Radiator control valve leaking when cold?

Hi everyone,

I apologize if this has already been asked and answered but I couldn't find anything specifically about it already on here.

Last November I did a DIY project on my home's hot water radiators. We have a gas powered boiler that feeds a bunch of cast iron hot water radiators throughout the house. The control valves on several of them were completely broken, so I replaced them myself. As you can imagine it turned out to be a bigger project than I had expected, but I'm pretty sure they were properly installed and sealed - at one point we had a plumber come in for something else and I asked him to take a look and he said they looked fine. I also closely monitored the ceilings underneath to look for any signs of leakage and never saw a thing that suggested any leakage.

Until last week. It was the end of March, so the new valves have been in place and in heavy off use (both off and on) for months with no visible problems. As it's getting warmer we switched off the heat for the system - and the next day, I saw a small but definitely new, still wet yellow stain in the ceiling underneath one of the radiators. I looked at the control valves I had installed and they felt wet - there seemed to be a small amount of water flow coming through the screws in the tops. I turned the heating system back on and the problem seems to be fixed, the stain is dry and hasn't grown.

I'm just wondering if anyone has experienced this before or has any particular advice on how to fix it or what might be wrong, since eventually I'm going to want to shut the system down for the summer and don't want a repeat of this happening.

Thanks!

-Mike

First post a picture of the valve. 2nd it may be leaking even when it's hot and it just evaporates do to the hot water.

Thanks for your reply - here's a picture of the valve:



And this is the stain underneath:



The big stain is about 4" wide and 6" long at the widest/longest points.

Thanks for replying (I thought I answered but it didn't post). Here's one of the valves, all three are the same type.


Do you think a hot water based heater (as opposed to steam) would get hot enough to evaporate the leak?

Here is the stain, it's 4" wide and 6" long, and appeared overnight:



Thanks!

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Sorry if it's not clear what's going on in the pictures

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