What do you mean by "hydronic radiators?". I am assuming you have old cast iron radiators, not newer panel radiators or fin tubes. Are you asking because you want to fix it yourself? Are you a good DIYer? If this is your first plumbing project, might be easier to just call a plumber. It's not rocket science, but dealing with getting old fittings off, refilling and bleeding a heating system, etc are a bit harder than things like fixing a dripping faucet.
You can probably just add a valve where none exists. The valves have a union one one side (radiator side) and standard pipe thread on the other. How the swap out goes depends on what you currently have. The valve has to be bought at a plumbing supply place--usually these only sell to licensed plumbers, but there are also outfits on the net that sell such parts. (Don't waste your time at Home Depot). Check out
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On the radiator that won't come on, there are several things that could be wrong. Most likely on an old system is that the valve is stuck shut. (I had to change every valve when I bought my house). Could also be full of air. Plan A would be to bleed it. If that doesn't work, then the valve has to be changed.
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