Confused by Boiler Ratings

[Mushroom Man]

If a wood boiler has a rating of 60 KW (205,000 BTUs) what does that actually mean? Sounds like a naive question BUT:

My confusion stems from the fact that different woods release differing amounts of heat. Add to that the fact that moisture content impacts the release of heat, or at least its useful energy, and you have an almost bewildering range of heat that can be released.

Doesn't the amount, kind and wetness of wood in the firebox have a greater impact on the heat that is produced than the machine itself.

Using this logic, a 40KW machine packed to the gunnels might easily put out more energy than a 60KW machine loosely packed. Is this logic correct or have I got it all wrong.

Take this calculation for example
I put in 32 lbs of white ash at 6400 btus per lb and I get nearly 205000 btus of heat. That's not a lot of wood in my firebox and yet that is its rated capacity. I could easily double that weight in wood. Does that mean I would have a 400000 capability?

As you see, I am quite confused. Anyone have answers?

 

[Nofossil]

It's pretty easy to get wrapped up in the numbers. In all reality, the output is a rough estimate. 205,000 BTU/hr should really be interpreted as "more than 150,000 BTU/hr and less than 250,000 BTU/hr".

My boiler is rated at 80,000 BTU/hr and I've seen outputs at full throttle from 60,000 BTU/hr to 90,000 BTU/hr.

A good rule of thumb is that you can expect a gasification boiler to give you around 75% of its rated output as a good working average over the course of a fire. It will be lower than that at the beginning and especially at the end. but probably near its rated output in the middle.

They're typically rated assuming dry hardwood.

 

[jebatty]

Based on experience with my boiler I assume that the rated capacity is btuh output under fairly ideal, peak or near peak conditions. Nofo's answer is pretty good on the 75% assumption as a good working average.

 

[Duetech]

You have some good questions and some good answers.Though it would be interesting so see which medium and it's moisture content is being used in the trial runs to determine the claimed btu output. Fossil fuels give a rated output based on an assumed average fuel quality but even there a difference in quality is present though it is rarely discussed and the general populace usually takes it for granted that the unit will perform as advertised. Couple the variations of wood boiler fuels used and the varied fuels available at given different destinations of the products it seems it would be hard to make any claims of potential output without a list of fuels used with a wide array of parmeters including length of burn per load for the fuels used. (perhaps this is a standard that customers should demand of manufaturers since part of the sales pitch is based on an assumed quantity /quality output and a "limited" warranty) I.e. areas that will have pine as a dominant fuel supply would not be expected to get the same results per load and btu output as white oak regardless of the near similarities of btu's output per pound of wood. Comparably pellet stove useres get varied results with brand A versus brand B pellets. Boiler makers cannot be held accountalbe for the type of fuel used but they shoud be required to divuldge the test data results that brought them to the numbers they advertise.

 

[jebatty]

I don't think in the abstract wood species has too much of an impact on the maximum boiler rating, but moisture content does. I would assume, rightly or wrongly, that the rating is based on the mfr's recommended MC for wood. Species does have a real world impact though, as the draft and air mixture impact rate of burn considerably. If a "sweet" spot for draft and air settings was picked for one species of wood, that might not be optimal for another species. The softwoods (and some of the soft hardwoods) which also are very dry burn hot on my normal draft and air mixture setting on my Tarm and the burn is somewhat cooler on an oak load (of which I have little). The result is not boiler over-heating, but is high btu output along with higher flue temps, which also means somewhat reduced overall efficiency. At the same time, I know I can reach rated capacity of btu output if, by adjusting draft and air mixture settings, I allow flue temps to go "high" (about 600F) while I cannot achieve that with "low" (400F) flue temps. My preference with regard to draft and air mixture settings is "to set it and forget it," and let flue temps and btu output vary according to the wood being burned.

It's impt not to forget that burning wood is not like burning gas or oil, consistent fuels. Variation in burn and output should be expected, are not harmful in my experience, and the I suspect the overall efficiency impact is less than 4%.

 

[Mid Michigan]

I must humble disagree and put my .02 cents in.

There are many variables when it comes to OWB efficiency. Here are four I can think of.

Type of wood: here is where I must disagree. I can burn Osage Orange (have no idea were to find it) that has 32.9 million BTU per cord or White Ceder that has 12.3 million BTU per cord. These are the two extremes. One can see a drop of 73% in efficiency from the fuel being used.

Second: I will agree moisture content also has a lot to do with OWB efficiency. Even the above figures are based on 20% moisture content. It take BTU's to boil the water out of the wood. The more moisture the more wasted BTU's.

Third: Creosote has a big impact on these type of stoves. The walls of the firebox are covered in it. The amount of creosote on walls has a direct relationship of how well the heat transfer is. If these stove do not have a secondary burn chamber, than it is getting some of the heat transfer directly from the fire and some from exhaust as it weaves it's way through the stove.

Fourth: How well your OWB drafts will have a impact on performance.

Check out this link for different type of wood and there BTU's available: Look at how a large a gap there is just between Sugar Maple and Red Maple. Not all maples are created equal.

(broken link removed to http://chimneysweeponline.com/howood.htm)

 

[stee6043]

I took some interesting readings this Fall when heating my storage tanks for the first time. When I was heating 1000 gallons of water from cold start (59 degrees) I was only averaging 52,000btu/hr output from my EKO 40 for roughly 12 hours straight. This was with an all day long, good solid burn with good dry wood. My real "boiler output" was likely well above 100,000 btu/hr but the water in my tanks was soooo cold that a significant amount of my boiler output was being used to maintain proper boiler input temps. As a result my system output was an uninspiring 52k per hour.

So for what it's worth - boiler output, as stated above, is almost as complicated as the mind of woman.

 

[Donl]

So for what it's worth - boiler output, as stated above, is almost as complicated as the mind of woman.

If that's the case I see no point in pursuing this any further!

 

[Hydronics]

So for what it's worth - boiler output, as stated above, is almost as complicated as the mind of woman.

If that's the case I see no point in pursuing this any further!

Well now, not even the Space Shuttle is as complicated as the mind of a woman...
Good thing my Wife doesn't read this forum! I hope...

 

[Como]

Altitude can be a big factor for some of us.

 

[heaterman]

Boiler ratings have little to do with the wood being burned, little to do with what size of fire in it and little to do with how it's operated. The distinction here is that we are talking about ratings not actual output. Output of course depends on too many factors to discuss in anything short of a masters program thesis.

Boiler ratings are just that, a rating that says the particular product has the heat exchange surface area needed to transfer xxx,xxx btu's. This of course is under laboratory conditions and a consistent, controlled set of operating conditions which are designed to level the playing field. Most reputable manufacturers use an independent lab to do this testing and arrive at a conclusion. These input ratings are done using standardized testing much like the EPA's emission testing, which btw, bears little to no resemblance to real world conditions.

So I guess the best way to look at a firing capacity rating for a wood boiler is simply to use it as a rough gauge for comparison to other like products.
Brand A sells for $xx.xx and brand B sells for $xx.xx, both are "rated" the same so the matter of choice boils down to product reputation, dealer service or lack thereof, installation help and support. The ratings should be looked at simply as a "yardstick". They are accurate to the extent that the testing was done correctly and the manufacturer has not fudged the numbers in their favor.
Not all manufacturers operate with the same level of integrity nor do all utilize the same methods of testing. I know that some of the European models for example are listing efficiency numbers that result from what is called the "high heat value" of the wood which counts the energy released in the form of water vapor even though it cannot be captured and used. Some US models like the Garn for example list efficiency using the more standard "low heat value" which does not count the energy in the condensate. These would be more accurate assessment of actual field efficiency.

Some manufacturers rate their output conservatively also. For example, I know from conversations with the factory and real field experience that Econburn's 150, rated at of course 150,000 will actually and consistently produce a higher output than that by a fairly large percentage. I clocked one as best I could using temp rise of a known volume of water at close to 180,000, the factory said 172 IIRC.

At any rate, a prudent buyer would use the btu rating for comparison but never count on actually achieving that rating on a day to day basis. Better to be a little over your heat loss than under.

EDIT:...... I just can't help but add here that the above is one of the main reasons that storage with a wood burning system is sooooooo important.

 

[Mushroom Man]

Bingo Heaterman. That seems plausible. The real "field leveler" would be a consistent testing regime. I doubt that manufacturers in Germany, Poland, the USA etc. have adhered to any such testing regime; but at least the heat exchange capacity consideration would be a starting point.

Measuring the heat exchange occuring in your storage would give a clue as to whether you are getting somewhere close to your boiler's rated capacity; assuming you know the wood type, the moisture content, the weight of wood, the efficiency of the boiler and the elevation. Too much guesswork for this rural dweller.

I guess its back to thinking about and plumbing the barn loop. Thanks for the thoughtful answers.

 

[jebatty]

The traditional water jacket OWB is inherently of such low efficiency that any discussion as to ratings and use of one type of wood or another is an exercise in futility. Kind of like trying to reduce water usage by getting a low flow shower head while leaving all the other faucets wide open.

As to the comment on the length of time to get a cold water tank up to a usable temp, perfectly normal, and think of it as btu output vs rise in boiler temp. I will experience the same situation but with this data: boiler output temp to storage 165F, boiler return (Termovar return water protection) of 140+F, storage return to boiler 65F. That's a 100F temp rise. I have a flowmeter on my boiler, and flow to boiler may be just 2 gpm. But 2 gpm with a 100F temp rise = 100,000 btu output to storage, and balance of boiler output being used to provide return water protection. Not a bad result. At 2 gpm will take hours to raise the temp of 1000 gal storage for sure, but the boiler is still performing likely not too far from capacity, depending on burn rate.

 

[heaterman]

Heat transfer example and firing rate comparison:

We recently did the final wiring and fired up a Garn WHS2000 that a customer had installed himself. No provision had been made to pre-warm the water in the tank so 1900 gallons + the 4000# of steel was sitting at 43* in his shed. Nothing to do but put a bucket under the exhaust and fire it up so......that's what we did.

