help...mod con buffer tank

Discussion in 'Boiler Forum' started by ronbo, Dec 29, 2011.

  1. ronbo

    ronbo New Member

    Dec 29, 2011
    newton nj
    I have a natural gas Weil McClain 155 ultra (128,000 btuh)Mod Con with ODR installed in my home to manufacture spec (primary /secondary) with an indirect as priority 1 but the boiler is 2 times the size needed. My heat loss is 62,500. I have the high fire rate set to 55% (70,400btu) and the low to 20% (25,600) which is lowest setting and might set the high a bit lower to 52%,.Boost is off and it modulates at default 5 on 5 off. Should i raise to 6 on 6 off ? I also have a 008 delt pump for my four zone pumping, is this useful in this application or a problem?
    I'm thinking about a buffer tank but not sure how they are installed to cure short cycle issues i am having.Its bumping off the bottom so to speak.I tried slaving two of my four zones together which works at temps greater than 144 targets but would like lower temps.I don't think more base board will do the trick as i have whats needed now to heat at 140/design 0.
    I would love some suggestions on buffer size. Is bigger better?How is it piped?
    How much lower will it get my temps down?Is a water heater tank a good option/insulated very well?
    I have an 1 1/2 manifold/header with 3/4 branches off the mains, can i just send it from supply via 3/4 branch to the tank and back to the return 3/4 branch with no zone valve so its always pumped through when a zone calls/pump turns on) for heat unless ball valves are closed(this allows me to shut off in summer).
    do i need to up size my expansion tank also to match the new volume?
    Ty in advance for any help,Ron
    Do you live near Newton NJ and want some work/lol?
  2. Dana

    Dana In the trades

    Jan 14, 2009
    A buffer tank will not lower your operating temp by enough to matter (if at all), but it can cure short cycling, if it is indeed short cycling. What do the minimum burn times look like? How much baseboard do you have on your smallest zone?

    If your output is 144F and the return water entering the boiler is ~125F and it's firing at min-mod, you'd be getting some decent condensing out of it:


    Averaging low-90s with fin tube would be about as good as you can do. With fin-tube baseboard as the heat emitters the output is unreliable and difficult to model when you take the temps below 125-ish an the ODR curves tend not to track there, independently of short-cycling related to it. To get into the ~95% range or higher would require more radiation, and radiation of a type that works more linearly with falling temp. (Cast iron baseboard would work, or low-temp panel radiators, etc.)

    The boiler's hysteresis, the output of the smallest zone, and the thermal mass of the water in the smallest zone determines the amount of additional mass you might need. If the minimum burns are longer than 3 minutes, forget about it. If they're under 100 seconds, it's probably worth fixing. Don't know what the hysteresis is on this boiler, but 5F is probably not out of the question. (If it displays the output temp, observe the difference in output temp between when it fires up and when it turns off as it's cycling). At 25K a 3 minute burn would be 1/20 of an hour x 25K= 1250BTUs. A 5F difference with 1250BTU of intput takes 1250/5= 250lbs of water, which is 280/8.43= 30 gallons. But in fact the zone's baseboard is using a fraction of the 1250BTU, and there is likely at least a gallon or two in the plumbing, plus whatever is in the boiler itself, so odds are you'd never need more than 20 gallons of buffer, if the guesstimate on hysteresis is accurate.
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