Yes - it is water. Sometimes if you are in a very cold climate and have a newer system, they will put an additive (like antifreeze) in it to prevent the pipes from freezing if the boiler is in the garage or a colder area. Since water expands and contracts as it changes temp, it is not a closed circuit - If you follow the pipes at the boiler you should see a water supply line which will allow new water to be added when needed. Newer models will also have an expansion tank and an automatic air release in case air gets in the lines. Older models you can usually release the air by turning a screw in the radiator. if when the heat is on, a part of the radiator doesn't heat up it probably has air in it.
It's generally just plain old water. An a/c may use freon or other gas, but heat can be transferred pretty easily just by using water.
If anyone else has this problem then I'd recommend checking to see if there's any air in the system - which may involve bleeding the radiators. If that does not work - or if you have not got the time, then I'd recommend STL Heating because when I used them they were able to solve a loosely similar problem in less than an hour, plus they did'nt charge a fortune - unlike the last nationwide I used!
