net stack temp

[emdog]

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?

[Infidel]

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.

[Infidel]

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.

[stickinit2thman]

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

[emdog]

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 so, until i can get a better answer, i'll consider it as an old school method that is no longer valid, unless you don't have a meter

[stickinit2thman]

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!

[Infidel]

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.

[Infidel]

I'm sorry emdog you're absolutely correct, I miss spoke

[Infidel]

You big poo poo head? I love it