Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Sizing a mod con unit.

LandlordEE Member Posts: 1
Ok so I have been having a lively discussion with a contractor who owes me money and is trying to work it off in labor. I have an 1100 sf (100 year old) house in Eastern PA (allentown). It has cast iron hot water radiators in only 6 rooms (small house). The house has replacement windows, a full basement, and attic is insulated. ONE of the long walls is shared with another house. There is a full basement and ceilings are 8' high. It is a 2 story house. Everything is on a single zone.

He is insisting on installing a mod-con combi boiler with a BTU output of 96,000 max, (I think the low end is around 33k). I am thinking that is way too high? The 30 year old high mass boiler was 100k BTU output. I do NOT know how that was because we just bought the house.

He did not do a J-Study, and neither did I. Just looking to get a feel for this.... is 96,000 too much? He claims you need extra to heat the hot water. There is only 1 bathroom. Should I stop him from installing this unit or will it be ok? (It is a Triangle Tube Challenger 125h combi boiler) ( https://www.acv.com/us/customer/page/V_ZChyQAAEICary-/new-95-energy-efficient-challenger )


  • Ironman
    Ironman Member Posts: 7,399
    That comes to 87 btus per sq ft. Definitely more than what the house needs X 2.

    This is the problem with combi boilers: since they're heating the domestic instantaneously, a high input is needed. They'll almost always be way over-sized for space heating.

    I'd recommend that you pay the extra bucks for a KHN 055 plus an indirect water heater. It will last a lot longer and be more economical.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    GordyZmanJean-David Beyer
  • Gordy
    Gordy Member Posts: 9,546
    edited July 2017
    Like @Ironman said. The low end of modulation on that combi could be the most you need on a design day. What makes that a problem is below design days the combi can't modulate down anymore so it short cycles.

    If you must have a combi seek one with a 10:1 turndown ratio. This could get the low end modulation lower. Such as the lochinvar Nobel. It has 10:1 TDR. So while having 110 k btu high end it can modulate down to 11 k .

    I'm still partial to indirects. At least if there is a power failure you still have some hot water, and you can get by with that KHN 55 boiler.

    Need to do that heat loss though. You can benefit from condensing with the envelope improvements , and ci rads.
  • kcopp
    kcopp Member Posts: 4,448
    The combi set up is why you need the larger boiler.... and that is prob his thinking.
    If you want to spend the extra money for a indirect water heater you can go w/ a smaller boiler to better fit the space.
    I actually like the TT Challenger boiler design... Simple design w/ only 4 moving parts. I would choose it over a firetube boiler any day.
    A TT Challenger Solo 50s would more than likely be perfect for the size home you have.
    A heatloss is recommended.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,578
    Download and use the Slant/Fin heat loss app for your tablet, and you will then know what your BTU requirements are.
    Then tell us what your hot water needs are for the indirect.--NBC
  • Alan (California Radiant) Forbes
    We are just installing this combi boiler in an 1,100 square foot house:


    It modulates down to 17,500 BTU and has a 25 gallon DHW tank. A low-loss header will also be used to keep the boiler happy.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Rich_49
    Rich_49 Member Posts: 2,766
    Have or perform the heat loss by someone qualified . make sure the installer knows that you will not hold him responsible for the sizing , that may put him at ease with a properly sized system . Maybe he'll learn something . Just varying a bit from Ironman's recommendation , pay the extra bucks for a heat loss to be done , then figure out what you'll really need . You don't know until you know
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • bob eck
    bob eck Member Posts: 930
    Lochinvar Noble combi boiler 10-1 turndown
    110,000 BTU input to 11,000 BTU input
    SS Fire Tube heat exchanger
    Domestic hot water output of 2.6 GPM at 77 degree temp rise.
    Check it out.
    JUGHNE Member Posts: 11,108
    Is there the possibility that the debtor has this boiler on his hands and wants to unload it?

    If not then if you pick the boiler/system he is off the hook.
    IIWM I would insist on every valve/drain/bleeder possible.
    He may try to tell you that some things are not necessary but if you furnish the material and him the labor then you should get every device needed possible at the expense of his time.

    Personally I would go for the heating boiler and separate hot water indirect tank....just me of old school....I agree with Ironman.

    Is this your house or a rental??