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    Possible Short Cycling? (9 Posts)

  • Med618 Med618 @ 11:17 AM
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    Possible Short Cycling?

    I've attached what I think is an interesting graph.

    This is a 6 story, one pipe steam system with about 90 apartments. All of the top floor apartments have thermostats(each apartment is indicated in different colors). When the average reading of the thermostats hits a temperature that we pre-set, the boiler cycle ends.

    The red means the burner is firing. The grean block that it sits atop shows the period when it is calling for heat. (The yellow also means that the burner is firing but that the super has bypassed the sensor system). Notice how the red(and the yellow) are appearing in streaks as opposed to a solid block? In our other buildings, when the boiler is running it shows up as a solid block for the length of the cycle. I'm wondering if these streaks are indicative of short cycling. I also note that some of the apartment temperatures do not seem to respond to heat at all.

    Any analysis/help you can give would be much appreciated.
  • Long Beach Ed Long Beach Ed @ 11:58 AM
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    Sure looks that way...

    According to your data, it would appear that the boiler's cycling three or four times every half hour.  That would be short cycling indeed and be wasteful of fuel.  

    Figure out what's turning the boiler off and correct it.  A common cause could be a faulty pressuretrol, clogged pigtail, or control problem. 

    Could also be a problem with the graphing.  Has anyone babysat this boiler for an hour and seen this actually happening? 
  • Med618 Med618 @ 12:08 PM
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    never verified but..

    I've never verified the graphing. We have tremendous problems with heat balance in this building. I'm wondering now whether our venting strategy(if we have any) is all out of wack and whether a lack of venting on the mains and risers might be causing a rise in pressure and shutting the burner down.
  • MrDvorak MrDvorak @ 12:07 PM
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    Neat charts!

    That is a neat visualization of data!

    I asked the same question a while ago for a much smaller house but the principles are the same. See http://www.heatinghelp.com/forum-thread/138326/Q-about-cycles-what-is-short-cycling-and-what-not

    The thermostat can ask for heat for longer time than it takes the boiler to reach the cut-out pressure. Pressuretrol will switch the burner off when the desired steam pressure is reached, and it will switch it on again later when the pressure drops below the cut-in setting. Heat is on as long as there is steam pressure in the system. So this cab be pretty normal as long as the building is heated evenly.

    A too quick onset of this cycling is an indication of an oversized boiler though. In my case, it happens only when recovering from a setback temperature.

    Edit:I have just read your response to the Qs above and your situation is obviously different from mine so you can disregard what I have written here. If your boiler starts cycling before steam fills the system and all radiators get nicely hot, you have a short cycling problem (bad venting, bad pressuretrol, etc.).
    This post was edited by an admin on November 11, 2011 12:20 PM.
  • Med618 Med618 @ 12:19 PM
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    but isn't it inefficient?

    Thanks for the reply. But wouldn't that mean then that the boiler is oversized and/or some other type of inefficiency is occurring?

    It seems to be that a burner going on and off in such short increments is incredibly inefficient.
  • MrDvorak MrDvorak @ 12:25 PM
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    Efficiency

    Not really. More steam or bigger pressure does not necessarily result in more efficient heating. When the system becomes saturated, there is no point in supplying more steam. But you said you have problems with uneven heat, hence your system is likely not saturated when the cycling begins, and you are suffering from all kinds of inefficiencies.

    Another edit: I should take more time to read and write responses. Oversized boiler = less efficient than a properly sized boiler, correct. But, you may have other problems - venting, malfunctioning controls, etc, instead. I think I am confusing you more than helping, I apologize for that. Dave in QC has some excellent answers in my original thread.
    This post was edited by an admin on November 11, 2011 12:37 PM.
  • nicholas bonham-carter nicholas bonham-carter @ 12:21 PM
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    graphology

    the problems associated with steam systems can be divided into 2 major divisions:
    1.controls
    2.steam supply with sub-categories
       a.dry steam production
        b.air removal
        c.condensate return 
    if the boiler were short-cycling, then that could more easily be seen in the boiler room, rather than by studying bits of graph paper with the room temperatures on them. as the boiler is firing. if it is cutting out on pressure before steam has begun in earnest, then the air is having difficulty in getting out, due to bad main venting. it helps to let the boiler cool down before firing to make this observation. if the boiler is short-cycling while steam is being made, then it may be over-sized, and needs to be down-fired. probably, it has a modulating burner on it for this size boiler, so is it working, controlled by the vaporstat? are the vaporstat settings verified with a good low-pressure gauge? as you only need 8 onces of steam pressure to get up to the top floor, the standard 0-30 psi gauge is almost useless for diagnosis, even thogh it must remain for code purposes.
    if there were only a single thermostat, this system would be easier to diagnose, but the addition of averaging thermostats makes it harder to see how the system is functioning. with one thermostat, the aim is for a steady temperature in that area. if not. then the steam supply side can be checked.
    i suggest you make sure for the moment that the venting is adequate [to the top of the risers], and that the pressure is low. when you override the averaging controls, does the steam arrive at all the top floor radiators simultaneously?  this has to be verified by feeling the radiator surface temperature--not the room temperature. there could be a down-feed riser piping arrangement which would supply steam to the top first, thus satisfying the thermostats too early. since you have so many apartments, the cost of renting an ir camera to "see" the movement of steam through the pipes would be justified.
    are the averaging thermostats occupant adjustable, or are they simply sensors? if sensors, then what sort of system is running the boiler?--nbc
  • Long Beach Ed Long Beach Ed @ 1:37 PM
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    Venting

    Med, if it's cycling on pressure before all the radiators are full of steam, there's likely insufficient venting. 

    If the tops of the risers are not getting hot at approximately the same time, there's likely insufficient main venting. 

    But have you looked in the boiler room?  The thing could be cycling for other reasons.  Like low water, clogged returns, dirty water, bad feed pump level control, etc.  
  • nicholas bonham-carter nicholas bonham-carter @ 5:47 PM
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    short-cycling boiler

    have you been able to see the cause of the short-cycling in the boiler room?--nbc
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