BTU'S

Brad White_73

Ed, you are right about this forum. We all learn from each other and teach one-another. I learn here everyday.

And what I liked about Kevin's question is that it falls into the category of "nobody tells you these things". Like acronyms and "common knowledge", there is an assumption that everyone knows but nobody has the fortitude to ask. I am glad Kevin did.

As for learning and teaching:

I was recently reminded of an old adage from medical training (EMT/Military Medic training -dang-30 years ago):

"Learn One, Do One, Teach One, Screw One." Meaning you take on a new bit of knowledge or learn a procedure, practice it, pass it on, then get brought to earth and humbled because you lost sight of something essential. Works in too many trades!

KAG

What's the difference

I have had this question in my head for a couple of years. When sizing a heating system which of the 3 btu rating should I use and what do they each mean as far as useage? Input, output and I B R. I have always used output ratings to size the systems

Brad White_73

Input, output and really conservative output....

Kevin,

Yes you want to use output ratings but the question then becomes one of "gross" and "net".

Fuel input will of course vary with the fuel used. Gas input is usually AGA (American Gas Association) and is listed in BTU's per hour (like all of the data). Oil of course is in GPH but may also be expressed in BTUH. The DOE (Department O' Energy) uses similar values.

BTU Output (gross output) is just that; steady-state albeit at certain conditions. The DOE figures are based (IIRC) on a boiler running for 20 minutes with certain inlet and outlet water temperatures or steam at a given pressure all in a room of a certain temperature. Basis of rating; never happens in real life, right??

I=B=R net output is the gross output de-rated 15% for hot water boilers and usually 33% for steam. Ostensibly this is for piping and pick-up from a cold start.


I find the I=B=R method tends to over-size boilers and it will unless you take pains to control the variables such as having insulated piping and figuring that you are not starting the boiler from scratch on a design day.

For example, a boiler with 100,000 BTUH input may have an 82,000 BTUH gross output (82% combustion efficiency). The corresponding I=B=R rating would be about 71,300 BTUH. The assumed cushion you have here is 10,700 BTUH to warm up the pipes or to get you to temperature sooner.


Bottom line, I use the gross output rating where I can but incremental realities of boiler sizing dictates that my boiler output will be somewhere between the gross and net outputs, as close to the gross output as I comfortably can achieve.

KAG

Thanks

I often wounder why the three ratings on the boiler, but never asked. Then I started to read P&M mag, then found this site and was blown away by the KNOWLEGE of heating and how it can really make a differance. I was brought up in the way of "Put this in. Is it going to work? Ya! How do you know. STOP asking questions and just put it in." It is nice to know I can finally ask the questions and not get something thrown at me. Thanks again.

Edward A. (Ed) Carery

KAG's good question.

Kag,

Your question was a good one, and Brad's answer was really well done.

You will never have anyone here bust you about asking a question. Sometimes a smart aleck like me will make a friendly joke about a question, but you will find that no one is ever laughing at you; they are all laughing with you.

I have found this site to be kind of a brotherhood of people with a mutual respect for all, regardless of age or experience. We are all fortunate to be here.

Best Regards

Ed Carey

S Ebels

Brad

I picked up on this thread whilst cruising another, must have missed it the first time around.

What's your take on boiler output ratings when dealing with the new modulating/condensing boilers? I have the feeling that IBR is completely obsolete for these especially considering that a lot of the systems connected to them are constant circ. Would you agree that constant circ pretty much eliminates piping and pickup allowances?

Brad White_9

Absolutely, Steve

The I=B=R method has it's place but I agree wholeheartedly that constant circulation (and it's newest sibling, the Vitodens Principle of which I am still learning) tends to negate any pick-up losses to speak of. Constant circulation especially with radiant and minimal if any setbacks by definition will eliminate "pick-up" losses.

It is for this reason that I use what are very close to combustion efficiency ratings when sizing a mod-con or other high efficiency boiler. I will skew a few points off for degradation of efficiency and base efficiency. By this I note that a design day may require the hottest water -of course it will- so I start with the efficiency at that higher operating temperature. Higher efficiencies toward spring and fall are a seasonal usage bonus.

For example, a purported 96% boiler may be so at 80 degree return water but will be 90% with 120 degree return water. My assumed boiler capacity if say 100 MBH input, would be 88,000 more or less. Two points off the ideal. But no additional 15% on top of that.

Of course if there is third party verification at various temperatures, I would rely on that rather than my judgement. But thus far, we are all awaiting that and rely on the experience of people like yourself, Mike Thies, Mark Etherton and many others we all know to write this new book!

And I admit the modulating feature gives me some leeway.

Brad