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Plumbing in the Home/Draining heat system to repair leaky valve


QUESTION: Hello Jay, how are you doing? I need to repair or replace leaky shut off valve to a radiator. That is no problem. It is on the inlet of a radiator for a force hot water system. My question is how much water do I need to drain out of the system so the level is below where I will work on? This radiator is on the first floor and there are others on the second floor. It is one big zone. I know to turn off the burner and shut off the water supply then drain, but how much how do you know when enough is enough? Also when do I bleed air from the system before or after I turn the supply back on? Thank you so much for any input. I will consider donating as I think this will be the second time you have helped me. Thank you.

Best regards,

ANSWER: David.
Based on you question, I am assuming you have free standing cast iron radiators. Be aware that you will need a special radiator  valve removal tool to replace the valve. The valves on these radiators are a two part system that uses a ground joint union connection. Half of which is threaded into the radiator. This is where the tool come into play.Finding one may be a bit tricky they are not common anymore. Check a plumbing & heating supply store. Whoever sold you the valve should know about these tools.
OK,.. Clean and scrape as much paint and crud off the threads and squirt  break-free or any good penetrating oil on the threads and let it work.The valves are brass and soft.  The radiators are cast iron and the dissimilar metals can create a issue when trying to separate them.  
As for the draining. You can do the trial and error method and get water all over the place or be safe and drain the entire system.(recommended) Open the drain  on the boiler and start the  flow then go to the second floor and open a vent on a couple radiators. When it's time to refill, close the vents and fill normally. start venting the radiators on the first floor starting with the one furthest from the boiler. take a full water glass full from each radiator. Then do the second floor the same way. When all is bled, fire up the boiler, let run for 30 to 60 minutes and then vent all radiators again the same as described above. The only difference is you only need to pull a cup or less from each radiator. If the water coming out on the second round is not hot, stop and allow the boiler to run longer and heat up before bleeding any more air out.

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Shut off valve
Shut off valve  
QUESTION: Hello jJay,

Here is a pic of my shut off valve.  I don't think I need that tool for this do I? Thank you...David

David, if you plan to replace this valve you will need that tool. UNLESS you get an exact replacement brand for brand, size for size then you won't have to remove the part that screws into the radiator itself.  That large nut is part of the valve assembly and comes with the new valve.To remove the old one you have to unscrew the nipple from the cast iron radiator. The tool is inserted into the opening and is used to unscrew the part out.  an exact brand replacement should mate to the old one.


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Jay Nicholas


Plumbing & Heating (warm air, hot water & steam heat)


40 years in the business, 40 years as a lic. master plumber in NY State. Retired

Graduated Magna cum laude at the School of Hard Knocks

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Commercial, residential and light industrial. You name it ...I have probably worked on it

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