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combi boiler

jrshanker Member Posts: 4
I have a cottage house about 600 sq ft in cold climate new jersey. I currently have a 30 year old tank boiler with baseboard heating and has a farthest run of 40 ' for a loop total of 80 feet. I have 7 base boards 4 6 ft 3 2 ft and 1 2 ft. my estimated btu need is about 50000btu. I am planning to replace the boiler with a combi boiler like a Bosch greenstar 100. Not very sure if this would be the right combi to use. the demand for DHW is minimal as this cottage is mostly used in summer and sometimes in winter. Any advice is well appreciated.



  • Gilmorrie
    Gilmorrie Member Posts: 160
    edited March 2019
    Double-check your heat loss calculation. 50k Btu/hr sounds high for 600 sq ft. Why are you replacing your boiler? This is natural gas, right?
  • jrshanker
    jrshanker Member Posts: 4
    The boiler is 30 years old and don't want it failing in the middle of winter. So I am proactively thinking about replacing it. This is the only lowest BTU rated Bosch combi model I could find
  • Mook531
    Mook531 Member Posts: 13
    edited March 2019
    Doesn’t sound like a great fit to me. You only have 32’ of baseboard, which makes the 50k really oversized. It’s going to be tough to get it to condense, and still heat the house with that little baseboard, IMO.
    HVACNUT Member Posts: 4,594
    edited March 2019
    Even at 25 BTU's per square ft. which is real real conservative, that's only 15K output needed.
    32 ft of Slant Fin 30 or similar will put out about 19K at 180°.
    Are you stuck on Bosch?
    Other guys know for sure but I believe there are a few boilers, not combi's, that can modulate down to 10K or lower.
    I'm partial to an indirect water heater over a combi any day.
    Even with a low output, a buffer tank might be needed. And of course connect the ODR.
  • Gordy
    Gordy Member Posts: 9,514
    First question is how does it presently heat?

    The installed baseboard is 17k at 500 btus a foot with 170 awt. Which works out to 29 btus a sf for a 600 sf cottage. On probably the coldest day.

    So if you want a combi to cover dhw it would make sense it could modulate down to the output you need for heating.

    Just think about your dhw needs, and size from there. Probably the smallest combi available.

  • superdave
    superdave Member Posts: 155
    What is your DHW needs GPM?? The Baxi has 2 sizes that might work best depending on your DHW. 30GA has a heat out put of about 85,000 down to 15,000 with a 2.5 GPM @ 70DT and the 40GA out put is about 112,000 down to 19,000 with a 3.9 GPM @ 70DT.
  • jrshanker
    jrshanker Member Posts: 4
    Thanks for all the input. I am a novice at this. The only reason I was going for Bosch was for them being known for quality and performance. Currently, I have a regular tank boiler. I am tight on space and the combi will work better. My DHW is nominal since this cottage is mostly used in summer and sometimes in winter. I just looked at the Baxi 30GA and that will work. Again thanks for all the input. I will update after I have it installed and get it working.
  • rhl
    rhl Member Posts: 93
    In New Jersey you are probably in a warm enough climate to use an air source heat pump. There is the Sanden CO2 heater, which can output 15K BTU, assuming your 50K btu load calc is wrong. It's primarily a hot water heater, but, can also be used to provide space heat. Then you can throw up some solar panels and go off the grid.
  • kcopp
    kcopp Member Posts: 3,990
    The installer is the important part... Use a unit that they know inside and out and can repair/ service.
  • I like the Rinnai e50c combi:


    It will modulate down to 13,600 BTU's which is about what your baseboard will need to heat the house and it will provide 2 gallons of domestic hot water per minute.

    We've only installed two of them, but they are now my go-to boiler for small combi's.
    Often wrong, never in doubt.
  • SuperTech
    SuperTech Member Posts: 1,792
    Just because a boiler is old doesn't mean it's going to fail. Did you ever hear the saying that they don't make them like they used to? A cast iron boiler is usually easier to work on and service than a combi and had less electronic components with potential for failure.
    Perhaps you could just renovate the existing boiler and have the reliability that you are looking for and save a bunch of money in the process.
  • TAG
    TAG Member Posts: 620
    Alan: The unit with the tank is nice .. like the Viessman 22.
  • nibs
    nibs Member Posts: 470
    We took Alan's advice and but in a Rinnai E50C and are very pleased so far.
    Thanks again Alan.