have 04 ford explorer that I purchased 2 months ago. I'm just starting to turn the heat on because it just starting to get cold. And it always takes about 15 min to get hot. After it gets hot it will last maybe 2 minutes than go to cold than back to hot and will just keep doing that it never just stays hot. What could make the heat do this. I've heard coolant can be low so I'm putting that in first thing in the morning. But could this be electrical or something else other than coolant.

Do you have an engine temperature gauge? The heater core uses engine coolant for the heat. It sounds like the thermostat in the engine could be sticking open making the engine run cooler than it should be. Either that or your HVAC controls are acting up. The thermostat for the engine cooling system would be my prime suspect though.

Replace the heater matrix behind the dash board.Heat in car taking awhile to hear up - 1

They have trouble with the blend doors. Very hard to get to.