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Replacing oversize boiler - atmospheric or power burner?

djackman Member Posts: 12
1940's 1200sq/ft semi-split ranch. Current system is a early 1980's Hydrotherm HC-145 nat gas boiler 116k net with two baseboard zones. Adding up all the baseboard there is approx 60k of radiation... previous owner was of the "bigger must be better" mindset. Have not done a Manual J yet but with new windows, attic insulation, significantly better air sealing, new stack damper, we are warm but the boiler short cycles due to lack of load.

Looking to replace the boiler with a properly sized and more efficient unit. Wondering if we should stick with atmospheric or go to a power burner setup. Had a Wayne conversion burner at parent's house for many years and it worked very well.

Currently venting into a lined chimmey. Planning to convert to a indirect HWH from the current standalone HWH when the boiler is replaced.


  • Steamhead
    Steamhead Member Posts: 16,736
    Do the heat loss calculation first. Then we'll see what's available in that size range.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • GBart
    GBart Member Posts: 746
    edited September 2018
    Atmospheric is not efficient so you can cross that off your list.

    To be in 2018 you need variable speed equipment with all the options including Outdoor Reset.
  • djackman
    djackman Member Posts: 12
    SlantFin app is coming up with 25k for a 70dF indoor 10dF outdoor note this is my first time 'doing the math'

    Current baseboard:

    loop 1 52 feet
    loop 2 58 feet + inwall heatvector on diverter t's
    loop 3 dedicated kitchen toekick heater

    We run loop 3 with loop 1 when it gets really cold (consistently below freezing for a few days)

    Thought the 25k number was a little low but with aquastat at 140dF we're comfortable. If understand the slant/fin ratings 110ft of baseboard @ 140dF is 35k

    If the numbers are right what would be an appropriate size replacement boiler? Have a never used takeout Superstor SSU-30 to install for DHW when boiler is replaced. DHW load is low just two adults.

    I'd consider replacing BB with panel rads but wife is firmly against it. Seeing math think I could get away with them just under windows in most of the rooms and eliminate the wall to wall BB