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Fogged Mirror

vinceM Member Posts: 81
Hello all, I'm steadily getting a handle on how my steam system functions. For curiosity I held a small mirror about an inch above the rad air vents on my second floor rads and found moisture would form on the mirror as it vented. Also, there seems to be a slight "congested" sound coming from the vent at the same time.
Any thoughts???
Thank you


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,564
    This is probably due to wet steam coming out with the air.
    Is the boiler water clean, and oil free?
    Is the supply piping proper?--NBC
  • Fred
    Fred Member Posts: 8,518
    Vents being what they are, will close but I'm not 100% sure they completely seal 100% of the time. Any little bit of dirt that may blow up into the orifice may be the cause. If the stem doesn't hit the seat exactly centered each time, it may leak a small amount on that particular closure but may close completely the next time. I suspect if any of us were to go around to each of our radiators and hold a mirror up to the vents, we'd find at least one, maybe more that fogs the mirror, on any given heating cycle and probably different ones on the next heating cycle.
    Keep the pressure set where it should be, the boiler clean, the radiators pitched correctly and if a vent has obvious issue, you probably won't need a mirror to find it.
  • RomanGK_26986764589
    RomanGK_26986764589 Member Posts: 229
    I have the same issue with my vents. Most of them fog the mirror up a little.
  • Fred
    Fred Member Posts: 8,518
    You guys have given new meaning to "Smoke and Mirrors" :)
  • Mark N
    Mark N Member Posts: 1,094
    The air being vented from a steam radiator vent will be warm and humid. So it will most likely form condensation on a colder mirror. That doesn't mean it is leaking. If the vent is known to be closed and this is happening it is leaking.