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Sizing boiler

kevkev Member Posts: 94
What is the proper way to size the boiler for a forced air with coils system.

I have the job of plumbing the boiler with supply and return piping to four coils. The HVAC person has split this house into four zones. Each zone has what appears to be the same 3/4" coil in the duct work. The customer also wants a BUDERUS GB boiler regardless if he needs it or not. The HVAC person has told the customer he needs   a 150,000 BTU boiler.  I am sure he pulled that number out of the air. I have plumbed these type of systems before using basic on off boilers with high limits set to 180 degrees and figuring 40,000 btu's per 3/4" coil. They have worked fine, but now i need to know the proper way to figure my actual loads. Thanks for any input.


  • Robert O'BrienRobert O'Brien Member Posts: 2,939
    Heat loss

    You need to do a heat loss calculation
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  • kevkev Member Posts: 94
    That is

    where it gets fuzzy for me. I am familiar with a heat loss  for hydronic  heating. Forced air uses the same formula?  How does it correlate to the fan coils if I figure my BTU? Not sure if I am asking the question correctly.
  • SWEISWEI Member Posts: 7,356
    Same thing only different

    You start with a zone by zone heat loss, then pick a design water temp which allows each coil to meet the demand for that zone, then size your circulator for the correct flow and ∆T for the system.  If you're lucky, the coils are oversized and you will be able to use a lower design temp.  Outdoor reset works beautifully on hydro-air, but the curve will fall off a bit faster as the water temperature drops.
  • RichRich Member Posts: 2,247
    Heat loss

    is heat loss . Whatever the building loses must be replaced no matter what technology or method is replacing it . Some ways just do it much better .   Since you have been instructed to use a GB and a heat loss has been given to you whether accurate or not the only boiler for you is the GB142/45 . It is right where you want to be but the next smaller option will probably fall far short .
    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
    Rich McGrath 732-581-3833
  • kevkev Member Posts: 94

    Heat loss is a heat loss. I need from HVAC a chart for his coils to tell me how much and how hot, to satisfy the heat loss. I was always under the idea that a coil in these type of systems do not work well under 150 degree supply temps.  Thanks for the help!
  • SWEISWEI Member Posts: 7,356
    Working well

    is not that hard when the air is only a degree or two away from setpoint.  Temperature modulation is ideally accompanied by smarter controls, which together create far greater comfort than most Americans have ever experienced. 
  • kevkev Member Posts: 94
    heat loss

    For building is at 108,000 BTU's.  Looking at the BUDERUS GB142 boilers as owner requested and it seems we land in the middle of two sizes.  I looked at the TRIANGLE TUBE condensing boiler and they also are around the same outputs. With a fan coil system such as this where  the smallest zone load i have is 18,00 BTU can I use the larger size and let modulation do its thing or do I use a slightly under sized boiler and assume my heat loss is generous?
  • ChrisChris Member Posts: 3,056
    edited July 2013
    Why Not A

    Viessmann Vitodens 100 WB1B-35 118,000 In, 109,000 Out....Better HX with a better warranty and simple control..Give the homeowner the choice. You cannot oversize a condensing boiler on the high end output it;s the low end you need to worry about.. The boiler will only provide it's full output once the design delta-t on the primary side is reached. When piped pri/sec or LLH that will only be achieved when you can exhaust the full primary flow rate into the secondary and your system delta-t matches the primary's design delta-t.

    If the boiler is over sized then size the primary or boiler pump for a larger delta then the secondary. It will keep the primary flow rate closer to the secondary flow rate need. Size the boiler for 30 delta and the primary for a 20 delta..

    One could say the boiler wouldn't be oversized but the primary or boiler pump may. The pump due to a large flow rate across the hx could make the boiler short cycle.. You only need that flow rate under design conditions. What happens the other 98% of the heating season?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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