It was an old peice of junk with the old hydronic heating system. First there is a boiler, and a curculator pump. Somewhere on the system is a tank with air in it. This acts like a pressure tank to regulate the water pressure in the system as hot water expands. Also somewhere it will(may be) conected to a water supply to easily add water. Air is purged usually by a valve on the radiator. There is a key temperature - how hot the boiler heats the water and then curculates it. Mine was completely manual and I had to learn over time the best temperature to circulate the water.
So I lived with my hot water heater (happily) and had it down pat in a few years. I had little luck with getting information but eventually I became a master of the beast. Here are some does and donts that I learned over time. 1. Dont ever consider draining all the water and adding fresh water with out some consultation. The water in the system has to be conditioned to prevent rust. 2. The curculating pump is the key to proper heating. Somewhere you will find a thermometer that measures the temperature of the return side of the system. You will want the return side temperature about 3 to 4 degress less that the out bound side. This means the boiler uses less fuel to heat the water. Basically, you may find yourself adjusting the boiler temperture manually daily or weekly. IE Its warmer today, I would lower the boiler temperature. 3. In early fall and late spring reduce the temperature of the boiler. This will save you bucks in fuel. 4. If it leaks and the plumber doesn't (or you)want to fix it, you can shut down a rad in the room by the valve and run a base board into the room. 5. Dont paint the radiators 6. And finally, I have since discovered they have cool little computers that measure the out side temperature, the boiler temperature and the return temperature and do all that stuff for you now. 7. Big old buildings still have hydronic systems, so there are guys who know all about them out there, so dont worry.