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Size of New Boiler?

insolentumbrella Member Posts: 4
edited July 2022 in Oil Heating

My current oil boiler needs replacing and so far all the estimates I have gotten seem to be for a bigger boiler than I thought the home might need.

I did a room by room manual-j / heat loss calculation for the home using CoolCalc and also did a fuel usage assessment over the last two years using degree days based on the method suggested on green building advisor. The heat loss calc came out to 45k BTU based on a 6 degree design temp (Framingham MA) and the fuel usage assessment came out to 32k BTU.

I have baseboard heat with 145 feet of exposed linear slant fin in 4 zones. 46 feet on second floor and 99 feet on the first floor. During the day we typically do not run the heat on the second floor (bedrooms). Currently have a tankless coil for DHW but planning to change that to an indirect tank with the new boiler. Assuming the baseboard is 3/4, I figure it can only actually put out 85k BTUs @ 580 BTUs/ft if all zones were calling, which they rarely are.

I have spoken to three contractors so far and they all are adamant I need at least a 120K boiler. Can anyone explain to me why it would be a bad idea to put in a Buderus G115WS3 which has a DOE rated output of 85k with a Riello burner?

Rounding up to a *very* conservative heat loss of 50k BTUs, would a DHW priority zone on an indirect 45 gallon SuperStor Ultra really need the extra power, and if so, why? Everyone says I need a larger boiler but is unable to explain to me why.

Thanks in advance for any of your expertise.


  • mattmia2
    mattmia2 Member Posts: 9,409
    IT depends on how you size the indirect and your hot water usage patterns. At 85k BTU the indirect recovery will be slow with low incoming winter water temps so it probably needs to be sized to store enough hot water for each period of use. If that isn't their reasoning then it is probably time to keep looking.

    I know there are a number of people on the wall from MA, maybe they can do it or recommend someone.
  • insolentumbrella
    insolentumbrella Member Posts: 4
    Thanks, as I understand incoming water is typically around 45-47 degrees in the winter around here. We have two full bathrooms, one 40 gallon tub. Almost never take two showers at the same time but do take two back to back showers with dishwasher or washing machine running at the same time. Family would likely be happy with 80+ gallons first hour DHW delivery with 45-60 minute recovery time. We had a heat pump hybrid water heater in our old home so somewhat use to longer recovery.
  • DJD775
    DJD775 Member Posts: 241
    Old school rule of thumb contractors. Keep looking for a contractor who will size the boiler appropriately.
    Always can run the indirect at a higher temp with a mixing valve and set it to priority if necessary.
  • GGross
    GGross Member Posts: 948
    Unless you expressed a desire to have substantially more hot water available to you than you currently have I can not see the reason why they would want larger equipment either. In general if someone can't tell me why they chose a particular size of equipment I just assume they are guessing, because if they are not guessing they can show me how they came up with the solution they are offering. Unfortunately many contractors do just guess on the size, so they guess larger because they are worried about under sizing equipment, just not so worried that they would calculate the BTU required to actually heat a space.

    Most 40 gallon boiler mates being fed by about 80k BTU will output a whole lot of hot water, but I do know many contractors who won't hook a boiler mate to less than about 100k BTU, doesn't matter if they pipe it so it only gets 60k BTU, they are convinced the extra BTU is a good thing
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,788
    Bonkers to recommend a boiler that big for your situation. Most Americans use separate DHW tanks which have nowhere near 120kBtu of power and get by just fine.
  • insolentumbrella
    insolentumbrella Member Posts: 4
    I did not express any need for excessive hot water, we are a typical family of four and politely made the manual-j report available to them.  

  • Jamie Hall
    Jamie Hall Member Posts: 22,897
    Bottom line: go with the Manual J. The contractors wanting to put in a bigger boiler... want to sell you a bigger boiler. They will be happy. You won't be, and neither will the boiler.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Alan (California Radiant) ForbesMikeAmann
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    Put in an EK At even the higher heat loss any boiler is too big. EK is the most efficient.
  • insolentumbrella
    insolentumbrella Member Posts: 4
    Thanks, have someone coming to quote an EK early next week.  Haven't talked to anyone who installs them yet but got the contact through the EK website. Will see if the price differential is worth it over the Buderus which is already a decently efficient boiler.
  • JayMcCay
    JayMcCay Member Posts: 32
    Thank you for your comment @STEVEusaPA, and for your interest in our products @insolentumbrella.
    Based on the information you provided an EK1 size boiler will be more than adequate, and there’s no oversizing penalty so it will run very efficiently in all seasons and for hot water alone.
    Please let me know if I can assist any way.

    Energy Kinetics
    Jay McCay
    National Sales Manager
    Energy Kinetics
    908 328-7154 cell
  • Labenaqui
    Labenaqui Member Posts: 72
    Hello neighbor (NH). Your perimeter baseboard heating configuration ideally compliments dedicated Taco VT2218 Delta-T managed hydronic distribution. Coupling a Commercial-rated Weil-McLain UO Boiler & Beckett NX Oil Burner has proved a "bullet-proof" combo. Low systemp operation with 150k hours operation thus far - no service calls! We're patented & Taco supported, but willing to share our findings.