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boiler sizeing on counter-flow system

i'm going to replace one of the boilers in my two family house, the one that heats the top floor.  the connected load is 320 sq. ft.. the system is a counter-flow with  a two inch  main 20' long.  i have a gorton #2 on the main and its wrapped in 2". 

i want to use a burnham mst series because of their effiency ratings.  the boiler in there now is an arcoliner rated at 360 sq. ft. of steam which has heated the place fine except that it drinks oil.  thanks to dan's seminars and TLAOSH i've taken care of the fine details in the system and it runs well.  even heat and not one bit of noise.

so, 320sq.ft. of connected load X 1.33 pickup factor=425.6 sq. ft. of steam

my question is; will the mst396 (396 sq.ft.steam) be adequate or should i go to the mst513.  even with a 3" drop header i' concerned about putting too much steam into that 2" counter-flow main.  i've read about how touchy the counter-flow systems are to oversized replacement boilers.

what would you guys recommend?

thanks in advance for reading this


  • Dave in QCA
    Dave in QCA Member Posts: 1,786
    pick-up factor already figured in

    When you look at the boiler rating for sq ft steam, it is telling you the amount of radiation that you can connect to the boiler.  The pick up factor is already figured in.  The mst396 is completely adequate, in fact you may want to down fire it a bit.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
  • steampunk_2
    steampunk_2 Member Posts: 28
    boiler sizing

    got it. thanks dave.
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