Attempt to Compute Mean BTU/Hr

[Tennman]

Will you guys check my math.

Started burning 11/21 to today (2/10) = 82 days
Consumed so far this season of ~10-12% moisture content wood ~ 8.5 chords
20% MC Hickory or Oak per Engineering Toolbox ~3600 lbs/chord
Lbs consumed this season = 30,600 lbs
Per Siegenthaler's Training Doc 12% MC wood ~ 6800 btu/lb
Total BTUs this season = 30,600 lbs x 6800 = 208,080,000 BTUs = 208 MMBtus
BTUs/day = ~2.54 MMbtu
BTUs/hr = 105,700 btu/hr

So the magic number is ~106 kbtu/hr for a typical winter.

I assumed the Biomass 60's mean output at 180 kbtu/hr back when sizing components. Now with storage I can watch excess energy being accumulated by the tank temps. A cold day for us is 25-35F and on those days it takes quite a while to bring the bottom temp up. So I know with decent wood our 60 boiler stays ahead of the demand, but not with lots of excess energy. Don't have a more scientific way of estimating demand or do I have realistic load calcs on our leaky old house. Our propane furnace is a 175kbtu unit and it doesn't run continuously on cold days but pretty close to a 50% duty cycle, so it all seems to sync.

We're considering a lamba upgrade in the next few years and a pellet or ? within 6-8 years (age/health driven). There are some energy improvements we will be doing but now with storage I'd hate to go less than a 60 Class to have decent recovery times. Please let me know if you guys see any errors. Thanks

 

[jebatty]

I didn't actually do the math but looks good to me for btu input into your system and should work relatively well for your purposes. Your calculation does not provide btu output, however. You do realize though that you have lots of "slop" in the calculation?

1) A cord as a scientific measurement is very imprecise. In fact it is so imprecise that around here mills do not buy timber by the cord and instead buy according to weight. A ticket still may show "cord," but that is just a conversion from weight to cord following an agreed standard.

2) Moisture content would not have been imprecise for Sig but likely is imprecise for you. Unless your wood was stored indoors for at least 3 years with good air circulation, as a general matter it is unlikely to be at 10-12% and more likely to be closer to 20%, which would be closer to typical for 3 year wood seasoned outdoors. I'm not saying your moisture meter didn't report 10-12%, but accuracy of the meter, variations in splits, method and location of storage, all add doubt to the 10-12% reading.

3) As to 6800 btu/lb, I don't know whether or not Sig calculated flue temperature in coming up with this number. If so, what was it? And how does that compare to your flue temperature? And imprecision here too because high, mid and low burn flue temperature all would be different. The higher the flue temperature, the less of the assumed 6800 btu/lb you actually are making available for delivery from the firebox to water.

6) Btu input vs btu output is another variable. Most gasifiers probably fall in a range of 80-90% efficiency in taking available btu's and transferring those to the water. But even here, there are "losses" which vary in each situation. Losses to the ambient environment from the boiler, plumbing losses, storage losses. If all of these are to the space heated by the boiler, probably can pretty much ignore them. But if not, these losses are variable and can be substantial

Now to the bottom line. Based on my calculations with my boiler (it and all plumbing and storage are in the heated space), which also have included flow calculations, delta-T, data logging, flue temperature compensation, etc., I would say that a gasification boiler can deliver on average about 70-75% of its rated output over the course of a burn. Your calculation of 105,700 for a 180,000 btuh boiler may be a little low, but it is an average and lots of variables particular to your installation can impact that.

Final analysis, your calculation resulted in a a respectable btuh number and doing that is a worthwhile exercise in understanding your boiler and how it performs in your situation.

 

[Tennman]

Thanks Jim. I really should have worded my first sentence, "Check my assumptions". I don't remember ever seeing an attempt at backing into a home's energy demand based on chords consumed. I was surprised when I checked my 2.5 year min seasoned oak and poplar tonight. Oak averaged 15-16%: poplar consistently 11%. Outdoors, under roof the whole time. So my 6800 btu/lb is slightly high. Agree it's imprecise but without lots of sensors its a gross check that our boiler's sized about right. Again thanks for your input.

 

[heaterman]

Figured another way. ...

