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Radiator not Heating

I've been troubleshooting the 22 radiators at my home one (sometimes a few) at a time to get them all working well again. I've gotten down to my last few and was hitting homeruns until the one in a large bedroom. The USR CORP radiator had a vintage Hoffman air valve that seemed a little loose. I went to screw it out and it fell out - not from shear, but because it was stripped so badly they could wedge it back into the radiator. Obviously a new valve won't go in, and during the period it was out for me to take these pictures and wedge it back in, it was heating.

How and what size tap do I need to use to either re-thread, so I can screw a valve back in. Also - at the top offset of the radiator, there is a screw that looks like the valve could either be top mounted or mounted in the middle. Should I try tap and insert the valve in the middle opening, or seal it and place the valve in the upper opening?


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,116
    edited November 2022
    Tough to see without a reference like a dime or a ruler next to the hole, but the standard pipe size for radiator vents is 1/8" NTP. That pipe size tap is usually a part of the standard SAE Tap and Die set, or you can purchase it separately at ACE or True Value hardware stores, or Auto parts stores that sell tools. The problem is, there may not be enough metal to take the new threads.

    You may need to tap and thread a 1/4" NTP pipe opening and install a 1/4" x 1/8" bushing
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Steamhead
    Steamhead Member Posts: 16,270
    First, try the 1/8" pipe thread tap. It may just need to have the threads cleaned out.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • bburd
    bburd Member Posts: 683
    edited November 2022
    Do not use the upper vent tapping on that radiator. That is for hot water systems. If you use it for steam, the radiator is likely to heat only partially.

    The only time it should be used for steam is if two vents are needed to get the radiator to heat quickly enough. That way the top vent will close first once the air is out of the riser, and leave the lower vent to fill the radiator more slowly so that steam and condensate don’t clash violently in the piping and cause trouble.

  • mattmia2
    mattmia2 Member Posts: 8,435
    It looks like the threads may have corroded/sheared off that valve, so you might get lucky and might be able to thread a new valve if you clean it up with a tap. It is hard to tell from that picture. Otherwise you will have to drill and tap to 1/4" and use a bushing as @EdTheHeaterMan said.
  • mgambuzza
    mgambuzza Member Posts: 10
    I want to thank all for their input on this. After a VERY frustrating search for a 1/8 tap, I easily found a 1/4. Drilled, tapped, screwed, and inserted a leftover Leader air valve, and voila - heat!! Grateful for the advice, and confidence to do this. This was the last radiator to repair and just in time for the sub 20 degree weather.

  • Jamie Hall
    Jamie Hall Member Posts: 21,547
    nice work
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ethicalpaul
    ethicalpaul Member Posts: 4,822
    Nice job. I was gonna say Ed was being a little optimistic thinking a 1/8" NPT tap is available at all those places. But it's easily available at Amazon.
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG