# Thread: Boiler Size based on Heat Loss Calc

1. Regular Guest
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I've used two on-line heat loss calculators. One used total cubic feet and square footage of exposed walls, windows etc. Result = 107900 BTU. The other used the linear footage of existing finned tube baseboards times a 600 BTU/Hr multiplier. Result = 106800 BTU.

Now the questions:
Do I use these BTU numbers for the input of the furnace or the output?

I'll assume output for the next question. If a manufacturer has a unit with 105000 BTU output and another at 132000 but nothing in between, do I go with the smaller, larger, or another manufacturer with a better match?

Thanks!!

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~ 100,000 BTUh
3 ( or more) large heat pumps and Dual Fuel.

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Originally posted by needinfo
I've used two on-line heat loss calculators. One used total cubic feet and square footage of exposed walls, windows etc. Result = 107900 BTU. The other used the linear footage of existing finned tube baseboards times a 600 BTU/Hr multiplier. Result = 106800 BTU.

Now the questions:
Do I use these BTU numbers for the input of the furnace or the output?

Fin tube rads with a furnace? btu's per foot of radiation based on a water temp of 180, are 580. This is the normal water temp for most residential boilers.
Sizing a home based on the rads will only give you the amount of heat that those rads can put out. Alot of times the heat demand can be less or more dependant upon changes done to the structure.
Performing a heat loss calc will give you the amount of btu's needed to properly heat a home without undersizing or oversizing the equipment.

4. How many zones may also make some difference

5. If you have an accurate heat loss of 107,900 and like a boiler with IBR of 105K, I sure wouldn't go bigger. The IBR output takes into consideration piping losses too and is very conservative.

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## Yes I wish is was uncomplicated also

Needinfo: Don’t know how to tactfully say this but simply pushing numbers through a “heat loss calculator” doesn’t constitute much in the way of a Heating and Cooling Load analysis. First and foremost you have to understand the theory behind the methodologies in order to appreciate the limitations and precision of each algorithm your attempting to apply.

Case in point: You are indicating results with precision down of four places (implying that your analysis is accurate to within +/- 50BTU/Hour (half the magnitude of the significant figures [non-zero values] you’ve stated above). If you take the time to study ACCA’s Manual J procedure (even the Seventh Edition) you quickly realize that even the “estimation” of infiltration rates alone would induce a margin of error larger than the differences you are indicating between the one’s you’ve arrived upon.

Now the good news, Furnace sizing does not require nearly as high a precision as Air Conditioning (see ACCA Manual S). Therefore my very sincere and heartfelt professional advice would be to obtain a copy of ACCA’s Manual J (Seven Edition, the Eight Edition is still in a gestation period) and ACCA’s Manual S (1995 Edition). Both of these publications are available FREE from your local Library via inter-Library Loan!

You can carefully read through these publications (about a weeks time @ 2-3 hours per day, which BTW is what a typical junior college student is expected to study every night) and become completely familiar with the processes involved. You can then do sample calculations varying the parameters and you’ll learn which factors are critically important in the final outcome.

Least you feel I’m belittling or mocking your attempts, quite the contrary my compliments for being proactive in your residential equipment selection process. Rest assured I’m earnest to the point of providing the ISB Numbers you need to give your local Librarian:

Manual J: ISBN 1-892765-01-2
Manual S: ISBN 1-892765-03-9

Thoughts on hydronic heat… You’ll have the splendid opportunity to zone the heat distribution a on per-room bases, thus even more reason to obtain a highly accurate load study… BTW: Is Air Conditioning a variable in the equation?

ACCA’s Manual J (Seven Edition) and Manual S (1995 Edition) can be easily comprehended with basic High School level algebra.

Again: Please seek the advice of a qualified reputable HVAC Contractor to review your findings. Since they deal with this professionally everyday they are keenly aware of many variables that I’m unable to dissertate upon on a Forum. The subject is fertile ground for an entire suite of textbooks, thus relying upon “innocent” use of “two on-line heat loss calculators” may possibly not be in your best long-term interest for a major home investment.

God Bless…

[Edited by faith on 10-18-2005 at 09:54 AM]

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Faith,

Thanks for all the references and good wishes. The problem is that if I wanted to be a heating & cooling contractor, I would begin reading all that information and going back to school. I'm already a specialist in a very technical field and need a new boiler, not another degree. However, it's this type of forum that is supposed to allow me to get answers from people who are the specialists rather than becoming one. I'm just trying to be an educated consumer to make the right decision about a product and contractor.

I know this wasn't in the background info I gave you, and everyone else on this highly recommended and well-reputed board, so I'll add it now.

I've had several contractors, all referals from people whose value I trust, come by to quote my project. They look at the current boiler, make some notes off the information stamped on the metalic label, ask me how many zones I have (when they can see the 5 valves right in front of their faces)and send me a fax the next morning.

So, let me ask a simple yes/no question. If my current 33 year old boiler is 160000 input BTU and 128000 output BTU, is getting a more efficient replacement as easy as finding another one with the same, or nearly the same, capacity?

If "yes", then I've already spent too much time looking into this important decision and will decide based on contractor rather than the products they are recommending.

From the hours I've spent researching and trying to do the right things like a heat loss calc to make an educated decision, I am inclined to think the answer is no. Therefore, does anyone know a reputable contractor (HVAC or plumber) in the Detroit area that will take the time to properly determine my needs?

Thanks

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The guys who read the labels are knuckleheads. They should have done a heat loss clac like you, but given the caliber of people they seem to be, it's doubtful that would valid either.

If you have a heat loss of 107,000 then select the boiler size closest to that in IBR. Example: some mfr boiler's have some range in firing rates; ie Burnham. Their V84 will meet your needs w/ the smaller firing rate, plus have some capacity for an indirect water heater zone, etc later. Or, if you want the V83 can do this at the upper firing range.

9. Most likely the boiler was oversized when installed. When installed the oil/gas was less expensive and if the system used 10% more fuel due to short cycling it really did not matter. I think you are really close in your estimation of the btus needed. My parents home i used the btu per foot of BB and it worked fine. They complain that it takes longer to warm up if they set back when gone for a few days. I went from a 160K input to 100K input, old was 70% new 80%.

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