Wood Weighing

[hiker88]

I meant to add something. I don't know how this affects things but where does the lambda component come in at all? During each of my summer burns the boiler will ramp way down in the middle to two thirds of the burn. The fan will slow down to 35% and flue temps drop to around 240-250f. However, the boiler will put out 193f water for two hours straight.

Just wondering...

 

[Coal Reaper]

getting on the wood weighing bandwagon tonight. let me know your thoughts please.
I have not yet crunched any numbers for how many pounds of wood I need to reach target temperature. rather, I just weighed my wood tonight throughout a burn knowing that about 2.5 loads brings me back to 200Ftop/180Fbottom. probably 1040 gallons of storage, boiler, and plumbing between. I shut down the DHW for this burn so I have no load. heres how the schedule went:
4:40, tanks 118/98*F, 80F boiler, load 50#s ash at 20%, stack temp 150*C
5:00, boiler is up to operating temp of 160* (this is what it cruises at until return water is greater than 140*F, water reaching top of storage reads 170* tho), stack at 200*C
5:30, added 20#s oak at 25%. do MM read high when it is warmer out? my usually 33% finger came in at 37% this evening and I thought this oak was closer to 20%...
7:30, tanks 164/110*F, added 30#s more oak
I only have temp probes at top and bottom right now so I have no idea where my stratification line is. will be adding ones at 1/4, 2/4, and 3/4 up tanks.
by my limited past experience that should be close but I will likely need to add a few more sticks tonight. perhaps not since I have been burning mostly ash and now transitioning to oak. but the oak is not quite as dry. I will check it before bed and report back in the morning. hopefully I can find this nirvana you speak of and not need to question myself.
how do you guys factor in heat load during the winter? outdoor temps?
here is some pics all taken immediately after adding the first load. 5-10 minutes after striking match and this thing is off to the races!

 

[Coal Reaper]

My helper and i just added two more sticks for a total of 105#s. that should be all i need. Tanks are at 172/154 and boiler at 195 so the rest of the water will cycle through quickly. Fire box is currently half full of all oak. Math time.

 

[jebatty]

Wood weighing is easy, takes the guess work out of how much wood to burn, and eliminates idling. The 6050 btu/lb is a quick starting point to estimate the amount of wood: tank delta-T x gallons of water x 8.34 / 6050 = lbs of wood. Then adjust as needed to account for your particular system, heat loss, etc. Of course, if you have a demand period while burning, you might need to get sophisticated and estimate what the btuH demand consumption usually is and add wood accordingly. For my shop, I know that a demand period will drop the storage tank 10F, so if I expect a demand during a burn I simply add 17 lbs of wood.

Tank stratification (my tank is horizontal) is very interesting. I have four digital sensors, top, down 1/3, down 2/3 and bottom) and data-log temperatures. Supply is at one end of the tank 6" down from the top and return is from the same end of the tank and 6" up from the bottom. So long as there is no system demand, during a burn stratification is extreme, so extreme in fact that the bottom 6" will still have "cold" water when the return is at 190F+ (tank fully charged), and unless I intentionally mix the tank that bottom 6" essentially will not be heated. My supply/return flow rate is about 12-14 gpm.

Also, as the tank remains stratified after a burn, I estimate average tank temperature for the purpose of determining how much wood I need to burn by using the tank temperature at 1/2 down (a separate sensor with a digital read-out temp meter). Since there will have been tank mixing during demand periods between burns, I assume that the temp of the water above this point is the same at the midpoint and that probably the temp below this point also is close to the same temp by reason of mixing.

I get mixing because I mix down supply water to 100F for the in-floor. Because I have a single speed circ and flow at about 2.25 gpm total through the 6 floor loops, I have quite warm water being returned to the tank. That return is to the bottom of the tank via a diptube, supply to the in-floor is from the actual top of the tank.

Mine certainly is not a perfect system, but it can't get much easier to have efficient burns without idling and heat storage to almost exactly the temp I want. My target temp normally is 185F, although I can heat the tank, top to bottom, to 193F, which I do if I plan to be gone for an extended period during cold winter periods.

 

[Coal Reaper]

tanks 195/160 this morning so i didnt get hot hot water down to the bottom of tanks as expected but i imagine the stratification line is pretty close. that gives me a deltaT of 69.5*F, calculated using 6050btu/lb is 73*F. not too bad i dont think? using these numbers means i need to change my floating btu/lb to 5741. dividing this by the 6050 gives me just shy of 95% efficiency. this dont seem right. too high, right? also, this puts me at 1.5#s/degree rise. need to determine exactly how much volume i have and get some more insulation for return pex lines between boiler and storage and get the other temp sensors hooked up so i can get a better average of the tank temp. am i outright wrong with anything here?

 

[Fred61]

A little off subject but:
I don't recall if you ever posted your heat loads or actually know what you're going to need for stored heat this early in the game but your numbers made me wonder if you actually will need to bring your storage up to 195/200. Those last few degrees can be very expensive in terms of fuel use and number of passes. If you're going to light a fire every day during the heating season, it makes sense to only bring the storage to the temperature you will need to cover the load for the period between burns.

 

[Coal Reaper]

i agree. i will back off on the charged temp when i start having fires closer together. my thought right now only using DHW is that it takes energy to heat up the boiler from room temperature before it starts pumping hot water to storage. figure while the boiler is at operating temp i might as well make storage as hot as i can so i can go longer between fires. it cools off to room temperature within 3 days anyway. speaking of, should we take into account the mass of the boiler going from initial temperature to operating temp when calculating pounds of wood required after lighting a cold boiler? hard to figure that deltaT with outside of boiler not getting as hot as the water and refractary getting much hotter.
this has been covered already but i just want to ensure i understand. when calculating efficiency using 6050, this more of a sytem efficiency assuming 20% and 400*, right? with these assumtions being actual and perfect insulation, this efficiency number could be 100%. to get boiler efficiency one would use a HHV of 8-10,000 btu/lb? i understand why this would not work for wood weighing. but my results of 57-71% using HHV seem low. curious how manufacturers come up with high eighties to low nineties efficiency when they market boilers. perhaps regular air and dried wood somewhere 0-20% MC. lot of variables and different ways to skew numbers...