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Traps Open or Closed?

Looking for some responses on what everyone has experienced. Do thermastatic radiator traps fail in the open or closed positions? I know Dan always says they fail in the open position and pressurize the returns, but I have some radiators that just don't want to heat up completely and I'm wondering if their traps are failed in the closed position. Everything else on the system seems to be correct. Majority of the rads have Sarco H traps and a few are Hoffman 17C.

Any input would be  helpful. Thanks.


  • Both!

    Traps can fail in either the open or closed position.They can also fail half way if dirt gets on the seat.  If a radiator with a trap doesn't heat up it's very likely the trap has failed in the closed position.  The other possibility is a problem with the inlet valve however it is the trap that is far more likely to be the source of the problem. If you ever replace the whole trap save the old body as you can then setup a trap testing station. The easiest thing to do is to swap the suspect trap element out with a new one and see how that works. Dan's suggestion is to start at the farthest end and work back towards the boiler when checking traps. - Rod
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