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Follow up question re boiler replacement

repleo Member Posts: 5
Follow up question to my earlier post re Sanity Check and thank you for the great answers I got.
I had a heating contractor come by this morning to give me a proposal for replacing my existing 100MBH oil boiler with new gas fired system. He estimated the block load for heating at 27 – 28 MBH which is actually the same as I calculated from loadcal.net and other sources. We discussed a couple of options – 1. Bosch Floor mounted Greenstar Combi unit. 2. Weil-McLain Gold unit. I think it has a cast iron primary heat exchanger and a condensing secondary exchanger. 3. Conventional cast iron boiler and 40 gal indirect water heater. (I haven't seen the full proposals or pricing yet.)
However when we discussed boiler sizes, I think he was talking about something in the 100MBH range. I keep reading how it is important it is to not oversize the boiler. Does 100MBH for 28 MBH heat load plus domestic hot water sound reasonable? Any other thoughts on these systems? I am 71 years old and I want the system to outlive me as troublefree as possible. I am more concerned about overall 'cost to own' than fuel efficiency..
Much appreciated.

1144 sq ft Ranch over unheated full basement built to minimal 'Contractor' standards in 1973 on Cape Cod, MA. Currently heated with a 25 year old 100,000 BTU +/- Burnham cast iron boiler through 60' of Slant Fin baseboard. The boiler has a hot water coil with no tank and has given us sufficient hot water. I have run heat loss calcs on Heatload.net and come up with heating load of 27,000 BTU


  • SuperTech
    SuperTech Member Posts: 2,124
    Keep it simple.  Go with a cast iron boiler with an indirect tank.  I have been on several no heat calls for customers your age who were sold a condensing boiler or combi. Every single one regrets the decision when the time comes that the boiler needs repair and they have to wait for some expensive proprietary parts to be delivered.  
    MikeAmannMad Dog_2clammy
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,798
    edited November 2023
    A modulating boiler can be oversized, most will have about the same minimum fire. A cast iron shouldn’t be. They make CIs that are right around that size so install something like that if you don’t want a modulating one 
    Mad Dog_2
  • pedmec
    pedmec Member Posts: 936
    modulating boiler is the way to go. work on them all the time. properly installed they work great. the only problem with mod cons is contractors dont know how to install and set them up. they live in the past
    Mad Dog_2
  • repleo
    repleo Member Posts: 5
    OP here. I think I could have phrased my question better. If I only need 28MBH for home heating what size should the boiler be (assume conventional CI ) to also run a 40 gal indirect water heater.
    EBEBRATT-Ed Member Posts: 15,335

    60 ft of baseboard will only need 33,000 btus which is close to your heat loss. The heat loss is the correct method the baseboard is just a quick double check.

    You don't need to add anything for an indirect unless you have something unusual like a hot tub etc because you can set the indirect to priority. It will shut the heat to the house down to heat the indirect. If you have been surviving with a tankeless water heater you will have no issues with an indirect even with a small boiler. I would go traditional Cast Iron. Chances are the smallest boiler available will be 45-50K anyhow
    MikeAmannMad Dog_2SuperTechclammy
  • jesmed1
    jesmed1 Member Posts: 554
    edited December 2023
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,214
    It sounds like a little Burnham 202 Boiler would be just right....28,000 btu/hr net rating. Something to remember about the indirect load.... you will only have a significant heat load when you are home using hot water. That hot water is throwing some heat into the home and the human body emits heat as does any light, cooking and etc. So when you are using hot water, even on the design day, you won't need the full design capacity. Design heat load assumes no one is home, no lights no computers and the sun is never out. Unless you get some really severe record temperatures ( Chicago design is 0F, though our record here is about -25F air temp and -80F wind chill) or you need heat in a basement, that 28,000 btu/hr is probably all you need. Also, I would try to use a higher mass boiler to help ensure longer burn cycles since the system mass is so low. I've also set up boilers to always pump through the indirect to provide that extra mass during heating system operation to get long burn cycles. This would need Primary/ secondary piping. You would need a mixing valve on the indirect outlet to make sure the tap water doesn't get too hot. Long steady heating cycles, particularly at lower water temperatures (but still above 130f or so) , provide for very efficient boiler and system operation. This wipes out most of the efficiency benefit of a modcon.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
    MikeAmannMad Dog_2
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,726
    It was just a few weeks ago, I posted about my Older Brother, Big Bart's 65 year old Gas Hot water "Baby 👶 " boiler as I call it, with a 90 K input and how its still cranking.  I'm replacing it shortly with A Burnham ES 24 (105 K in) gas, cast iron, 85% efficiency & a Turbomax 24 Indirect water heater.  Its reliable and a perfect 👌 fit for his Fambly..  Mad Dog 🐕 
  • repleo
    repleo Member Posts: 5
    Many thanks for all of the advice. This site is a great source of information.
    Erin Holohan Haskell