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    Lead lag set-up for two steam boilers?? (8 Posts)

  • Kmac Kmac @ 6:51 PM
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    Lead lag set-up for two steam boilers??

    Recently a client of mine had her 600,000 btu steam boiler replaced. The contractor, not me, installed two 300,000 btu burnhams. He told her that the boilers were set up so only one boiler would run, and if necessary the second on would kick in if needed. Both boilers piped into same 4" header. A quick look didn't reveal any special controls, tekmar etc. There's a single thermostat in old large house.
    Is it possible? I've never seen it done without actuated valves with special controls etc..
    Nothing like this on this system. House is vintage 1910.
    Any thoughts
  • Jamie Hall Jamie Hall @ 7:22 PM
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    That's one big setup...

    I contemplated the possibility of such a thing for the building I care for; gave it up as unnecessarily complicated.

    There is one significant concern: the piping must be arranged so that firing one boiler neither raises nor lowers the water level in the other.  This isn't impossible by any means -- but it does have to be done.  It doesn't need valves.  I think there may even be a diagram in LAOSH, which I don't have to hand right at the moment.

    As to control... it would depend on whether you wanted the boilers to alternate or not.  Much simpler if not.  If you don't need them to alternate, it is a matter of the settings on the vapourstats (I hope) or pressuretrols.  You need one (which you might think of as the maintaining boiler) set to some reasonable range -- let's say for the sake of talking a 12 ounce cutout and a 6 ounce cutin.  Now the other one would be set lower; let's say a 10 ounce cutout and a 4 ounce cutin.  What will happen is that both will fire when the thermostat calls, and quickly bring the system up to operating pressure.  Then the lower pressure one will cutout, leaving the other to maintain steam.  Now if the maintaining boiler can't maintain steam, eventually the lower boiler (which had cut out) will cutin, and bring the pressure back up.  In the odd circumstance that the maintainer was able to keep the pressure going up, it would cycle exactly as a normal single boiler would.  If things are sized right, this won't ever happen.

    I would suggest that both the vapourstats in question be piped off the header, with a snubber, rather than mounted on the individual boilers (your backup pressurestats would be on the individual boilers, as usual).
    Jamie

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Kmac Kmac @ 9:33 PM
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    Thanks

    Jaime thanks, when i think about it, I can see how setting the pressuretrols at the different settings will indeed set the boilers to run in the lead lag sequence. It's so basic- once you step back and look at it. Thanks
  • Tim_Hodgson Tim_Hodgson @ 7:55 PM
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    Double Trouble

    I have always found that parallel steam boilers without automatic boiler  valve and feed water control valves will have water level problems. Does it have a gravity return or does it have a condensate pump? I have installed float and thermostatic traps at the high water level on the equalizer pipe to drain the non-firing boiler into the condensate receiver when it overfills. Like you I prefer staging controls in a lead-lag set up.
    Good luck,
    Tim Hodgson
  • nicholas bonham-carter nicholas bonham-carter @ 4:00 AM
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    Twin boilers

    Does the system work properly?
    It would be especially important to keep the pressure very low (a few ounces), in order to prevent water from being blown into the inactive boiler through the return.--NBC
  • Dave in QCA Dave in QCA @ 11:05 AM
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    Another possible setup

    It would be so easy if we could see all of the details of this system, but we can't and therefor all we can do is make guesses as to how it might be set up.

    Jamie's explanation is certainly sound and would be essentially the same in operation as a single boiler with high-low control.  Most, but not all steam systems must account for a "pickup factor"  when sizing the boiler and in doing so the boiler is over sized out of necessity.  So, in this type of setup, the two boilers would fire until the system is completely heated up and all the radiators are filled with steam.  At this point, the pickup factor is satisfied, the pressure begins to climb, and the firing rate necessary to maintain steam in the radiators is less than what was required to fill them with steam.  One boiler in a staged system shutting down or a hi-low boiler going to low fire is more efficient than one single boiler going off an on every minute or so. 

    But, this system would require both boilers fire at the beginning of each cycle.  Kmac tells us that the owner says she was told that only one boiler would fire and the second would fire only when needed.  So, I am thinking this setup is probably not the way this boiler system is set up.

    However, there is also another approach and it has been used, or perhaps pioneered by Dave Bunnell, aka the Steam Whisperer, formerly known as Boilerpro.  It goes as follows:    He has successfully set up two small boilers to fill the total needs of the system.  Of course, piping was done as necessary so that water levels remained equal between the two boilers.  Controls to stage the boilers were accomplished by one simple 2-stage thermostat.  First stage would fire only one boiler.  With the thermostat left at a constant setting, the second boiler only fired when the first boiler did not maintain the space temperature. 

    OK, red flags are flying all over the place because we all know the problems that would happen by ignoring the pickup factor. We know that part of the system would heat well and part of the system would remain stone cold.  However, This application was on a two-pipe system.  Ideally, such a setup should be on a fully orificed system or a system that works with orifice valves such as a Moline system or other similar setup.  Keep in mind that orificed systems should NOT have a pickup factor included when sizing the boiler.  The system that Dave Bunnell installed was not orificed, but balance was achieve by pinching down the radiator inlet valves.  He was going to go back later an install the orifice plates, but I'm not sure if that was determined to be necessary or not.

    Dave's report on this system, which used a pair of Slantfin TR series boilers with gas guns was that it was very satisfactory.  It lowered the heating cost compared to the old system very dramatically and also provided very even heat to the large home.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
    http://grandviewdavenport.com
  • Jason Jason @ 8:37 AM
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    Piping

    Just remember when piping the boilers to drop into the top of the common header which will keep water from dropping down the boiler riser of the off boiler.
    A duplex condensate pump for return is a good idea but have seen many gravity systems work well.
    This post was edited by an admin on April 1, 2013 8:38 AM.
  • Kmac Kmac @ 7:01 PM
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    Update

    I was at the house this week to do some plumbing service and she asked me to re program her thermostat -she was complaining that it was not maintaining the temp. Right. So I'm going over times and temps with her when I realize the heat should be one. Go to basement, boilers off on low water. Able to take a long look at what was done, like Jaime said the pressuretrols are set at different cuties and outs so, both boilers fire on a call for heat, one boiler will shut down while one will continue to run to try and maintain psi. Both boilers off on low water, "main boiler" close to bone dry secondary boiler just needed a jab of water to get back on. Told her to call installer for some feeds that should have been included inhis price. Thanks to everyone for their help and ideas.
    I'm sure ill be back with some more steam problems.
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