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Boiler Sizing Questions - Steam Sq FT

DonKennedyDonKennedy Member Posts: 2
edited September 10 in THE MAIN WALL
I have 5 quotes from 5 contractors on my new boiler for my one pipe steam system on my 1929 house. All quoting the same boiler, same supply house. Some charged me to to quote me . It was the first week of May when boiler gave up its life by cracking at the seam.
30 years - would have like longer but still it was a good run. Peerless G-461-W-S 120 BTU, 300 Sqft of Steam and 96 BTU of Output. I live in Grand Rapids, MI

I did a replacement boiler sizing chart walk around myself of my radiators and figured out my heat loss and my total radiation per sq ft.

I came up with 403 sqft of Radiation and total btu h load of 96,672 and a heat loss of 44,803 @ 60 Deg and 52,867 @ 70 - we try to keep day time temp of house at 69/72 because of a 88 year old living in house.

All contractors used the supplier boiler quote of New York CGS-C 50C which has a 354 sqft of steam seems like an potential issue down the road from what I have learned on the site.

I was told not to worry the new boiler could provide the steam needed.

Maybe it can I just don't see how that is so when it is about 40 - 50 sq ft short. Maybe I am not getting it?

Thought please?


  • Jamie HallJamie Hall Member Posts: 13,209
    40 to 50 square feet short is about 10 to 15 % of your load. The EDR rating of a boiler has as 30% "pickup factor" built into it. You are within that -- though I should caution you that while it most likely will work just fine, that assumes that your pipes are well insulated and that the controls -- thermostat and pressure -- are properly set for your system.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • DonKennedyDonKennedy Member Posts: 2
    Jamie Hall- Thanks for the response. The pipes are insulated, what I can see in the open basement, additional insulation would be put on the first 3 feet of outgoing steam supply. Tne boiler outgoing supply was insulated at about 2 ft above the boiler with encapsulated asbestos insulation. The thermostat is a 2 wire on/off with night time turn down Honeywell. I am sure the system steam/air values are not as tightly balanced as they could be - nothing has been done with the values during my 22 years of ownership. There are two steam/air values on 2 side by side condesated return lines that t into one run back into the boiler. All eleven radiators have Hoffman Values.

    If it is close does going to the next size up cause what issues in system design? I am thinking CSC50 has 135 input BTU and can get the job done then the CSC60 with a 172 input BTU less btu = lower gas/fuel costs than the next size up that has more deliverable EDR. The most knowledgeable contractor said, "it will be the correct system for all but the 3 - 4 days that are the coldest days of the winter" That caused me to question his thinking. Since all of the contractors quoted the same New York Boiler sized by the supply house it just got me wondering - what if the supplier house was wrong. The supplier was using my old nameplate date - but no one did any calculation or even looked at the radiators or asked. Just asking a follow-up question, since when I asked the supply house the question they won't answer me because I had to pick a contractor. I just felt like they had me by _alls. Thanks for your help.
  • Jamie HallJamie Hall Member Posts: 13,209
    Steam systems are a little odd... they must be sized to the radiation, not the heat loss, plus a bit. How much "a bit" is is open to some debate, but if the steam lines are insulated, the 30% pickup factor is more than enough, and many would advocate for less.

    The difference in efficiency -- and thus the amount of fuel you will use -- is very small, provided the boiler is in that range. If you go too large, then efficiency will drop because the boiler will short cycle on pressure. If you go too small, it may not be possible for the boiler to fill all the radiation -- which is obviously undesirable. The range is from a boiler the rating of which exactly matches your radiation on the large end, to one which is only slightly in excess of the actual ability of the radiation to condense -- in your case, 96,700 BTUh. The CSG-C50 has a net heating capacity of 133,000 BTUh which, using a pickup factor of 1.33 gives the net rating they quote of 85,000 BTUh, or 364 square feet -- but that rating includes that pickup. Your radiation load is considerably less than the net output, though more than the rated square feet -- thus it is in that range.

    If your system has Hoffman valves on the radiators, does it also have Hoffman radiator traps? And you mention two steam/air valves on the 2 dry returns? Look at the ends of the steam mains near the related dry returns. Are there traps which connect the steam mains to the dry returns? There should be. These are called crossover traps, and they are absolutely essential to the correct operation of the system The main vents are the two steam/air valves which you mention. Can you determine what model or manufacturer they are? Or post a picture? Because I'd be kind of surprised if they were actually big enough.

    Are there any vents on any of the radiators? If this really is Hoffman equipped, and two pipe steam, there shouldn't be.

    Further, if this really is a Hoffman equipped system, it is also a vapour system, and designed to run on very low pressure -- all the more reason not to go too big on your boiler. But it also means the you must control the boiler with a vapourstat, to keep the pressure down where it belongs.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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