Hello and welcome to JustAnswer!
Does the temperature gauge warm up to it's normal position, or does it stay very low near the bottom while driving?
Do you get any heat at all - does the air warm up a little but not enough?
Does it get any warmer when you are accelerating?
So were you changing the coolant, and that's why you were filling it up? Or was it low and you were topping it off?
Was the heat working before that?
Did you bleed the cooling system when you filled it back up?
I was topping off because it was low.
The heat did work prior to the last couple of days.
No I did not bleed the cooling system before filling it back up.
Again thank you for your time.
I think you probably just have air in the heater core. If you had a thermostat stuck open the engine wouldn't get to normal operating temperature. Bleeding the air out will probably take care of the problem.
With the car cool to avoid burns, remove the radiator cap and make sure the bottle is near full. Start the engine and leave the cap off. While it's warming follow the top radiator hose to the engine and you will find the coolant bleeder housing where the hose attaches. It will have what looks like a brake bleeder screw. With the engine running open this bleeder and leave it open until you get a full stream of coolant with no air and then close it. Allow the engine to run with the cap off until it reaches full operating temperature. Have the heat turned on so you can feel what is going on. When it gets to operating temperature if you still have no heat raise the rpms a bit and hold them there for 20-30 seconds, and you will probably start to feel heat. This will cause the water pump to force coolant through the heater core and purge the air out. Throughout the whole process you'll want to keep an eye on the coolant level, and periodically open the bleeder screw to check for air. When you have good heat and are no longer seeing any air bubbles come up in the coolant bottle or from the bleeder, then make sure the level is correct and put the radiator cap back on. Throughout the process be aware of the moving belts and pullies, and be careful of the hot coolant and steam.
If you had to add very much coolant to start with you may have a leak somewhere. Keep an eye on the coolant level for the next few days even if you don't see any leaks on the ground.