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Vacuum and Vents

AlexR Member Posts: 61
Questions about vacuum during short cycling on a one-pipe system.  I have a vaporstat set low (why burn more gas) and I tried to set it so the pressure stays just positive during the cycle.  But if it's too low, and the system falls into vacuum during each burn cycle (about once per minute towards of the end of the call for heat)

1) does this tend to cause Gorton vents to re-open a little after they've closed?

2) does this cause long term damange to Gorton vents?

I'm asking because some of the radiator vents don't close well and I'm trying to decide if it's the vacuum, whether I've damaged them, wet/dirty steam, etc.



  • MrDvorak
    MrDvorak Posts: 63
    edited December 2011
    Some thoughts

    Hi Alex,

    I am a HO with a pimped single pipe system and not a pro.

    Here are my observations:

    * G2 are notoriously slow to open when hot. In my system, I was experiencing "deep vacuum" at the end of "massive cycles". See this Garry Gill's video for explanation how bimetal-based G2s work: http://www.youtube.com/watch?v=Wl8vQ2MYhMs

    * IMHO, while this may affect main vents in the long term, the vacuum has a worse effect on your radiators and their vents as they do experience more rapid pressure swings. I could hear the rads vents click frantically and suck air like crazy.

    * This IMHO worsens the efficiency of the system. During heating, why would you interrupt the steam phase, displace steam with air, cool the radiators etc?

    My "short cycle" interval is about 2 minutes and I did two things to alleviate this issue: I set my vaporstat to cut out at 8oz and cut in at around 4-5oz for the pressure to never drop below ~1oz while thermostat calls for heat. I have an oil burner with a non trivial momentum, hence the high cut in. The second thing I did was an installation of a vacuum relief valve which now causes the vacuum to never go stronger than approx. -1.5 oz. My radiators are now much quieter and happier and my 3PSI gauge does not have to experience the misery of "deep vacuum" any more.
  • Long Beach Ed
    Long Beach Ed Member Posts: 1,188
    Short Cycling

    If your system is experiencing cycle times of one and two minutes, you are wasting more fuel than all the venting in the world will ever save you. 

    Find out the cause of that short cycling and fix it.  It's very bad. 
  • Not too bad

    If you are referring to me, the system cycles after 30 minutes when it gets fully saturated (in mild weather). The reason is simple - oversized boiler installed by previous home owners. I do not think my situation is unique based on what I have seen in this forum already.
  • AlexR
    AlexR Member Posts: 61
    Short cycling is from oversized boiler

    The short cycling is only at the end of the call for heat once the radiators are hot and because the boiler is oversized (rated for ~350 sqft EDR and the radiators are only ~220).  I can't do much about it short of a new, correctly sized boiler.  There's no short cycling while the system fills with steam.

    The observed problem is the boiler losing water faster than it should, probably because some of the vents don't close well- I can hear them still venting once the radiator is hot and a cold spoon held above the vent shows a lot of condensation.  So the question was whether raising the cut-in to eliminate vacuum will help, or whether repeated exposure to deep vacuum has damaged the radiator vents (bent the bimetal strip) and I should replace them.

  • Low pressure gauge?

    Do you have a high precision low pressure gauge installed on your boiler? Many folks here install them because the default code required 30PSI gauges register pressure only when then the boiler is about to explode. I could see the wild swings from 8oz to like minus 20oz clearly on the 3PSI gauge, the main vents were simply too slow to break the vacuum alone. I do not think such vacuum does the radiators vents any good. I would try to measure what is really going on in your system, alleviate the vacuum problems, then replace the leaking radiator vents.
  • Long Beach Ed
    Long Beach Ed Member Posts: 1,188

    The vacuum will cause no harm to the radiator vents. 

    If they are hissing, the pressure is too high or the steam too wet. 

    Correct these problems, along with the short cycling in your system.  Your problem is not vacuum.
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