How best to close-off this hole?

[Nehmo]

This (un-threaded) hole is covered in mineral deposits, and the 1/2" pipe holds hot heating water. I want to close the hole, but I'm not familiar with the procedure. (I realize I need to clean it first.) What can I use as a plug? Do I have to make something? Is there a copper fitting for this purpose? Should I try just plumber's putty? Will epoxy and something that can screw-in work?

 

[Jeff Handy]

The whole thing looks totally corroded and ready to burst or crumble.

You will likely be throwing good money after bad, if you waste money and time trying to get it in service.

 

[Mitchell-DIY-Guy]

This looks like some kind of radiator. You would block the hole with a cap, or crimp, and seal with solder.

In doing so however you will change the flow through the unit—basically turning it into a brick.

The only thing you should do to this is replace it.

 

[CT18]

You need to braze a plug into that. Contact a HVAC tech as i am sure they will have the necessary equipment to do the job

 

[Nehmo]

This looks like some kind of radiator. You would block the hole with a cap, or crimp, and seal with solder.

In doing so however you will change the flow through the unit—basically turning it into a brick.

The only thing you should do to this is replace it.

The piece of equipment is part of the heating system of a large ( ≈ 40 units) apartment building. I don't live there, but someone I care for does. Yes, I realize the management is responsible for the maintenance, but they are not doing it, and they are unlikely to get around to it anytime soon. I plan to fix it myself.
Actually, electric space heaters are allowed. The electric bill is paid by the management.
If there were a plug in that hole, and if a nearby valve were open, then a thermostat would control another valve which would allow hot water to go through the coil.

 

[Jeff Handy]

If you personally do any repairs to that crappy old radiator or air handler, then management can shift blame and financial loss onto you when (not if) it ruptures and floods, maybe taking out a boiler or water heater in the process.

 

[Nehmo]

If you personally do any repairs to that crappy old radiator or air handler, then management can shift blame and financial loss onto you when (not if) it ruptures and floods, maybe taking out a boiler or water heater in the process.

I thank you for being concerned about my legal/financial safety, but it's not the issue.
I happened to run into one of the two maintenance guys on the elevator. He was in a partial stupor, smelt of alcohol, and was carrying, with his tools, a cup whose contents he was accidentally spilling on the floor. One of the other passengers advised him, "Hey, you're spilling the good stuff." The maintenance guy awakened from his stupor a bit and realized that he was spilling his drink. He corrected its level and said, "Damn, I lost a lot."
I saw him again later and I asked about the heater. He said it needed "a new coil-thing". He then added that "We can't do that." I don't know if he meant the inhibiting factor was his skill, budget, or license.
I'm going to try to find some kind of plug I can solder into the hole. A rubber one might not be good considering the pipe gets hot.

 

[Jeff Handy]

It is so corroded, soldering seems unlikely to hold water or pressure.

Unless you can get the surface shiny bright clean.

 

[Dshow]

I wouldn’t touch it, When anything fails... You will be the scapegoat.
Even if you can convince someone... It was an old system... I was just getting it to work in my space... I couldn't have caused that other issue... The time and effort, even if you succeed to convince someone, is not worth it.

If they pay the electric bill... I would spend my time/money looking for my new electric heat source!

(Looking at the picture again... It must not have fluid in it now that runs through the radiator. If you do seal it, do you have control over the entire system and could get it working? If you get the cap on there, it may just be the tip of the iceberg to get the system running again.)

 

[Nehmo]

I wouldn’t touch it, When anything fails... You will be the scapegoat.
Even if you can convince someone... It was an old system... I was just getting it to work in my space... I couldn't have caused that other issue... The time and effort, even if you succeed to convince someone, is not worth it.

If they pay the electric bill... I would spend my time/money looking for my new electric heat source!

(Looking at the picture again... It must not have fluid in it now that runs through the radiator. If you do seal it, do you have control over the entire system and could get it working? If you get the cap on there, it may just be the tip of the iceberg to get the system running again.)

The system runs for the other units and provides them heat. At every apartment, a local valve, which is currently closed, controls the water through each individual unit. If the picture I took were larger, it would show the valve.
Since the apartment is surrounded by other apartments, I don't suppose it will get really cold. And, of course, a space heater helps.