(206) 949-5683, Top Rated Plumber, Seattle
Page 2 of 2 FirstFirst 12
Results 16 to 21 of 21

Thread: What size boiler?

  1. #16
    In the trades Dana's Avatar
    Join Date
    Jan 2009
    Location
    01609
    Posts
    2,720

    Default

    The seasonal deltas are easy. Converging consistently, not so much. Knowing whether the occupant habitually kept it at 74F with a couple of windows cracked, or how often they used the wood stove or when and how long they left on vacation (and what they'd set the T-stat to) are purely speculative. Month to month working some reasonable assumptions on background load it doesn't always converge- you HAVE to know the habits & occupancy rate of the place to narrow it down, which isn't always knowable the way it is at your own place.

    My manual-J on my mother's place came in at ~18K, which was (uncharacteristically) at the low end of the 17K-26K range I was coming up with using reasonable guesses based on the prior owner's billing history & HDD data. Third party company proposals for mini-splits running other heat loss calculation software ranged from 17.6K-20.1K. (Maybe that overheated HW heater in the uninsulated utility closet WAS skewing the result, or the prior owner was a fresh-air freak? We'll never know.) Reality may be as low as 15K with the windows closed, but probably not much lower. Without consistent occupancy and use information there's only so much faith one should put in a fuel use calc for sizing the equipment, unless the data are rock-solid consistent from one billing period to another. With diligence a Manual J approach can still overshoot by 25-50% , but almost never by 100%, despite it's many shortcomings.

    Working from a heat pump power use is fraught with error, at least on the heating season end when auxilliary resistance strips can cut in, even if it's somewhat consistent for cooling season use. Modeling a heat pumps COP curve against the actual vs. average daily outdoor temperature already has error bars as big as anything in a Manual-J. Most mechanical equipment is oversized and will keep up no matter what it's state of decrepitude, and even undersized equipment doesn't often fall short by much.

  2. #17
    DIY Member Buffalobillpatrick's Avatar
    Join Date
    Sep 2011
    Location
    Stonewall Colorado
    Posts
    38

    Default

    The DOE online program RESCHECK is pretty easy to use. Uses entire house envelope inputs.

    https://energycode.pnl.gov/REScheckWeb/

    RESCHECK gives my house Total: UA = 397

    397UA x (100* my delta-T (-30*f -> 70*f)) = about 40Kbtu/hr boiler

  3. #18
    In the trades Dana's Avatar
    Join Date
    Jan 2009
    Location
    01609
    Posts
    2,720

    Default

    And that's exactly 35BTU per square foot, right?

  4. #19
    DIY Member Buffalobillpatrick's Avatar
    Join Date
    Sep 2011
    Location
    Stonewall Colorado
    Posts
    38

    Default

    Dana, R U asking me? If so it was 14btu/ft2
    I'm not getting what your saying?
    Last edited by Buffalobillpatrick; 01-24-2012 at 05:43 PM.

  5. #20
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,405

    Default

    Someone earlier said to just use 35btu/ft2...obviously that WAG as a 'standard' would have produced a boiler MUCH bigger than you need, which is why you need to run the calcs - a WAG is rarely correct.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #21
    In the trades Dana's Avatar
    Join Date
    Jan 2009
    Location
    01609
    Posts
    2,720

    Default

    Jim's right- I was making fun of the rule of thumb presented in an earlier post.

    Your true number of 14BTU/ft makes my point- those BTU/ft rules of thumb (in this case 35 BTU/ft had been presented) reliably oversizes real loads, often by more than 2x, which is why you should NEVER use them. Old skool heating contractors like them though since it means they never have to actually think about it, and it's never undersized, avoiding the 6AM call from the irate freezing client. But it's the client that pays- more up front for the larger unit, and lower efficiency/higher maintenance down the line as it short-cycles itself into an early grave.

Similar Threads

  1. Flue liner size for boiler?
    By NH Homesteader in forum Boiler Forum
    Replies: 4
    Last Post: 09-16-2009, 02:42 PM
  2. Replies: 3
    Last Post: 09-10-2009, 12:11 PM
  3. boiler vent size problem
    By lurch in forum Plumbing Forum, Professional & DIY Advice, Tips & Tricks
    Replies: 11
    Last Post: 06-09-2009, 11:34 AM
  4. What size boiler
    By JOE1934 in forum HVAC Heating & Cooling
    Replies: 5
    Last Post: 06-04-2007, 10:39 AM
  5. Back boiler & oil boiler to twin copper cylinder
    By sheen in forum Plumbing Forum, Professional & DIY Advice, Tips & Tricks
    Replies: 2
    Last Post: 12-07-2006, 04:34 AM

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •