Wood Weighing

[hiker88]

In a few weeks I will start burning a known quantity of wood. I thought that would be a good time to start weighing the amount of wood I burn as I have seen recommended on this forum.

Can someone give me a primer on how to go about doing this? My wood is about 18% mc. I have 820 gallons of storage, and my boiler is rated at 105k btu\hour I believe. However, the boiler will limit it's output to that of a 20k boiler anytime the setpoint is exceeded by 2c (my setpoint is 88c). I don't know if that is a variable I need to factor?

By the time I start doing this, heating season should be over, so I don't think I need to account for heat load right? I will need to learn how to do that at some point I realize.

What I'm hoping is that I can say, "storage is 45c top and I want to get it to 85c, I need to burn 40 lbs of wood".

Thanks.

 

[Nofossil]

It all depends on how obsessive you want to be. At the simplest level you could keep a journal of wood burned and degree-days. Unless you instrument your storage with sensors at several levels you won't really know how much heat is is storage. Don't know the details of how your storage is plumbed, but with good stratification you could raise the temp of the top few inches while the rest remains cold (that's a good thing, by the way). Some of us have been known to go off the deep end. Not naming any names, mind you ;-)

 

[goosegunner]

I made a spreadsheet that I have on my iPhone or iPad. I enter top of tank, bottom, boiler temp. It does the math.

During heating season I know my heat load depending on outside temp. I also enter that and the hours I will burn. It adds that amount of wood in also.

Based on the amour of wood needed I set my timer, I can hit it right on the head leaving coals for the next fire.

gg

 

[Nofossil]

I built a spreadsheet years ago to calculate theoretical energy available from wood. It pulls data from the SQL database where I log all the sensor data, and calculates 'system efficiency' based on how much heat I can measure going to different places. Here's one from a few years back:

 

[jebatty]

Everyone knows that I am not one of those "obsessive" people referred to. But ... I have done weighed wood burns for two full heating seasons. While not exact, and nothing really is exact anyway, I use 6050 btu/lb at 400F stack temp as the realistic "available" btu's from burning 20% MC wood. Now, my stack temp is not always 400F, but it is a pretty good average, and my wood is not exactly 20% MC, but it truly is well-seasoned and likely is quite close to 20%.

As to the 6050 btu/lb, see Energy in Wood

Weighing the wood is easy and fast. Simply make a cradle hanging from a bar, use a digital luggage scale device (available on *bay for about $10), load the wood and see the result. The advantage, as goosegunner notes, is that you can determine almost exactly how much wood you need to burn to have your boiler coast to coals as storage tops off, and all of this without any idling, assuming you have adequate flow rate to handle your full boiler output at boiler delta-T of 20F or greater.

Determining storage tank temperature and how many btu's you need to reach your target storage temp charge also is easy but you need enough sensors to get an accurate read on this. I have sensors on my tank at about 1/3 from top, 2/3 from top, and very bottom of tank. Based on experience I can average the 1/3 and 2/3 sensors to get an "average" tank temperature. For my 1000 gal tank, if the 1/3 and 2/3 sensor average is 120F and my target charge temp is 185F, then delta-T = 65, btu's needed are 1000 x 8.34 x 65 = 542,100, and lbs of wood needed are 542,100 / 6050 = 89 lbs.

But ... this assumes that 100% of the heat energy is going into storage. System draw and boiler/plumbing heat loss on the way to storage also are taking some of that heat. For my system, and again based on experience, I know that to actually reach that 185F target temp, and no system draw, I actually need 110 lbs of wood to account for boiler/plumbing heat loss. This works out to 1.7 lbs of wood for each degree that I want storage to be raised: 65 x 1.7 = 110. This boiler/plumbing heat loss is of no concern to me because my boiler/plumbing/storage are all in the heated space anyway. I also know how much additional wood is needed if there is a system draw during my burn period.

So, in the end I use the 1.7 lbs of wood / degree to calculate how much wood I need to burn. Simple, very simple. I would suggest for you to start some weighed wood burns, use an X lbs of wood per degree guesstimate (maybe 1.7 to start), and see what happens. Adjust the X lbs of wood as needed, and you will eventually be right on in determining how much wood you need to burn to accomplish what you want.

 

[Fred61]

I also batch burn but have been using the "seat of the pants" method to estimate how much wood is needed. My unpressurized tank gives me a little cushion since I only need to have the top of the tank at 175 to185 after each burn to maintain my heat load for the next 20 hours. If I add a little too much wood that 180 degree water temperature seems to just become deeper approaching the center sensor. Bringing that 180 to185 degree temperature from approximately one foot deep to three feet deep (6 foot tall tank) takes alot of energy and will more than likely deplete any overload of wood in the firebox.

 

[hiker88]

So, in the end I use the 1.7 lbs of wood / degree to calculate how much wood I need to burn. Simple, very simple. I would suggest for you to start some weighed wood burns, use an X lbs of wood per degree guesstimate (maybe 1.7 to start), and see what happens. Adjust the X lbs of wood as needed, and you will eventually be right on in determining how much wood you need to burn to accomplish what you want.

This is exactly what I was referring to. Thanks.

 

[jebatty]

For 820 gal storage, try 1.1 lbs of wood per degree, plus a factor for boiler/plumbing heat loss. This will depend on your setup. Maybe start with +10%, so 1.25 lbs wood per degree. You also will want to add for system demand during the burn, learned by experience.

 

[Blue Tornado]

As to the 6050 btu/lb, see Energy in Wood

This publication that Jim has referenced is truly a good read.

This formula is what works for me, based on the information from the above linked article. Beginning with the 6050 btu/pound of wood referenced in the article, I assumed that flue loss and boiler inefficiency loss would reduce the btu/pound of wood actually transferred to the water. So, I began with an even 5000 btu/pound of wood in the formula. It took a half a dozen firings to dial in to the actual btu/pound of wood transferred to the water (roughly 5350 in my case). What it tells in the end is that the boiler is working at 88% efficiency.

1 gallon of water = 8.3 lbs

Total volume of water in system, in your case about 850 gallons

Target temperature minus current system temperature average, let's say 190 - 140 = 50

850 gallons x 8.3 weight per pound of water x 50 temperature rise = 352750 total btu needed.....divided by 5000 btu/pound of wood = 70.55 pounds of wood required

In place of 5000 btu/pound of wood, you could use any number up to 6050. If at the end of your burn the target is not reached, you would use a lower btu/pound of wood in your formula for the next batch. Or, if you have a surplus of unburned wood at the time the target is reached, you would use a higher btu/pound of wood in your formula for the next batch. Eventually you will get it tuned to your boilers output.

 

[Floydian]

What it tells in the end is that the boiler is working at 88% efficiency.

I think it is worth mentioning that if you were actually seeing 88% efficiency, you would be dangerously close to condensing-something to be avoided at all cost.

And if a non lambda boiler could achieve 88%, lambda models would probably be around 93-95%(definitely condensing territory) and Nofossil's brother's condensing gassifier would be over 100% efficient. Now were talking!

Here is a good read from a few years ago:https://www.hearth.com/talk/threads/wood-boiler-efficiency.35487/

And another:https://www.hearth.com/talk/threads/moisture-content-and-efficiency.11921/#post-213033

My point is we need to be careful with efficiency claims as there are a lot of variables here.

I totally agree on weighing wood when it comes to dialing in batch burning.

Noah

Edit: this one is good too:https://www.hearth.com/talk/threads/gasification-boiler-efficiency.56938/#post-712422

 

[Blue Tornado]

My point is we need to be careful with efficiency claims as there are a lot of variables here.

Ok Noah point taken, thanks for making my day. LOL Those threads are definitely worth reading.

Using LHV in my calc gets the 88% while using HHV nets 63% efficiency. That 88% sure looked good while it lasted..............Cripes that is no where near the 91% claim from Eko Orlanski. Does 63% seem low to you?

Well anyway, to the OP I can weigh the load based on the 5350 btu/pound of wood in the formula and hit the target temperature at 63% boiler efficiency.

 

[hiker88]

I started a burn tonight before seeing anything after my last post. I went with 1.7 lbs of wood per degree. I started around 530 tonight with a pretty cold boiler and the top of the storage at 42c (last burn ended 4/6 @ 10 am). Goal was to bring the storage top to 85c which will usually put the bottom of the storage around 78c. I can easily hit 85/78 or so with no idling with a boiler setpoint of 88c. So, (85-42)*1.7=73.1lbs of wood.

The boiler is running at about 42% fan speed right, now and it is slowly working it's way back up to max normal output (safety door closed) of 85%. Top of storage is 78c right now and my guess is that I'm probably going to get it back up to 80-82. So, I missed it a little bit, but I had a bit of heat load plus dishes and baths\showers tonight.

I have a lot more to learn here, but this is the first time I felt like it was more than just a seat of the pants thing. I'd stay up and watch it more, but I'm on day 7 of a 13 day work stretch so I am going to call it a night.

I appreciate the input and I'm going to keep working at it. I feel like I am maybe one step closer to attaining wood burning nirvana.

g'night.

 

[jebatty]

I have made several posts on efficiency using wood at 6050 btu/lb assumption (400F stack temp and 20% MC), which I think makes real world sense rather than using HHV, which is around 8-10,000 btu/lb depending on species, or some other standard.

HHV as I understand it is something like wood at 0% MC combusted in pure oxygen with 100% of the released energy recovered, such as stack temp at ambient temperature and all energy recovered from the condensing water resulting from combustion. Currently this is not achievable except in the laboratory.

The 6050 btu/lb fairly represents recoverable heat energy under "normal" conditions involving stove wood. So an efficiency calculation based on recoverable energy makes sense. Obviously, design, engineering or technology which can reasonably raise the recoverable heat energy is positive: thus lower than 400F stack temp is good, but not so low that condensation results unless the design can handle condensation, and MC lower than 20% is good, if the heating appliance has a design that can handle this. Lamda controls, variable speed combustion air, etc. are steps in this direction.

At 6050 btu/lb assumed as the standard, efficiency in the mid to high 80% range is very possible, and I think might even be possible into the very low 90% range, as measured by the increase in temperature of a known volume of water. To achieve the higher efficiency %, heat recovery from the burning device would have to be included, as well as losses from plumbing and storage if a tank of water is the measuring device.

Efficiency is a moving target based on the standard employed. For me it makes most sense to base it on a standard that a user might expect when operating his heating appliance under every day, real world conditions. Statements regarding efficiency that cannot be reasonably duplicated on an every day basis by a user are very misleading and not helpful, IMO.

 

[Blue Tornado]

Hiker:

Glad to hear your burn went well and close to your target. You'll have it dialed in soon and find the number that works with your system. "Wood burning nirvana".....too cool

 

[jebatty]

Me: Statements regarding efficiency that cannot be reasonably duplicated on an every day basis by a user are very misleading and not helpful, IMO.

I feel that another member thought this comment was a personal criticism, which it was not intended to be, and I apologize for that. I meant this comment to apply to manufacturers of wood boilers that make claims all over the board (pun intended) about the efficiency of their boilers. Efficiency determinations in a laboratory do not, IMO, fairly represent what a user can expect to obtain using cordwood of varying species, cut and split in varying dimensions, and of uneven and often unknown moisture content. Now, I hope that I have not opened another Pandora's Box by this comment, but if so, I'm getting used to the taste of foot in mouth disease.

 

[Floydian]

Apologies on my part for taking this thread in this direction in the first place.

Blue Tornado, it was not my intent to shoot your numbers down, I just felt the need to point out some variables when it come to efficiency claims. Sorry for being the efficiency bummer man.

Jim, your contributions to the site are as good as it gets-Thanks. FWIW, I agree that the 6050btu/lb, 400 flue temp, 20% MC methodology does closely resemble real world efficiency results vs controlled lab testing with 0% MC fuel, flue gas analyzer, etc. In honesty I am out of my realm here, though I find it all quite fascinating.

I'm curious, have you had a chance to play with these kind of numbers with the Deep Portage Frohling set up? (using the 6050,400,20 parameters)

Noah

 

[TCaldwell]

set up a luggage scale for wood weighing, using jim's 6050 btu value for a lb of wood, it seems for my garn with approx 2000 gal of storage the. number works out to be 2.9lbs per degree raised. curious how other wood weighers are making out.

 

[jebatty]

What Garn do you have? My system works out to 1.7 lbs of wood per degree in the 1000 gallon tank, or 17 lbs (the wonders of math) per 10F degrees.

 

[heaterman]

What Garn do you have? My system works out to 1.7 lbs of wood per degree in the 1000 gallon tank, or 17 lbs (the wonders of math) per 10F degrees.

