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New Gas Boiler & DHW - Unsure of heat load and calculations from company

Wisey Member Posts: 3
Moved into a 1906 built house last June, which had an old cast iron FHW boiler that had been converted from oil to gas and appears to be from the late 70's, paired with a 7 year old AO Smith DHW that already is corroding and needs to be replaced.

We're planning to switch to a ModCon paired with an indirect DHW, but am a little hesitant on trusting the one set of Manual J calculations I've received, as other contractors we had in for quotes seem to have differing opinions.

The house is 2591 sq ft, with cast iron radiators on much of the first floor (slant fin baseboard it looks like in the kitchen), along with radiators on the 2nd floor and then the baseboard again on the smaller 3rd floor. We live in Mass, so climate zone 6a, and currently have basic insulation (from what MassSave could tell) in terms of the walls and attic area.

We're planning to insulate the attic/roof deck and rim joists with closed cell spray foam, and like doing the walls with blown in cellulose, so assume we'll see a good reduction in heating needed, and here's where I need the help.

Does a heat load of 79743 btuh sound right for what I've described? I am just a normal homeowner, so don't have the expertise, but wanted to get the opinion of others before moving forward. The contractor is recommending we get a 125 000 btu ModCon with a 5:1 turndown ratio, but if their Manual J is right, I am thinking a 90/100 btu unit is more appropriate, and will prevent short cycling for smaller load areas like the Kitchen (which is on its own thermostat).

Any and all comments and input are welcome, and happy to provide more information or even a redacted copy of the Manual J we received if it helps.


  • Zman
    Zman Member Posts: 7,493
    Your Manual J is probably a little on the high side. If you post it, you will get plenty of opinions :). You can also do your own with the free apps like the one Slant Fin has.

    The combination of cast iron and slant fin radiators creates a bit of a design challenge as the water temp requirements are likely quite a bit different. That can be a conversation for later. Do you still have the original huge piping from the gravity system?

    I would suggest designing around the typical heating day rather than the coldest (design) day. With that in mind, the turndown ratio is far more important than boiler max output.

    Check out some of the 10-1 turndown models in the 80k-110k range. Lochinvar is a good one but there are many others.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Wisey
    Wisey Member Posts: 3
    Here are screenshots of the Manual J. I was thinking the same thing about the turndown ratio, but most of the units we've been recommended from the various contractors all seem to be 5:1. We've been quote a Bosch Greenstar, as well as the Viessmann Vitocrossal 300 (that one by a few people). Hope the screenshots help, and thanks again to everyone in advance for your thoughts/guidance.

  • Ironman
    Ironman Member Posts: 7,222
    The manual J comes out to about 30 btus per sq. ft. which sounds maybe a little high. Without knowing the actual construction data and measurements of your house, we cannot say if it's accurate.

    If you still have the large iron pipes from the original gravity system, then there's a lot of buffer there and going up one size on the boiler probably won't hurt and would give a little better response time. 125k btus is too big, though.

    You only need the full output of the boiler at design conditions. When it's 35-40* outside, you only need 1/2 of the full output.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Wisey
    Wisey Member Posts: 3
    We definitely have some large iron pipes in the house, but guessing those are mainly for plumbing. The two of you both seem to agree that 125 is likely too big, and with two zones using only baseboards, that if it's too large btu wise and doesn't have enough of a turndown ratio that short cycling could definitely occur (if I understand things right).
  • Ironman
    Ironman Member Posts: 7,222
    You could add a buffer tank to the micro zones.

    Look at the HTP UFT080. 10 to 1 turndown, fire tube heat exchanger.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    EBEBRATT-Ed Member Posts: 14,505
    79k sounds right. I would use the SlantFin app and double check the manual J
  • hot_rod
    hot_rod Member Posts: 20,740
    Be good to know the load number after your insulation upgrades, that is the number you should size to. Most of the mod cons allow you to derate then on the control to best match your load.

    It would be nice to do a radiation assessment and determine the lowest possible operating temperature, that will maximize a mod con efficiency.

    Here is a guide to help with that.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream