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Steam boiler sizing dilemma

Ed59Ed59 Member Posts: 12
Hi folks, I've read every article I can find here and elsewhere and every post on The Wall about boiler sizing at least three times now, but I still feel unsure on my situation.

I've got a choice between a new Burnham IN4 (271 sq ft) and IN5 (358 sq ft).

I calculate I've got 253 EDR of radiator and the pros who've given me quotes get about the same (maybe slightly more).  Seems obvious the IN4 is the right choice and was recommended by one contractor I got an estimate from.  Everyone else recommends the IN5.

Here's where it seems a little more complicated.  This is a two family house and the current boiler (787 sq ft) heated the whole thing.  The other unit has disconnected from the system and removed their radiators, plugging most risers in the basement, but there may be one or two plugged up on the third floor, so closed unvented pipes going up the walls. 

And there is a lot of main piping, starting out with two branches at 2 1/2" and 3" then branching out more (getting smaller).  Maybe 100 feet or more.  (Yes there is a T into the main, and no I'm probably not going to spend the extra $1000 to replace it now.  It is 8 or 9 feet above the floor, so I'm hoping not too much wet steam gets up there).  Maybe this counts as particularly "unusual" piping.  (The mains are not well insulated now, but I'd plan to do that especially well if I got the smaller boiler)

Using Dan's suggestion from L.A.S.H. pg75 of increasing the piping-pickup factor to 1.5, that gives (253*240) * 1.5 = 91K BTU, 4K over the DOE capacity of the IN4. 

I'm also planning on getting a 45 gallon SuperStor indirect water heater on it, though I understand this shouldn't matter?

If anything, we could reduce the size of radiators since the house is better insulated (it's 1905 construction) and windows are being replaced.  But I don't know if that's really going to happen.

Does any of this justify bumping up to the IN5 (358sq ft, 115 DOE heating)?  Would that be a more conservative choice to be sure we can fill the mains and radiators (quickly), start up cold (fall/spring), maybe recover from a setback of 5 or 6 degrees?   If you needed 92 DOE would you go up to 115 rather than down to 87?  Any chance more than a 1.5 pickup should be used?

I'm concerned with being fuel efficient.  But we're also pretty concerned about being sure the house heats up when we want it to, and that we don't have to be calling back the professionals a lot to keep tweaking the venting to make it work well.

Any thoughts?


  • nicholas bonham-carternicholas bonham-carter Member Posts: 6,540

    Go with the one whose Sq ft of steam rating most closely matches the total of your EDR-the in4. The boiler rating already includes the pickup factor.

    Spend the money saved by that choice on a nice drop header, using both tappings out of the boiler, and a larger header. Don't forget the installation of large main vents.

    Try doing without the 5 degree setback and select a lower constant temperature.--NBC
  • BobCBobC Member Posts: 4,454
    Go with the smaller boiler

    If you insulate the piping in the basement all you have to do is select a boiler that has the sq ft rating to feed the radiation you have. That rating already has a 30% pickup factor figured in so you don't have to worry. Using a larger boiler would just be inefficient.

    It sounds like the installer that told you to use the smaller boiler knows what he is talking about. Make sure the new boiler is installed per the installation manual or better and that all steam carrying pipe is threaded steel. Have the pressure set as low as possible (1.5PSI) and you should be all set. Make sure any lines that are not going to be used are capped off so no mischief can come from them and insulate any piping you can reach with 1" rigid fiberglass pipe insulation.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Steve NicholsSteve Nichols Member Posts: 124
    Go with the 4 section

    I have close to the same square feet.   The contractor I hired put in an IN5 instead of an IN4, despite having only 260 sqft of steam. Not going to even get into that whole mess.  Suffice it to say that 4 section will work very well.  Mine is oversized and builds pressure rather quickly when all the vents shut.   Like the well learned individuals say above, size it to the SQFT of steam.  The BTU already has pickup factor built in.
    striving for peaceful coexistence with an oversized boiler....
  • Ed59Ed59 Member Posts: 12

    Thanks,Makes me worry less. I was just getting worried about the book suggesting 1.5 pickup factor on old buildings with a lot of piping.
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