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Burnham Megasteam sizing

USA498 Member Posts: 45
I am still trying to wrap my head around the size of a replacement boiler.

Here is the scenario. Mega Steam wanted.

EDR of system = 407  10 Radiators - most 3 col 26"  a couple of 5 tube radiators.

<strong>Current boiler</strong> - Peerless JOTTW 175   646 edr 1.75gph -  apparently way oversized but never trips the vaporstat. Runs at 2-3 oz throughout the heating cycle even on a long recovery from setback. Riello F-5 burner  1.25 nozzle ( assuming 140 psi at pump ). Oil guy gave it an 80% efficiency rating after servicing and recommend it be replaced

<strong>1st possible replacement</strong> - Burnham MST 396 - 396edr 1.05 gph - EDR says its the right size but the difference ( 39%) from the current boiler is huge. I am a little concerned that it will not get the job done.

<strong>2nd possible replacement</strong> - Burnham MST 513  513 edr 1.35gph - EDR says it should be too big but it is 21% smaller than the current boiler.

Any insight would be greatly appreciated.

link to current system <a href="http://www.heatinghelp.com/forum-thread/136919/Almost-done#p1234763">http://www.heatinghelp.com/forum-thread/136919/Almost-done#p1234763</a>


  • JohnNY
    JohnNY Member Posts: 3,229
    This is one of the hardest things about my job (the one when I'm wearing the Salesman hat)

    The size of the old boiler means absolutely nothing. Put it out of your head.

    The EDR calculation represents the system's ability to convey and condense steam effectively and efficiently.

    The number it gives you is the number you need with a little bit of fudge factor built into the boiler ratings.

    Install the right boiler, insulate your pipes, and never think again about that old, oversized hunk o' junk you've been using for way too long.

    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    Only thing I might add

    to JohnNY's absolutely correct remarks -- that monster you have is rated at 646 EDR at 1.75 gph.  You are running it at 1.25, although possibly with a slightly higher pressure.  That half a gallon per hour does make a difference... if the pressures were equal, it would come out to about 460 EDR.  Which is still oversized, but not so radically.  And as your oil burner guy noted, not so great on efficiency (should be able to get up to 85% with a good burner).  I suspect that your first choice is probably the right one, based only what you say. 
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JohnPatFram
    JohnPatFram Member Posts: 1
    Sizing a steam boiler

    Most of the steam boilers I have encountered have been oversize for the system. The main reason was that there was not a small enough boiler for the system. Now I am working on a steam system that has failed after 7 years and I think the boiler is too big and does not run enough. So if the boiler is down sized a little it will just have to run longer and running longer is a good thing. What am I not thinking about?
  • Henry
    Henry Member Posts: 998
    Boiler sizing

    Use the EDR! An option instead of replacing the boiler is putting a modulating burner. Even if you replace the boiler, a modulating burner is ideal.
  • USA498
    USA498 Member Posts: 45
    Thanks for all the info !

    What effect will the Hot Water Coil have on the boiler EDR.
  • vaporvac
    vaporvac Member Posts: 1,520
    Usually none...

    Usually adding a HW coil will not affect the sizing of the boiler which should still be based on the attached (or planned) edr. It merely uses the excess capacity that's already included in the boiler size, usually 30% This is needed to warm up all the piping leading to the radiators and the cold metal. It's referred to as the pick-up factor. When you figure out your edr and size the boiler based on that alone, it is ALREADY included in the size of the boiler. Just be sure not to add it in twice.

    Two-pipe Trane vaporvacuum system; 1466 edr
    Twinned, staged Slantfin TR50s piped into 4" header with Riello G400 burners; 240K lead, 200K lag Btus. Controlled by Taco Relay and Honeywell RTH6580WF
  • Jason_13
    Jason_13 Member Posts: 304
    DHW coil?

    Forget that idea. It is the most expensive way o make hot water. Use n IWH as it can pipe into the boiler.
  • Henry
    Henry Member Posts: 998

    Making DHW with a steam boiler is a very energy inefficient way, specially during the summer. Get a dedicated hot water tank.
  • Jason_13
    Jason_13 Member Posts: 304

    You can add an Indirect water heater tot he Megasteam. There are control and pipe tapings(tapings K & L) for doing just that on the boiler.

    Avoid the domestic hot water coiler. very expensive way to make hot water as the boiler maintains temperature all the time.

    The boiler is cold start and only runs in the summer for a call for hot water.
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