Tom's Garn is a 2000 which he has "tinkered" with.....
It is probably the most advanced wood fired heater on the planet in terms of combustion control

 

[jebatty]

Tom's Garn is a 2000 which he has "tinkered" with.....
It is probably the most advanced wood fired heater on the planet in terms of combustion control

Excellent results, Tom. Based on the assumptions, 95% efficiency in transferring the wood btu's to Garn storage of 2000 gallons. I don't recall, but I assume your Garn is well insulated, and as Heaterman says, yours has been modified with a highly advanced combustion control. Regardless, a superb outcome.

 

[TCaldwell]

thanks for the kudos but I think I can do better, those results were with flue tubes that were dirty. My unit is a model 1900, not sure but I think the volume is 1950 gal. I will burn again in the next few days with clean flue tubes and post the results, also try to pin down the actual capacity. the garn is designed to burn basicaly full out, according to the testo fluegas analyser seems to run best at 4% residual o2 in the fluestream, any lower and the co substantially increases dropping effiency, higher creates excess air increasing flue temps. This is boiler specific, others have their own sweet spot.

 

[TCaldwell]

Tubes cleaned, 100lbs oak not as dry as i would have liked, start water temp[ mixed tank to top and bottom temps read 141.5deg, mixed tank throught burn with no load to end temp of 174.2, adelta of 32.7deg= 3.05lbs wood /degree raised.The gaarn 1900holds 1906 gal= 15896btu/degree raised/3.05=5211 btu per lb of wood into storage, divided by jims 6050btu/lb of wood =approx 86%effiency, hope the math is correct. Will try next burn with better wood. Jim and other froling users what efficiencies have you averaged?
thanks tom

 

[TCaldwell]

Not sure if the math is right?, seems high but maybe Jim could check. starting temp for 1906 gal was 147, burned 100.6lbs to end up at 183.9, coming up with 2.72lbs/deg rise, 5844 btu/lb and effiency transfer into water of 96.5%? This wood was dryer than last night, o2 setpoint was 4%.

 

[jebatty]

Math is good enough. Two assumed variables can skew the results: 1) actual wood MC, drier than 20% will increase the assumed efficiency and wetter the opposite; 2) stack temperature lower than 400F will increase the assumed efficiency and higher the opposite. Each 100F change in stack temp impacts efficiency 1-2%. This simply means that less than 20% MC and/or stack temp less than 400F means more than 6050 available btu/lb of wood.

Also involved is the rate of heat loss from the Garn itself and the plumbing which would be circulating the water in the tank. And the warmer the ambient temp, the lower the heat loss, vs the opposite. Regardless, the Garn burn/heat transfer design (no plumbing between the hot burning gases and the surrounding storage tank water) combined with Tom's combustion control should result in high burn efficiency.

My only direct Garn experience is with the WHS3200, and this raises questions which relate to realized efficiency, as compared to my direct experience with a Wood Gun E500, Tarm Solo Plus 40 and Froling FHG 50. These latter 3 boilers burn a load of wood down to very fine ash, and even after extended periods of operation there is very little remaining ash, and what remains is mostly dust-like. The Garn, when burned with multiple loads of wood to supply demand, produces a much larger volume of ash and unburned coals between periods of operation. So the question is: what is the combustible content and weight of the ash/unburned coals remaining after a batch burn temperature test? Any remaining combustible material would reduce measured efficiency. I have never measured that with the 3 boilers, because none are operated mostly in a single load batch burn operation, and at least during cold weather these may be operated more or less continuously, at which time unburned and accumulating ash/coals become both quite relevant and would negatively impact realized efficiency.

I think most of us would agree that when efficiency measures start approaching the magical 100% level, all other factors become increasingly relevant and, at least in my mind, suspicion starts to raise its ugly head as to the accuracy of the finding, and I start to look more closely for what else is impacting the finding which might skew the result. Yet, given Tom's attention to detail and his work to maximize the efficiency of his Garn, I also believe he should come out with a finding of high efficiency, and a result in the mid 90% range certainly would be possible.

 

[hiker88]

I'm in a weekly pattern where I burn 100 lbs every Sunday. Top of the tank is usually 45c by then or 113f. I use a multiplier of 2.5 and a goal of 85c tank top temp and I can get it within a degree or so without any idling.

So I think we are pretty close? You are heating twice the water I am (I have about 900 gallons between storage and the boiler itself) but your delta is about 37f and my starting delta is 72f.

I haven't checked the MC of my wood lately, but it was about 15% a few months ago and I am still burning the same load that I brought into the basement back in March, so it is only drier now.

I hope that helps.