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Undersizing a steam boiler

Shane_2 Member Posts: 191
Just looked at a job yesterday. Weil-McClain PEG-40 with copper risers and copper header. Leaking between sections. Nice smaller three bedroom home. Recommending boiler replacement with all new near boiler piping(steel).


Measured all radiators and came up with 520 sq feet, using charts from Lost Art. Not even considering existing boiler, but with chimney size, physical dimensions(getting into basement), and cost, Could one use a boiler with less EDR than charts say should be??


If this was a hot water system, heat loss would be less than 60,000 BTU. Having a hard time recommending a 200,000 BTU boiler for this job.


What would you guys do? What have you done in a similar situation?


  • Jamie Hall
    Jamie Hall Member Posts: 22,972
    Presuming this is steam...

    It is possible, but risky, to use an undersized boiler.  The problem is that if the boiler is too small it may not be able to raise enough steam to satisfy the rate at which the attached radiation can condense the steam, never mind the "losses" in the pipes (I hate that word "losses" -- it isn't really; just it's heat output which goes to the basement, not the radiators).  If that happens, it is quite likely that some radiators will get very little, if any, steam, and the system will be very very hard to balance if you can do it at all (and will only be balanced for one particular outside temperature, at that).  This result leads to unhappy customers.  Which is not a good thing.

    However, I have a question on the 200,000 Btu/hr boiler vs. the 520 square feet (presumable EDR): that sounds high to me, although I don't have the tables handy at the moment.  Even allowing for a 1.33 pickup, I get around 165,000 BTU/hr at the most... and I know the boiler in the building I super is around 340,000 Btu/hr, but for 1500 square feet EDR and works fine -- just about right -- which suggests you might be OK with even something as small as 120,000...  Anyway, something sounds fishy there.

    On the other hand, you should examine why the heat loss is so much less than the installed radiation.  House has been reinsulated?  New windows?  What has changed since the system was first installed?  It might be possible to get away with less radiaton -- depending on the system, simply turning off a radiator or two or three might do it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Put TRV's on the radiators....

    I have stumbled across systems in the field where the boiler was significantly undersized, and have put TRV's on all the radiators, and the system works great.

    In reality, the boiler is oversized for 98% of the heating season in most cases. In a lot of cases, the radiator need not be on, so if you can control it based on real time needs, the system should be balanced out, and the radiators will work in unison with the load, instead of dumping unnecessary heat into the space.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Ideas..

    The first question I would have is whether it is one pipe or two pipe.  "Undersizing" the boiler in a two pipe system has enormous historic precedent.  Going back into the manufacturer's literature for the early two pipe vapor systems with orificed inlet valves you'll find they directly address the situation when the boiler is not producing steam at a full rate.   Google Frank Gifford and you should come across an article he's written on this very subject.  In it he states he has numerous systems where the boiler are far smaller than the radiation load and they heat exceptionally evenly and efficiently.  I have some myself and am working on more currently. 

    For one pipe systems, I believe it is more difficult to downsize the boiler, and maybe it simply can't be done to quite the extreme.  However, if you apply the basic principals of venting steam mains very quickly AND RADIATAORS VERY SLOWLY and in proportion to the heating needs of the space they are in,  you can get a system that operates very evenly with exceptionally long firing cycles that produce excellent fuel economy.  If you consider that one pipe steam radiators rarely, if ever in most systems, heat completely, that then begs the question why is a boiler needed that is big enough to produce enough steam to heat them completely.  A safe alternative, some where in the middle of all the posssibilites, would be to size the boiler to meet only the actual heat losses FROM THE SYSTEM, including piping, and take out the typical pick up factor.  so if you have 520 edr = 124,000 btu/hr and then add whatever the loss is from the piping (let's say 20,000 btu/hr)  then lou need a boiler with an output of  144,000 btu/hr.  What this would do is always make sure that the boiler can continue to inflate the steam balloon of the system, so distribution should remain in balance.  However,  with loads so low, I would suspect that a smaller boiler is safe to use because during the typical heating cycle the boiler  would be shut off by the thermostat before it would run out of capacity to continue to fill the "balloon" .  If you are planning on setback, particularly steep setbacks, a bigger boiler would be necessary to allow a constant inflation of the steam balloon during the heating cycle.

    As others have stated, getting the system in balance is the key to making smaller boilers work...especially making sure that the steam fills the mains first before heating the radiators.  Big venting on the mains, and small venting on the radiators.  ie>  on a typcial one pipe steam system , I use Gorton #2's as main vents and then the rads get Hoffman adustable vents that are never set higher than about 2.5.   Take a look at the venting charts available on this site in the store.  You are looking to vent the radiators at rates used during the coal era....Think Hofmann #41 not Gorton D.

    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert

    Noisy Radiators are a Cry for Help
  • Shane_2
    Shane_2 Member Posts: 191
    Thank you

    Thank you gentlemen for the thoughful and helpful answers.

    This is a one-pipe steam system, paralell flow with wet returns, 2 - 2" steam mains. The 200,000 BTU came from the Weil-McLain chart which was input from an EG-55 with a 521 sq. ft. EDR rating.

    Usually in my area, the boilers I replace are 2-3 times oversized. I am not one to cheap out, I would rather not get the job, than install something that would not work as it should. In this case, I believe a boiler with less of an EDR rating than the radiators measure will still heat the space. 

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