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Question on vaporstat

Alex265 Member Posts: 41
Is it normal for a steam boiler, controlled by a vaporstat, to short-cycle only at the end of the heat cycle?

For example, it will take ~30 minute for the boiler to satisfy the thermostat setting. First ~20 minutes boiler is on without interruption with pressure running couple of ounces below cut-out, then the pressure builds up, and for the last 5 minutes vaporstat starts to cycle the boiler on/off every 40-50 second - this is what it takes the boiler to go down from 8oz. (cut-out) to 0oz. (cut-in) and back again. Thank you.


  • Short cycling?

    My boiler does the pretty much the same. It will run for 13-14 minutes till it hits the 13oz cutoff and then cycle 6-8 more times between the 3oz cut in and the 13 oz cut off; in this phase it fires for 45 seconds and then shuts off for 110 seconds until the thermostat is satisfied.

    In my case the 13 year old boiler is almost 3 times as big as it should be and, short of replacing it, there is not much that can be done. Mine is oil fired and you just can't downfire that much. When the time comes I'll replace that 5 section boiler with a 3 section, but untill then I'm afraid I'll have to live with it.

    How many sq ft of radiation do you have and how many sq ft is your boiler rated for? The two number should be pretty close for optimum operation. If you are using gas you may have other options with the boiler you have now.

  • Alex265
    Alex265 Member Posts: 41
    This is exactly the behavior

    Bob, thanks for your reply. Yes, this is exactly the behavior. I don't have the exact numbers right now, but I do know that the boiler is (significantly) oversized, it's 2 years old and came with the house. It's gas. I though about trying different cut in-cut out ranges, but I have a feeling this won't change the picture much.

    Can you please tell me more about the options you mentioned?
  • gas boiler too large

    With a gas boiler there is an option to modulate the gas so you can feed it at a lower rate (resulting in less BTU's and a longer burn). I believe they use a vaporstat to cut down the gas flow as soon as the boiler starts to make steam. This is called a modulating burner and I really don't know much about retrofitting them to an existing boiler. The fact your boiler is pretty new is a plus.

    My knowledge on this area is limited because I've always had an oil fired system. Try searching this site for modulating boilers.

    Perhaps someone more knowledgeable on the subject can offer some assistance?

  • Steamhead
    Steamhead Member Posts: 14,864

    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Alex265
    Alex265 Member Posts: 41
    Question on vaporstat

    Thanks, Steamhead.

    By the way, Bob, can I ask you why your cut-in pressure is set on 3 oz., and not 0 oz.? Won't lower setting slightly decrease the short cycling even if by several seconds? Why is 3 oz. better than 0 oz. in your case? 
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Minimum cutin pressure

    It may not be a good idea to set your cutin pressure all the way down to 0 oz. If the pressure drops that low before the burner fires, the system may drop into vacuum and start to suck air into the vents before pressure builds again. Leaving the setting higher, at 2 or 3 oz will insure that the system is still filled with steam when the next burner cycle begins.
  • Alex265
    Alex265 Member Posts: 41
    Question on vaporstat

    Mike, thanks for your reply,

    but is it really relevant to our 'last 5 minutes short-cycling' case? At least, if I set my cut-in to anything greater than 0, it will just reduce the 'off' time by 5 or 10 seconds.
  • David Nadle
    David Nadle Member Posts: 624
    Is it really so bad?

    My boiler does the same thing after cut-out and it is definitely not oversized. But isn't any boiler sized for a cold load going to be way oversized for a hot load anyway? In my experience it doesn't much matter what you set the cut-in pressure to; my system is under vacuum less than a minute after cut-out. A more complex control system could address this issue but I have doubts that this end-of cycle chatter hurts overall efficiency enough to worry about. Some empirical data would be nice. 
  • Why not 0 PSI?

    I think it comes down to not wanting the steam to collapse completely and thus draw air into the system. The question is how low can you go so you can keep the burner off for the longest amount of time during these end of cycle roller coasters. I think you want to keep some pressure in the pipe but I don't really know how low I should go.

    I have an elapsed time meter on my boiler so I can track the burner 'on time' down to 1/10th an hour. I can easily just lower the cut in and see if it makes any difference in fuel usage. It is 55 right now so I'll have to wait till we see cooler temperatures because the system only calls for heat early in the morning with temperatures like this.
  • AlexR
    AlexR Member Posts: 61
    edited March 2010
    3oz may not be 3oz

    On my boiler, there's a delay between the vaporstat turning the boiler on and ignition- it has to wait for the automatic damper to open and the boiler also seems to have a little delay of its own.  So I have my vaporstat cut-in set at 3oz but the actual cut-in is much closer to 0; I arrived at 3oz by watching the low pressure gauge during operation and turning down the cut-in as much as possible without going into vacuum.

    My boiler cycles just like yours does.  It's on continuously until the radiators are all hot, at which point the condensation rate drops and the pressure starts to rise. Then it short cycles until the thermostat is satisfied.  You'll get varying opinions as to whether a smaller setback wil save fuel, but I don't think there's any problem with your situation (other than wearing out the autodamper quicker than you might otherwise).
  • Alex265
    Alex265 Member Posts: 41
    Thanks for the link

    to the thread, it's really interesting what you are doing. And I understand yours and Mike's explanations about danger of too low cut-in, makes sense.
  • Alex265
    Alex265 Member Posts: 41
    Thanks, this makes sense.

    In my case though there's no delay (just half a second, may be) between the vaporstat turning the boiler on and ignition. And actually, my main concern here is not wasting fuel but - just like you said - possibility of wearing out the autodamper.
  • Alex265
    Alex265 Member Posts: 41
    Another question

    I'm sorry for this question, and it's definitely not worth opening a new thread, so I'm asking here. What is the purpose of pressuretrol on a float-type low water cut-off? What should be its settings?
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