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Gas Wall Hung Boilers

Greg Maxwell
Greg Maxwell Member Posts: 212
I need some input. I have 27 years experience in the Htg - AC supply side of our industry. This latest fad of wall hung boilers is giving me fits. Most of the jobs we are seeing are retro-fit, removal of an oil boiler and replacing with a wall hung "mod-con" type. 2-3 zone baseboard jobs. 180 degree supply temps. I have a real problem convincing contractors that they need thermal storage on these systems so the boilers can run long enough to condense. On these small heat exchangers, the boiler doesnt run long enough without it to get to steady state efficiency. It just keeps banging off limit.

There are jobs out there that are being figured by us properly, using radiant systems, so we have reset capabilities, but on some new systems, the guys want cheap & fast, so its 2-3 zone baseboard jobs, no extra baseboard to allow for lower water temps, and no buffer tanks.

Comments

  • Tim McElwain
    Tim McElwain Member Posts: 4,480
    First question I have would

    be concerning a heat loss and measurement of lineal feet of emitter. Most old system were over sized and in some cases over radiated. That if so could allow for lower operating temperatures along with outdoor reset.



    See Brookhaven national Lab, Dr. Thomas Butcher paper on the subject at:

    www.bnl.gov/isd/documents/28709.pdf
  • CMadatMe
    CMadatMe Member Posts: 3,085
    edited July 2011
    Haven't Run Into

    Many systems that were not over-radiated. Find that most are about 35%. A sound approach might be to work with your customer on a long term heating plan. Year one, put the boiler in and then begin changing out the board to hi-cap to decrease that design water tempdown the line. Being able to begin your curve at 160 puts you condensing most of the season. I also prefer the use of low loss headers vs pri-sec piping. If you ever have installed a Viessmann Vitodens you will see that the boiler logic creeps or coasts to setpoint thus decreasing short cycling. Another reason why they predicate LLH over pri/sec piping.



    From reading many of your posts on this matter I would say your main issues are with how these boilers are being installed which is not a boiler issue, its a contractor issue. Let's face it, cast iron boilers short cycle even more in most applications why is that an acceptable practice? Why are buffer tanks not predicated in these installs?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Wall Hungs

    The reason buffer tanks are not installed on cast iron boilers, is because they have water volume, unlike these giononi heat exchangers that hold about 1/2 gal water. Even on low flame, they limit out very quickly. The manufacturers of this type of equipment are not very forthcoming on how they get their advertised efficiencies, so homeowners, and contractors alike, think that this is a 1:1 swap, and voila! Money savings! We are finding that this is not the case. Average efficiencies, are more in the range of 88%, then you take into consideration the BTU output of oil vs propane, and there isnt much there to brag about.
  • CMadatMe
    CMadatMe Member Posts: 3,085
    Disagree

    A conventional cast iron boiler fires to or trys to fire its full load everytime. If only one zone is calling that boiler is 3 to 5 times bigger then what is needed. Boiler never reaches steady state. Burner fires full open where as a condensing boiler starts low and goes slow to where it wants to be.



    As for mfg not putting out the data. I agree literature misleads. The attached is from Viessmann's original draft tech data manual for the newer Vitodens 200. Its not in the current manual which I feel should be as it gives your the opportunity to explain how water temp and modulation rates effect efficiency of this boiler.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Thermal Mass

    That oil boiler may fire every time to its full capacity, but that pump runs a lot longer circulating water between the zones and often times, in the short season, the boiler may not even fire again and that zone will satisfy, where a gas wall hung will fire multiple times without the proper water volume in the system.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    edited July 2011
    Wall Hungs

    I'll put my Biasi oil boiler, and the system that I have, running the way it is, .65, outdoor reset, with a boiler efficiency of 87.5, against a wall hung propane boiler, in the same situation, any time.
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Gas Boilers

    You know, I think I have pounded on these wall hungs long enough. The proper boiler, such as an HTP Pioneer, or Versa, with a low temp applicaction, such as baseboard, rads, or radiant, which I am quoting, can and will be a good fit. I think I am just dissatisfied in the way that the boilers have been misapplied, and the homeowner then becomes the bearer of that burden. 'Nuf said.
  • CMadatMe
    CMadatMe Member Posts: 3,085
    So Your Using

    A buffer tank with that low mass boiler to prevent short cycling right? I'm game. What's the loss per zone by room with heat emitter sizes?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Plumdog_2
    Plumdog_2 Member Posts: 873
    I agree with Tim and others

    Your installers need to use the Outdoor Temp Compensation feature. Only time you need 180 degrees is at design temp! I see mis-applied mod-cons regularly...the installer simply replaces the boiler without using the Domestic Priority feature, or the Outdoor Reset, and lets it run 180 all day long. It's just wrong. Tell 'em to read the installation guidelines!
  • Charles Johnson
    Charles Johnson Member Posts: 24
    Don't over pump!

    Keep the flow down to where the temp diff between supply and return stays 20° or more. As said above many houses have more than enough radiation so 30° delta will often work just fine. So even on those coldest days when the setpoint is 180, you will get no warmer than 160 back which will increase the time the boiler spends condensing. Taco VDT pumps will vary their speed to maintain a set temp diff.



    If the smallest zone has a lower load than the lowest turndown of the boiler a buffer tank will prevent short cycles.



    And: ALWAYS USE OUTDOOR RESET!
This discussion has been closed.