The overflow tank is only to keep air out of the system. Since hot liquid (and gases) expand it has to go someplace or the system would overpressurize (bad). Then when the car cools off the coolant contracts sucking coolant from the overflow tank. If the overflow is empty it will suck in air instead of coolant, filling up your cooling system with air.

You were/are getting heat because now coolant is flowing everywhere it's supposed to be. Believe it or not your car's interior is heated using the coolant, which is why most people tell you if your car is overheating turn your heat on full blast. And you may not have all the air out of your system if you didn't turn on your heat when you filled your car with coolant, since there was air in the heater core. Your cooling system was probably almost bone dry, which is why you were overheating.

When was the last time you checked your coolant or topped it off? This is why basic maintenance is important to follow. I usually do this:

Once every other day or so:
1) check for flats
2) check for odd scratches or rust spots

Once a week:
1) check air pressures
2) check oil levels

Once a month:
1) check coolant levels
2) check washer fluid
3) check battery terminals
4) check engine area for any weird fluids or anything leaking

I'd still have someone do a pressure test just incase to see if your headgasket is gone. If you let the car overheat too much or too long you could have warped the head which is also bad. Honda engines are made out of aluminum which is great for saving weight, but bad since they are delicate.

 

Right. Thanks for the info. Honestly, I am not sure when was the last time the coolant was checked and/or topped off. The civic is my girlfriends car, and she is pretty hands off when it comes to her vehicle. Believe me, I have already lectured her on the necessity of doing the basic maintenance inspection laid out in the last post.

Do you think there is any reason why neither fan is kicking on though? Could this be a thermostat problem? water pump? timing belt?

 

One fan is the condenser fan for your AC (usually driver's side), the other fan is for your cooling system. The fan should have come on at least once in the 15-20 or so minutes you were warming up your car. If it's not then you might want to get that checked because that could be the overheating problem.
1st thing I would check though now that she/you have driven the car is to open up the radiator cap (of course when the car is cool) to make sure the coolant level hasn't dropped. If it has I would top it off. And check it again maybe in a day or so. If it's still dropping after a week then we have a problem.

 

Excellent. So far so good. I have drive the car a few times in about 20-30 minute intervals and the coolant levels have remained the same in both the radiator and the reserve tank. I still have not heard or seen the radiator fan kick on, so I will definitely get that checked out asap. I will continue to monitor the coolant levels diligently. Thanks again for all your help SIRacer, your input into this problem is very much appreciated.

 

let the car idle and wait say about 15 to 20 mins and the fan should kick. If not, you will be looking for electrical problem, maybe a fan relay.