In 2005 chevy impala the heater only gets warm sometimes. Other times it blows cold air. I put fresh antifreeze in. And there is no water on the floor of the car

You need to fix the coolant leak, then refill and bleed cooling system, then you will get heat and cure overheating problem too

providing your coolant is full cause low coolant will cause no heat
and your temp gauge is normal in the middle then most likely u have a bad blend door actuator usually on the passenger side

It could be your heater core or check your over flow tank for the radiator to see if it is your antifreeze is up to the line marked on the side of tank and check the over flow hose, make sure the clamps are tight.

Try flushing or blowing high pressure air through the heater core lines. its sludge build up and need to be flushed through heater core only. check antifreeze fluid level. only put in straight antifreeze, do not mix. most people do not know what mixture should be, so be safe and add straight, it will not hurt it, i run 100% straight antifreeze and never had a problem or worry about it ever freezing and cracking my block, heads or radiator cracking.

please check the fuses AC/cruise and AC/fan Located in the fuse panel on the drivers side end of dash

check your thermostat and make sure you have antifreeze 50/50. My 2000 Chevy impala was blowing cold air, but the previous owner had put some radiator stop leak in the radiator that clogged the whole system. I used a garden hose and flushed the heater core and the water pump.

did you put the thermostat in backwards ?

Quite a few idea's come to mind.
1) Thermostat is stuck open, causing the engine to run cool.
2) The vacuum valve that opens and closes the air "flap" is stuck open, allowing cool air in all the time.
3) The vacuum valve that opens and closes the line that feeds the heater core isn't opening all the way.

once truck is warm check both heater core hoses to make sure they are both warm if not then check the cold water valve could be stuck shut not allowing coolant to travel in to heater core.

Does it blow warm air when you drive the car? If so its the water pump.

Could also be air in the coolant system

Also have the cylinder head pressure tested to rule out a hblown head gasket
Hope this helps

Regards
Steve