Even though I am in S. Texas it does get cold enough to use the heater.....
2001 F-150 - 5.4L

I am not getting consistent heat from HVAC.
With temp dial turned to heat, I will get a nice flow of hot air for about 10 sec, then it cools off, never to outside temp, but not enough to keep cabin comfortable. If I turn fan off and wait a couple minutes, I can get another burst of hot air ,then cool.
Almost like the fluid is not circulating well?
A/C works great.
Temp meter is perfect, right in middle of gauge, Anti-freeze level is good.
So I think that eliminates the water pump & thermostat?
Thanks,
Ed

 

That does not necessarily eliminate the thermostat - the gauge is not linear, it's an idiot gauge that stays in the middle unless it overheats. Your heater core could be plugged.

 

Could also be low on coolant or the water pump might be failing as well as any restriction in the heater coolant loop.