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how much oversizing is too much?

I'm on a boiler finding quest and i'd love to use the triangle tube prestige solo 175.  however, i only really need about 79000 BTU.  This is rated at 134K btu.  Their next size down is too small (76K btu). 

Is this too much of an oversize?  we do plan on an addition in the next 5 years... is it worth just going with the bigger one, or would it make sense to get a different brand?  i really like the stainless hx and the controls for what i'm doing (two different radiant installations, floor and ceiling).



am i killing efficiency?  the house will have 7 zones (2 small bathrooms, 2 medium bedrooms, two large bedrooms and a large living/kitchen aread).

Comments

  • Gordy
    Gordy Member Posts: 9,514
    Too much

    I would say baring the addition the 110 would be fine IF you were very diligent about your heat loss calculation. In real life you would find the 110 easily handling the design load.



    On the other hand there are other choices with same HX design that may match your load closer, but on the heavy side. Mod/cons do best working up hill...a little.



    Lochinvar WHN 85 79K DOE

    Weil McClain 97+110 CT 101K DOE

    HTP elite FT 80 79K DOE





    Addition plans are how big, and what would be the heat loss? 5 years is a ways out, and things change.
  • Zman
    Zman Member Posts: 6,807
    Triangle Tube

    It sounds like you are using the IBR number for the propane model.

    You should be able to use the DOE number and you will be right in there.

    As far as oversizing goes, It really depends on the way you are zoned, how much system mass you have and how outdoor reset effects the load.

    If you have many zones and a low mass system, don't oversize.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Rich_49
    Rich_49 Member Posts: 2,603
    Protect

    your equipment and make room for future growth .  Truth is it's hard to oversize Mod Con equipment . You have 3 choices ,

    Use a low mass mod con , oversize and even when you've completed all the additions and such still short cycle to some extent . Unless you have employed the services of someone really sharp .

    Add mass to that low mass mod con by doing the math and installing a properly sized buffer tank . Again you should have someone do that math with you .

    Use a product that has mass already and can handle your NOW house and can be programmed a bit differently later to optimize efficiency in your then house .

    2 of these are GOOD solutions , 1 is not so desirable .

    Mass is the friend of systems that will perform optimally with what is available now . Very few homes are a perfect fir for all but the smallest low mass mod cons , they do exist however but to determine that you again will need help from someone qualified . Do not be talked into the myth that a large mass slab or other install method is a substitute for fluid mass , this only works a couple times a year after a decent , when system is first charging your slab or other and at times after a warm spell during the heating season .  
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • SWEI
    SWEI Member Posts: 7,356
    Sorry to disagree, but it's NOT hard to oversize mod con equipment

    My current rule of thumb is keep the MINIMUM firing rate of the boiler at one third or less of the design day heat load.  Your 79k is more than three times the 25k minimum rate on the PTS110, so the boiler will run on nice long cycles.
    Jean-David Beyer
  • Rich_49
    Rich_49 Member Posts: 2,603
    edited September 2014
    7 zones

    divided by 79,000 = 11,285k , with 2 calling your good @ design . What will the heat loss be at 35 or 40 * ?  Maybe half that and don't forget that the wind is not always blowing 15 mph on a design day either which changes infiltration immensely . So maybe for 45% of the heating season the load will be 50K divided by 7 = 7,140 . If 4 are calling you'll be alright. Then there's the shoulder seasons where the load will possibly be as low as 15,000 . Should we not insure that these systems are optimized for all running time guys ? I will agree to disagree Swei .

    By the way Avionator , how will you make DHW in this house and how many baths are there and what are your use habits ?
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • avionator
    avionator Member Posts: 10
    thanks for the responses folks. my project got side-lined with baby number three... but things are starting to calm down (or maybe we're just pro's at time management... three kids under three...!) and i'm looking to get into this again. I'll be doing a detailed heat loss and looking into other boilers (the Burnham K2 looks interesting... i'll be posting a general thread about that).

    again, thanks!
  • Rich_49
    Rich_49 Member Posts: 2,603
    3 kids under 3 , I would say is not good time management . Congratulations !
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
    Gordy