2x6 rate

i have a Small Brother’s Lightning 2x 6 raised flue. i have vermiculite with insulation blanket on top even to rails, tapering 6” from back. i run dry hemlock some pine. i fire about every 10 minutes or when back pan starts lagging. i have a 15’ stack, no blower or steam hood. stack temp measures about 300 degrees, not sure of significance of that. only a little smoke to front when i first fire it. i seem to consistently boil 18 gph. this year is when i retinned my arch and raised insulation level. before, insulation level was about 2” below rails under back pan. i have 1” blanket all over with 1” brick in firebox and 2” brick on arch. i was really expecting to get over 20 gph. any suggestions? i thought about adding air over fire, but not sure how to do that with insulation so high in back. i guess i could put it just before insulation gets too high at the top of slope. i thought it would do better by raising the insulation level.

Easiest thing to do would be add air under fire. Either put a blower in your draft door location or cut a hole in the back of the ash chamber and put one in. Any squirrel cage blower would work and help. Also, fire a little more frequently, if you are noticing your boil slow down in your rear pan, then that means you should have added wood already.

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Originally Posted by small_operator

i have a Small Brother’s Lightning 2x 6 raised flue. ... i run dry hemlock some pine... i fire about every 10 minutes or when back pan starts lagging. i have a 15’ stack, no blower or steam hood. stack temp measures about 300 degrees, .... i seem to consistently boil 18 gph. .... i was really expecting to get over 20 gph. any suggestions?

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- I'd suggest you fire more often and not allow the flue pan to start losing the boil...you're taking 2 steps forward and then 1 back...
- Hemlock and Pine are fast-burning woods and not real high in BTU's, Lots of heat but not for very long...again the need to fire more often.
- Stack temp seems low at 300, we run closer to 800-900 deg, but your stack gauge may be off.

I think the biggest culprit is your wood and firing technique.

18 gph is far under what a 2x6 raised flue should get, regardless of the type of wood burned. First, what size do you split the wood? Then for fueling do you use a timer? Back when I had a wood fired2x6 evaporator, it was a drop flue, raised gets slightly more than drop flue. I fired every 7 minutes both on that 2x6 drop flue and on my next evaporator which was a 3x8 raised flue. Every seven minutes, before I added AUF/AOF, high pressure air under fire and air over fire. Once I had that I changed my fueling time to every 9 minutes. If your 2x6 has double doors, only fuel one side at a time and have the door open as brief a time as you can without tossing wood in and hitting the flues, fill the firebox up to about 2/3 of the distance from the grates to the pan. Close the door as soon as you can. You will notice that the boil slows or even stops when you open the door, so keep it a brief as you can. Split your wood to wrist size and make sure it's fully dry. Even just AUF will help, but both AUF and AOF are needed for best results. Keep the sap level at 1-1.25" deep, not more. If this list of points to follow doesn't get your evaporation rate up to more than 25 gph there is something wrong with what you are doing, my guess is it will be 30 gph +/- 2 gph. Make sure you give the arch all of the air it can take, open that draft, until you can add AOF/AUF and never have a damper in the stack on an evaporator for wood fired, if one is there, make sure it is full open. It takes lots of air to burn hot enough to get a boil burning real hard, the faster the fire the faster the evaporation rate.

thanks for the info guys. i tried a timer every 8 minutes, and got a little better than 20 gph. i bet i just need more air flow. i used a laser pointer thermometer on the stack to get temp. sometimes it goes to 320 at peak burn. my stovepipe has a slight lean to it, and some leaks along the way, that probably cuts draft too. Also, i’ve been jamming the firebox too full from what Dave is saying. i’ll try with 2/3 full

I used to get 60-65 gph boiling rate on my old 2x6 with good wood, filling the firebox about 10” under the pans. This was an airtight arch with AUF and raised flue back pan with a rear hood and preheater. Try to keep your stack temp at least 900 degrees. On my current evaporator, I average about 1200-1300 degrees using mostly pine slabs. Even if it’s a standard arch you should be at least around 25-30 gph.

Anyone know a source for a squirrel cage blower. i saw some cheaper plastic ones online, but was afraid they might melt if close to arch. Maybe like an old furnace gun without the fuel pump?

On my home built 2x6 I'm using this blower. Got it off amazonDayton 1TDT6 PSC Blower, 2 Speed, 115V

Works pretty good for us.

On my 2x6 raised flue when I had added AOF/AUF (air under fire/air over fire) I hads stack temperatures with a temperature probe right in the stove pipe of 800-950 most of the time.
I suspect 2 main issues, but without actually seeing it i'm just guessing. 1. wood split too big or not fully dry
2. over filling the firebox, both can cause low evaporation rates. Even without a blower, if you give it enough air it should burn better.
Keep in mind, you need a real hot fire to boil fast, give it lots of air. If that doesn't help enough find a blower to push more air in, look for a blower that will move between 350-550 cfm (cubic feet per minute). If you find a bigger blower, use it, but you may need to use a slide gate to restrict eithen inlet or outlet air flow, if that's the case make it adjustable. Next, try criss-crossing the wood, a layer front to back, then one side to side and repeat until it's up to 2/3 full. Keep the door open as little time as you can without risking throwing wood in and denting the pan, especially the flues, but dents in eather pan is not good.

You should get 25-30+ gph once you get the operation right. in fact 35 gph is possible.