Results 1 to 13 of 13

Thread: net stack temp

  1. #1
    Join Date
    Jan 2008
    Posts
    14

    net stack temp

    still/always learning - i attended a seminar this week, specific to oil burners if it matters, and when the discussion turned to net stack temp, there wasn't an explanation for it's value regarding the overall performance, and the instructor, in an aside, didn't have a valid answer. the book even had an asterisk after saying that the subtracted value must not be missed. well....the book also didn't provide a use for the value. ie - that value tells you xyz. or, now you can plug that value into x formula. asking questions of vets and searching the various net sites didn't help. with a typical hand-held or big $ box, the needed values are already provided. what say yee about the value of this subtraction lesson?

  2. #2
    Join Date
    Oct 2009
    Location
    Somewhere in Arizona
    Posts
    30
    I'm not sure if this helps, but I believe the value you're talking about is room temperature, room temperature must always be subtracted from net stack temperature as the air for combustion started off at space or room temperature.
    UA 469

  3. #3
    Join Date
    Oct 2009
    Location
    Somewhere in Arizona
    Posts
    30
    Failing to do this step while calculating efficiency would not only give you a false higher efficiency, but simultaneously ruin the boiler over time with incomplete combustion, and not to mention all that wasted heat energy going out your stack.
    UA 469

  4. #4
    Join Date
    Feb 2010
    Location
    Landis North Carolina
    Posts
    528

    net stack temp

    correct it will give an inacurrate false high reading resulting in possibly too low of stack temp,this can lead to improper draft and flue gases condensing ruining flue pipe and heat exchanger. net stack temp. is measured stack temp subtracted by ambient room/space temp= net stack temp. or actual flue gas temps. hope this helps

  5. #5
    Join Date
    Jan 2008
    Posts
    14

    thanks for the replies

    but in my post, i'd already stated that i know the subtraction to get there. while i can see that it can give reason to deduce efficiency and heat gain across the exchanger, but any 'new', standard unit will already tell you the efficiency. so, until i can get a better answer, i'll consider it as an old school method that is no longer valid, unless you don't have a meter

  6. #6
    Join Date
    Feb 2010
    Location
    Landis North Carolina
    Posts
    528
    I think i know what you mean now, higer stack temp.= lower efficiency due to fact more of your heat escapes up chimney. Lower stack temp.= higer efficiency less heat escapes up chimney and goes into your house. Did that help? Its used to set up burner,yes new units have it published but those numbers are only correct if burner is set up properly,ie:no oil furnace comes out of box ready to fire up and leave. Unlike gas furnaces, due to the design of burner and properties of the fuel all burners must be adjusted, the peramiters you measure include o2,co2,co,excess air,flue gas temp.,ect. these adjustments must be done prior to leaving job complete or dangerous operation and inefficiency, the flue gas temp. is used along with other peramiters to properly and safely adjust burner, so in order for the appliance to meet the rated efficiency, yes it is very valid today, and should always be used,it is not an obsolete measurement and shouldnt be negated! Just last week went on new customer oil furnace and it wasnt set up correctly very high fgt. and other peramitors out of adjustment, gave problems from day 1. Please learn the correct was to service these for you or your customers sake!

  7. #7
    Join Date
    Oct 2009
    Location
    Somewhere in Arizona
    Posts
    30
    I guess I don't quite understand, are you asking in other words that once you've done the simple math, how do you plug the difference in to give you some kind of idea of system performance? Remember, when doing any kind of combustion analysis your efficiency percentage rating is pure garbage and should be the least of your worries, unless you're in the residential market I suppose, otherwise you should be worrying about your other readings.
    UA 469

  8. #8
    Join Date
    Oct 2009
    Location
    Somewhere in Arizona
    Posts
    30

    Cool

    Quote Originally Posted by stickinit2thman View Post
    I think i know what you mean now, higer stack temp.= lower efficiency due to fact more of your heat escapes up chimney. Lower stack temp.= higer efficiency less heat escapes up chimney and goes into your house. Did that help? Its used to set up burner,yes new units have it published but those numbers are only correct if burner is set up properly,ie:no oil furnace comes out of box ready to fire up and leave. Unlike gas furnaces, due to the design of burner and properties of the fuel all burners must be adjusted, the peramiters you measure include o2,co2,co,excess air,flue gas temp.,ect. these adjustments must be done prior to leaving job complete or dangerous operation and inefficiency, the flue gas temp. is used along with other peramiters to properly and safely adjust burner, so in order for the appliance to meet the rated efficiency, yes it is very valid today, and should always be used,it is not an obsolete measurement and shouldnt be negated! Just last week went on new customer oil furnace and it wasnt set up correctly very high fgt. and other peramitors out of adjustment, gave problems from day 1. Please learn the correct was to service these for you or your customers sake!
    That's what I was trying to say, well said.
    UA 469

  9. #9
    Join Date
    Jan 2008
    Posts
    14

    yikes!

    except for the moonlighters, we're strictly commercial/industrial

    infidel - you sound confused
    "room temperature must always be subtracted from net stack temperature"
    subtract room from gross stack temp
    "efficiency percentage rating is pure garbage"
    huh? efficiency percentage is what you're 'manually' shooting for by using the net stack temp

    stick
    "no oil furnace comes out of box ready to fire up and leave"
    of course not. but if you you use a 'modern' analyzer, there's no need to calculate anything, including nst. the analyzer does it for you

    and after speaking with a beckett tech, he agreed that the 'old school' is a thing of the past, bec the new analyzers produce the necessary values. in real time, and with more sensitive precision. especially digital. he also said that he didn't know of an actual formula in which to plug values for a calculation

  10. #10
    Join Date
    Oct 2009
    Location
    Somewhere in Arizona
    Posts
    30
    I think you're the one who's confused, tell me what exactly you're using efficiency for? On any modern analyzer while performing combustion test just for the hell of try manipulating your other values out of range, what happens to your efficiency, not a whole hell of alot, point being on the list of concerns when performing combustion readings, efficiently is at the bottom. Take a look at your original post, not very clear. You sound like a douche
    UA 469

  11. #11
    Join Date
    Jan 2008
    Posts
    14

    oh my, name calling.

    you big poo-poo head
    quoting you again - "room temperature must always be subtracted from net stack temperature" WRONG!
    Born to be wasted? yeah it sounds like it from the above quote

  12. #12
    Join Date
    Oct 2009
    Location
    Somewhere in Arizona
    Posts
    30
    I'm sorry emdog you're absolutely correct, I miss spoke
    UA 469

  13. #13
    Join Date
    Oct 2009
    Location
    Somewhere in Arizona
    Posts
    30
    You big poo poo head? I love it
    UA 469

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event