05 chev. silverado LT. Problem is the heater fan all of a sudden

Because u have a bad fan sending unit that stays on...trace wire from fan to the block and thats the sender u need to replace (stuck on drains batt and stuck off car over heats so i guess u was lucky)

It sounds like an alternator, yet you say the fan still worked after you disconnected the battery? And the car was still running? Or the car died out. Sorry, need some more details on this one.

In some vehicles, the radiator cooling fan is controlled independently by a thermostatic switch. What you perceive as a problem actually is normal operation. When the temperature reaches a certain point, the fan turns on. When you stop and turn off the ignition, the fan continues to run until the temperature decreases to a point where the fan turns off

Most likely you have something draining the battery. Quick way to tell, disconnect the ground (negative) cable to the battery, let it sit for a few hours, then reconnect it and try to start it. If it starts OK, you know the battery is holding a charge, but something is draining it.
To drain it in just a few hours requires a serious draw, like headlights or the cooling fan. It is possible that your cooling fan (behind the radiator) is staying on all the time. Often they are controlled by a thermostat, and will run for a few minutes after you shut off the engine, but it should shut off within about five minutes. If it is still running after ten or more minutes, you have a bad fan thermostat or a bad fan relay. If that is not the problem, you will have to get someone with an ammeter to measure the current draw, and start pulling fuses until you find the culprit.

There are sensors and relays for the fan [s] , sounds like you have a defective one.

My fans did the opposite and wouldn't come on at all. Last year I went through the same process (KMANIAC again being a big help), but it turned out to be the temp sender unit. Maybe a good bet is to replace them both at the same time. I think the TSU was only about $15-20

replace the speed control resistor of the fan.

Yes pull the cooling fan relay in the fuse panel. It might be bad.

Disconnect the wires going to the spring loaded switch in your hatchback.

Having the fan run on for a bit after the engine has stopped is not abnormal in itself, but 20 minutes sounds excessive.

If the fan does eventually shut off, I'd consider changing the thermoswitch (this is a sensor screwed into the radiator somewhere near the bottom, with a pair of wires connected to it - to change it, you'll need to drain the cooling system).

If the fan stays on continuously until the battery is completely flat, see what happens when you unplug the wires from the thermoswitch. If this stops the fan, you need to replace the thermoswitch. If it doesn't, then you need to examine the wiring associated with the switch and the fan, looking for damaged insulation.

A.