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Boiler Size

We have a 4B Burnham gas boiler that needs replacement. At some point, someone removed the two outside burner tubes, which reduced the total amount to eight (I'm told this was not a good idea). The system runs wonderfully, though. It's a two pipe Moline system with all of it's components removed except the radiators. The radiators have a valve at each end and no steam traps. The house has a total of ten radiators of varying sizes.



The burner typically runs for 30 minutes (sometimes as long as 45). The pressure gauge never moves off it's rest and the pressuretrol never trips. All the radiators warm to 200 degrees. The return lines get only warm to the touch.



My question: the original boiler steam BTU is rated at 107,300. When the two outside burners were removed (two out of ten), did this reduce the BTU output by 20 percent? If so, that would make it 85,840.



Burnham's IN5 boiler looks to be a perfect match at 86,000.



<a href="http://tinyurl.com/4j2ynkj">http://tinyurl.com/4j2ynkj</a>



Thanks,

Herbert

Comments

  • JohnLaPlante
    JohnLaPlante Member Posts: 49
    EDR?

    Hi Herbert,



    I asked almost the same question a couple of months ago, and the great people here first directed me to the amount of radiation I had connected to the system.  Do you know how many square feet of radiation you have connected? 



    I used this spreadsheet to calculate mine:





    http://www.usboiler.burnham.com/contractors/tool-box-sizing





    Good luck! 
  • Wendell
    Wendell Member Posts: 61
    edited February 2011
    Thanks for the link...

    I should probably calculate the radiation. But I'm not sure it's important in my case. It's my understanding that the Moline systems ran on ounces of pressure not pounds. They had a condensation loop with an injector thingy that would make a negative pressure on the returns (guessing). The radiators are adjusted by a series of orifices in the inlet valves. If I go by standard tables, I fear I will be over sized.



    I have no steam traps to prevent excess steam from entering the returns. If the pressure is too high the system will hammer. Last season I found the old burner tubes in the attic and installed them. The system would hammer if it ran for more than 25 minutes. The pressuretrol would trip at 1.5 pounds and cycled off for half a minute and then on for one minute. That's a lot of cycling!



    With two of the tubes removed the system purrs like a kitten. The pressure is unmeasurable with the 30 pound gauge and, as mentioned, the presuretrol never trips. I'm amazed how well the radiators heat with such a low pressure.



    Herbert
  • Youv'e got the right idea, Herbert

    The maximum boiler capacity that you should need is 80% of the radation size, since this is how the system was sized in the first place.    Since you have the orifice valves, I would be more inclined to size the boiler  to the current heat loss (or at least closer to) and then balance the system using the valves.  I've sone this before on a Moline and it worked beautifully.
    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert





    Noisy Radiators are a Cry for Help
  • Wendell
    Wendell Member Posts: 61
    edited February 2011
    I suspected my set up was unusual, but not alone.

    So... does removing two out of ten burner tubes reduce the steam output by twenty percent?



    Or is it not that simple?



    Herbert
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Should be close

    If you removed 2 out of the 10 burners, the gas input would be reduced by 20%, but this may not exactly correspond to the change in output. Removing the burners will reduce the efficiency slightly, so the actual output may be slightly less than what is implied by the reduction in input. It will be close though.



    Another way to determine the actual input is to clock the gas meter by timing how long it takes to consume a given amount of fuel. By determining how many cubic feet of gas is used per hour, the actual BTU per hour can be determined. Comparing this figure to the BTU input rating of the replacement boiler will give a useful figure, provided the relative efficiencies of the boilers are comparable.
  • Wendell
    Wendell Member Posts: 61
    Does 126 cubic feet/hour sound about right?

    Thanks, Mike. I counted the 1/2 foot dial for 5 minutes three separate times and averaged 21 revolutions.



    21 divided by two = 10.5 cubic feet in five minutes. 10.5 times 12 = 126 cubic feet/hr.



    How does 126 cubic feet convert to BTUs?



    Herbert
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    BTU per Cu Ft

    100 Cu Ft of natural gas has a heating value of about 102,000 BTU, therefore your input is 128,520 BTU/Hr.



    The Burnham IN5 is rated at an input of 140,000 BTU/Hr, so it would have a slightly higher output than your existing boiler.
  • Wendell
    Wendell Member Posts: 61
    edited February 2011
    Considering the present boiler is rated at 175,000

    (with all its tubes installed), I think we're headed in the right direction.



    Herbert
This discussion has been closed.