Radiators

My house was built in 1870. I have no idea how old the heating system is. The heating guy estimated the boiler to be 50 years old. This is a big house, with 13 radiators in all.
I went to turn the radiator on in one room and the valve starting leaking. There's a screw in the top, and a big nut underneath. I got it to stop leaking by cranking everything as tightly closed as I could. Screw, valve handle and nut. Now the thing is off.
I am a home repair idiot, but I'm willing to give this a go. How do I fix the valve? How do I fix the valve without having water everywhere? If I shut the whole system down, will the radiators be empty?
I was always under the impression that these were hot-water radiators. Are there both steam radiators and hot water ones? I've never had any hissing, knocking, etc from the radiators.
Thanks Jena
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You don't even know if you have steam or hot water? If there is a sight glass on the boiler, and you can see air above the water, it is most likely steam. If it is steam, there should not be pressure at the valve when the boiler is not running.
To fix the valve you can get something that Ace Hardware calls Faucet Packing Graphite (graphite impregnated rope). Back off the nut where the stem comes out of the valve body, wrap the rope around the stem under the nut. Tighten the nut so it is snug, but not too tight, so you can tighten it more if or when it leaks.

The old cast iron radiators were originally steam. Some people convert them to hot water, but some radiators work better for that than others. For more info about steam or hydronics see http://www.heatinghelp.com/ and The Wall forum. Dan's book The Lost Art of Steam Heating can help you understand how your system works (if it is steam).
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Why would a "home repair idiot" buy a big old house built in 1870? Hope you have deep pockets.
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