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On Delay Timer for a steam boiler

In all these years as a thermostat rep, I've never heard of setback causing short cycling. A thermostat is just a switch. When the temp is below setpoint, the switch closes to bring on the heat. I just had an idea. Wen the Th8000 series is calling for heat, there's a little flame symbol on the display beside the word Heat. As the short cycling is happening, you could see if that flame is there. If so, the stat is calling for heat, and isn't causing the equipment to cycle off.

Comments

  • Tom Hopkins
    Tom Hopkins Member Posts: 552
    Recommendation on an on delay timer

    My house is heated with a 2 pipe steam system, which works very well all in all. The one problem it has it the boiler short cycles on pressure if reovering from more than a 2 degree setback. These off-on cycles are approximately 60 seconds in duration.

    I thought of 2 possible solutions (and welcome others)
    1. A thermostat that has more than 4 programmable temperatures per day so I can step the temperature up in say 2 degree per hour increments. I currently have a VisionPro 8000, but from what I can tell, it only has 4 set points per day. We usually only need a step up in heat in the morning and another in the evening.

    2. Installing a on delay timer or realay in series on the off side of the vaporstat circuit that could be set to delay the boiler from cycling back on for a preset time once it cuts off on pressure. The burner circuit is 125VAC and the current draw looks to be around 8 amps. Would a device such as this unit work? http://www.grainger.com/Grainger/items/6A855

    Thanks,

    Gary
  • carol_3
    carol_3 Member Posts: 397


    The Vision Pro 8000 has a function in it--called adaptive intelligent recovery--that brings the heat up slowly from night setback. So unless you went into installer setup and removed that feature, it's already doing what you're thinking about. I suggest you post your question the main part of "the wall" at this website so the steam guys can see it. They may know something.
  • carol_3
    carol_3 Member Posts: 397


    The Vision Pro 8000 has a function in it--called adaptive intelligent recovery--that brings the heat up slowly from night setback. So unless you went into installer setup and removed that feature, it's already doing what you're thinking about. I suggest you post your question the main part of "the wall" at this website so the steam guys can see it. They may know something.
  • Tom Hopkins
    Tom Hopkins Member Posts: 552
    Adaptive Intelligent recovery

    Thanks for your reply, Carol.

    We have chatted about short cycle times on the Wall quite a bit, actually. The general fedback is that the short cycling is a consequence of the setback. One suggestion is to set back less, but looking at the math, even the 20 or so short cycles (1 minute off, 1 minute burn) that I see would consume as much oil as just one maintenance burn at night. SO, I am hoping to basically devise a system where the boiler cuts out on pressure and then cuts back in based upon off time (assuming the tstat is still calling for heat.

    The Adaptive Intelligent Recovery is enabled, but the short cycling continues. The issue is that when the VisionPro 8000 is in a recovery mode, it does not seem to recognize the time when the boiler is off via pressure. I am a little surprised; I believe the thermostat voltage is different when the boiler kicks off by pressure as opposed to when it meetis the thermostat set point.

    The other option I was condidering (but know only enough to be dangerous) is to program the VisionPro for a single stage heat pump and connect the white tstat wire to both the compressor and fan terminals, then set the compressor on delay to 5 minutes. A 5 minute off time would be a big improvement over the current 1 minute. Am I asking for trouble?

    Thanks

    Gary
  • Gary Segal
    Gary Segal Member Posts: 37
    The thermostat is doing what it is supposed to ...

    Hi Carol, I do not think I explained the problem too clearly. I believe the thermostat is doing what it is supposed to - that is call for heat when required. The problem is that on a steam boiler, there are multiple ways to end a cycle. The two that concern me are:

    1. Setpoint is reached - thermostat cuts off the boiler.
    2. Vaporstat cut-out pressure is reached before temperature setpoint is reaced. In this case, the vaporstat cuts off the boiler and will not let it restart until the pressure in the boiler drops to the cut-in pressure.

    So in my case, the boiler needs to run for a long time to recover from a setback, so it cuts off on pressure before reaching temperature setpoint. After about a minute, steam in the system condenses sufficiently to reach the low pressure cut in point. Then it cuts back in, runs for a minute and the cycle repeats.

    This off-on cycle is what I am trying to lengthen via a timer. The reason I feel it will be more efficient is that for these short cycles, what is primarily happening is the steam main is getting repressurized. It is not making my radiators hotter.

    I did try to set the VisionPro up for a heat pump, connecting the white whire to the compressor (was either Y or G). I then set the compressor on delay to 5 minutes and tried manually cycling the vaporstat to see if the thermostat would keep the boiler from firing for 5 minutes. It did not. However, when I cycled the boiler on an off by lowering and raising the tstat temperature, it did hold the boiler off for 5 minutes.

    So in summary, what I really want to have is a system that shuts the boiler off on pressure, but starts it based on a set time delay. I am trying to trick the VisionPro into doing the task for me, but it is not quite working.

    Thanks again,

    Gary
  • heatguy
    heatguy Member Posts: 102
    no diffrence

    the tstat is a switch just like the pressure control there is no diffrent voltage its either open or closed.post on main section sounds like you may have your pressure control set a little low or maybe some steam in return side of system.if you reset your pressure control remember low pressure is best and you shoundnt mess with it if you dont know what your doin.
This discussion has been closed.