If you crunch the numbers, we were working with about 15,800 pounds of water (approx 1900 gallons of water) and 4,000 pounds of steel. Lumping them together for the sake of simplicity (although the specific heat is different for water and steel) you can figure we had 19,800 pounds of "load". The Garn is rated at roughly 450,000 btus's per hour firing rate and we had the opportunity to feed this one while we did other work on the install.

We started with some kindling and really dry pine that the owner had stacked up. All of the pieces were relatively small, under 6" diameter and filled the firebox about 1/4 of the way full for the first burn. When that was burned about halfway down (maybe 20-25 minutes, we loaded the Garn up with a mix of the same wood along with some dry oak to about 1/2-2/3 full in the firebox and kept feeding it a few more sticks of wood about every 30 minutes.
Condensate poured out of the exhaust until water temp read 90* or so. From there to 100* it tapered off to nothing. We collected about 3 gallons in a 5 gallon bucket. I was curious as to what the actual output might be so we were clocking the whole ordeal to see how long it took to hit 180*. Total time from first fire to 180* on the tank thermometer was 4 hours and 11 minutes.

Working that out using the 19,800 pound number tells me that we were getting an average output of close to 640,000 btu per hour. Far in excess of the rated output.
The math goes like this...... 19,800 x 137* temp rise = 2,712,600 btu's Divide that by the time needed to reach that temp rise (roughly 4.2 hours) and you get 645,857 btu/hr input.

Of course at the beginning of the burn the Garn was hitting well over 90% efficiency judging from the fact that flue gas temp didn't even register until the water temp hit 65*, and the volume of condensate running out of the exhaust.

So, does that mean all Garns will do 645,000/hour? Certainly not. That is a pretty extreme example starting with 43* water and we were pretty much maintaining optimum firing conditions through out the burn. So you can see that actual output is a moving target and depends on a host of factors. The factory rating is probably, or maybe I oughta say "should be" an average rating of normal field conditions while factoring in the transfer area of the HX.

 

[julien]

Specific heat capacity of steel is 9.42 X lower then water by weight, 4,000 pounds of steel are equivalent of 424 pounds of water. 1,900 gallons @ 8.35 ponds/gallon give 15,865 + 424 = 16,289 X 137° = 2,231,593 btu's divided by 4.2 hours = 531,133 btu/hr . I presume air space in the tank for expension is higher than 5 % and maybe there have not 1,900 gallons in.

 

[Gooserider]

Specific heat capacity of steel is 9.42 X lower then water by weight, 4,000 pounds of steel are equivalent of 424 pounds of water. 1,900 gallons @ 8.35 ponds/gallon give 15,865 + 424 = 16,289 X 137° = 2,231,593 btu's divided by 4.2 hours = 531,133 btu/hr . I presume air space in the tank for expension is higher than 5 % and maybe there have not 1,900 gallons in.

I think you may be using those weird "Imperial Gallons" Julien... I'm pretty sure Heaterman was using US gallons, which changes things a little. I would also assume that a WS 2000, would be a 2,000 US gallon tank from the model number, so having it filled to 1900 gallons to start with sounds about right...

Gooserider

 

[heaterman]

Julien, I should have looked up the specific heat numbers before I posted. I knew that steel was less than water but not by that much. Good catch.

As to the actual amount of water in the Garn I would say it was all of 1900 US gallons so I was being conservative there. Garn specs show the nominal or average expected fill to be 1870 gallons. This one was full to within a couple inches of the overflow tube so it was likely higher than that by maybe 50-60 gallons. I would say we lost about 10 gallons or so through the overflow tube as it warmed up.

The point being made was that rated heat transfer capacity is often different than what is produced in the field especially with wood fired equipment.

 

[julien]

I think you may be using those weird "Imperial Gallons" Julien... I'm pretty sure Heaterman was using US gallons, which changes things a little. I would also assume that a WS 2000, would be a 2,000 US gallon tank from the model number, so having it filled to 1900 gallons to start with sounds about right...

Gooserider

«I think you may be using those weird "Imperial Gallons" Julien»

No no no..., imperial gallon weigh 10 pound, not 8.35 !

And for your info Goos, all canadian use a metric scale since ~30 years...

http://www.thecanadianencyclopedia.com/index.cfm?PgNm=TCE&Params=A1ARTA0005262


----

I just believed that Garn whs2000 have a 2,000 gallon volume, include air expension. And to keep 5% of water expension you requires a other % compressable air volume.


@ Heaterman, I agree, that rated heat transfer capacity is often different than what is produced in the field especially with wood fired equipment. My post is just to increase a bit a precision. And effectively, is surprising the highest capacity of water to keep energy, even in volume is beat the steel!

http://en.wikipedia.org/wiki/Specific_heat_capacity

 

[heaterman]

I appreciate the adjustment to my figures Julien. Water is truly an amazing substance when you think about it. It's heat and energy carrying capacity is incredible when you compare it to moving the same number of btu's with an air based system. A person can easily move more btu's through a 1" copper pipe than you can through a duct measuring 8x16" and do it with far less power consumption.