EIA quotes 20MM btu /cord as an average
If your boiler is running at 75% efficiency that's 15MMbtu net
8.5 cord x 15MM = 127.5MMbtu
127.5MM / 82 = 1.554MMbtu per day
1.554MM / 24 = 64,786 per hour for the winter.

I'm guessing that is closer to reality for you Tenman. 106,000 is a LOT of heat for a home, even a drafty one. Especially in Tennessee.

That 23,000 sq ft shop we put the 3 pellet boiler in is averaging 138,000/ hour since fire up old years day. They open the overhead doors in that building at least a dozen times a day and virtually loose all the heated air in the place each time.

 

[Tennman]

Thanks HM. Yes, using that shop as a comparison in a much colder climate indicates something wrong in my assumptions. What started this exercise was my concern about the biggest Froling being a 50 vs my current 60. I hate the idea of downsizing now that we have storage. Thanks for the input. The day's coming when you and I will be discussing a pellet boiler.

 

[maple1]

I was thinking it was kind of high also - but that's just a seat of pants feel. Mainly by how much wood you've burned so far.

We're in a cold spell right now, down to -20-25c over night, maybe up to -15c day. Yesterday, my storage was fully depleted when I started a burn (was in the 115 range top & bottom). About a half hour after lighting, I turned all the stats up so all 4 zones in our 2700sq.ft. two storey were running. The house wasn't 'cold' when that started, but had maybe 3 or 4°c to regain in 3 of the zones. I burned about 2.5 loads of crappy wood (spruce windfall with a bit of white birch mixed in) at about 3cu.ft./load, over about 8 hours. When the burning was done the house was fully up to temp (21c average?), and storage was back up to around 175/165. I'll repeat again later today.

I have never done a heatload calc on the house - the installer had one done 20 years ago when we built but no idea what it said or where it went.

What kind of temp swings are you seeing in your storage between burns? i.e. how cold when you start & how hot when you finish? I think I am on track for around 6 cords for the winter (a lot of junk), but always hard to tell until it's over.

 

[Tennman]

Maple, It's been surprising how storage has allowed us to maintain our winter temp of 67F at much lower water temps feeding the WTA HX. In the past when the supply got to ~130F (fire out) the HX rapidly sucked the remaining energy out of the small water volume and the house quickly got cold. This morning the fan was running continuously with only ~110F water supplied and the interior was at 66F or 1 deg down from the tstat setting. Never could have done that last season. Storage is allowing us continuous low temp air much like a heat pump. But I really don't like to let the bottom temp in the tanks get below 110F or so which means supply temps of ~140-150F. The warmer air means less fan cycling and a more comfortable house in general.

One problem I'm now having that seems common to the BioMass is the controller setpoint is set at 192F but all the gages and sensors say I'm getting about ~180-182F out. So there's a 10F thermocouple calibration or position error that is not letting me charge storage with 192F water (i.e. the controller is reading 10F high). The max setting for the RK is 195F. If I could set to 202F we'd be good. I may get some heat for this, but with my wife at home we maintain a small fire most of the time, but when we see the FUEL light we have anywhere from 4-6 hours of heat for our moderate climate. So big fire before bed and in the morning with occasional feeding during the day to maintain a coal bed and ~130 or so at the bottom. We mainly fire/feed by watching the bottom temp. Storage has literally been life changing and that's not over dramatization. We're no longer slaves to the boiler. I tell people running our boiler before storage was like a dog in the house and constantly worrying about getting home to tend it.

Best wishes. Stay warm way up there.

 

[maple1]

Agree completely with your assessment of what storage does for you & your life - I was a slave to my old setup for 17 years, and darned near burned right out by the end of winter. Then there was the slippy roof chimney cleaning experiences...

I forgot you had a WAHX setup.

I think you're likely having some heat loss too, by keeping the boiler hot most of the time with more fires going. That is likely skewing your numbers some - but no idea how much or how you would it out. If you did have low temp emitters, that could run down storage further, then burn only when storage needs charged & charge it only to about what you are charging it now even with the controller temp issue, you would likely see a reduction in wood useage & a lower final number in your calcs. But everyone does what they have to do, and sounds like you're doing good.