From Leslie and Ianto's book "rocket mass heaters", I built one of the described Bengali Pit Stoves. It worked great, but I did make a mistake or two and I have some questions. The book described the diagonal tunnel used as a feed tube for the wood and also as the draft. I ended up just depositing the wood into the vertical hole, as the diagonal tended to be difficult to continuously feed as the pieces got shorter. I also found this to push soil into the bottom of the fire, and thus didn't burn so well. I think I could have connected the diagonal to the vertical a bit higher so that as the wood was fed to the fire, it was fed directly into the fire and not into the coals. I also wonder about sizing the pit and the tunnels for different uses.

This type of stove was amazing to me. When I got this really cranking, it produced very clean burning, very high heat, and perfectly directed heat. I cooked several meals this way, and regulated the heat by raising or lowering the pot which I was cooking in. I highly recommend trying this out!

Another thought I had was to use this as a low cost, low pollution, efficient way of boiling sap down for maple or birch syrup. Just wondering if anyone has done this, or if anyone has thoughts on what sort of vessel to use so that I don't spend some money on a material that will just burn through or not be appropriate for the process for any other reasons. I was thinking stainless steel, and maybe a couple pit stoves in a row with a large ss vat above them with a lot of surface area. Thoughts?

Is this something you can build around this time of year? I'm hoping to make birch and maple syrup when the time arrives but I don't have anything built for boiling yet.

the diagonal tended to be difficult to continuously feed as the pieces got shorter. I also found this to push soil into the bottom of the fire, and thus didn't burn so well.

a piece of skinny sheet metal laid in there might allow you to slide wood down more easily and without pushing dirt into your fire.  I think dropping the fuel into the vertical hole would decrease your efficiency somewhat as hot oxygen-poor air pyrolyzes the fuel that isn't in the burn zone.  sounds like fun, though.  thanks for sharing your success.

Do you have a picture?

So the birch sap is flowing and things are going great. I dug a new pit stove for boiling the sap, and I made the vertical hole about 3 feet deep and flared it out at the bottom with a hoe. It it about a foot in diameter. I dug the diagonal tunnel so that it intersected the vertical hole about 8 inches above the bottom, so that the wood fed through would go directly into the flames, not the bottom of the coals. I also used a 6" diameter piece of stovepipe to line the diagonal with, thanks for the idea Tel, it worked great! This stove cranks the heat out bigtime! I was able to throw all kinds of green and wet wood in there, a bunch of black locust bark, and the incredibly high heat easily burned everything. I was given a 55 gallon stainless steel barrel that I cut in half, which I use for boiling sap over the pit stove. It tolerates the heat wonderfully, and the sap boils vigorously from twigs if that's what I feed it.

I was able to make some birch beer and some birch syrup so far. For the beer, I boiled 20 gallons of sap down to 4, while adding barley, malt extract, hops, and yeast. For the last 25 minutes of this boil, I added about 3 gallons of sweet birch bark, which carries the strong essence of wintergreen. Filtered it and it is now fermenting! I also made a tea from the red inner bark, just as described in "Stalking the Wild Asparagus" by Eulle Gibbons. It was delicious, and I felt like it was the spring tonic he described it as.

The syrup was quite the treat. I started with 23 gallons of sweet birch sap in the barrel. I ended up boiling this down to just about a pint of syrup, knowing when to stop boiling with the use of a refractometer. This took way longer than it should have, because I was cautious not to burn the sugars by too vigorous of a boil. I learned through this process that this risk is only present during the very final stages of the boil. I was boiling from noon till 3 a.m. Next time, I will feed the pit stove so that it roars and roars, and I'm sure I can cut the time way down. The syrup is amazing to taste, I've had it in coffee, on waffles, on ice cream, and I love it!

To conclude, I think the pit stove is the perfect way to boil sap for small scale sugaring operations. You can get extreme heat, a clean burn, use any old wood scraps lying around, and do it in bad weather if needed. The Bengali pit stove is a great way to do any kind of cooking as well. Thanks again to the authors of "Rocket Mass Heaters", Ianto Evans and Leslie Jackson. Here are a couple pics:

That's really cool, Jesse!  Glad you're having such success from such a simple set-up. 

I only wonder if you need a pretty high clay content in the soil to support the hole under the barrel? 

I've never heard of sweet birch sap.  Is it different from maple syrup?  And the beer experiment sounds really fun! 

Well I don't know what the clay content of my soil is, I hope it's high because I'd like to start experimenting with making some cob structures this season. I did try to dig one of these pit stoves in some really gravelly soil and it didn't work at all.

Sweet birch, black birch, cherry birch, all the same tree that is Betula lenta. Birch syrup is drastically different than maple syrup, and I have read that it is often used by chefs in gourmet kitchens! The sugars in birch sap are mainly fructose and glucose, while in maple it is almost purely sucrose. I have also heard you can tap the black walnut for making syrup, but I don't have access to any large walnut trees.

I also want to try making some birch wine and some birch sugar candies. We are getting about 40 gallons a day right now from 14 trees we have tapped, so there is no shortage!  Peace!

Oh, one question I have for anyone who has made syrup by continuously feeding the sap during boiling, is there a limit to how long I should keep adding sap to the boil? I wonder if the sugars do burn after being in the same boil for a long time. Seeing as I'm on the road until Monday, and all of the sap that's collecting is getting stored until I return, I'm going to come home and get boiling! Can I make one large batch or do I need to do several smaller ones? Thanks! -Jesse

I have also heard you can tap the black walnut for making syrup, but I don't have access to any large walnut trees.

Waaahhh?  We have two huge black walnuts and I can't really see us eating a ton of their nuts in the future.  Small, hard to crack, hard to get the meat - which is delicious, but very hard won.  I'd LOVE it if we could tap their sap!  Thanks for this info, gonna research that some more. 

Very interesting information regarding birch sap.  Let us know how your beverages turn out!  I thought the return on sap to syrup was low (probably typical for tree saps - I know nothing about it), but if you're getting 40 gallons a day, and now have a sorta fast and very cheap way to boil it down - more power to ya!

How did you dig that three foot very narrow hole, anyway?  A post hole digger and your arm? 

We have super silty soil, and I'm kinda thinking that this stove wouldn't work here. But we might try -  we need some set up for scalding pigs here in the next few years. 

Whelp, I have this handy post hole digger with an extra cross in the arms way down low, so I'm able to dig deep without pulling the arms so wide. I widened the hole down low with a hoe. After boiling for about 7 hours today, the hole has begun to widen a bit near the top, so I'll fill this one in and dig another one before I do any more. The sap I used today was a bit cloudy, and had a slightly unpleasant smell, so I was very skeptical that it was spoiled. There were a few buckets that I didn't use that smelled awful. So after boiling it down from 30 to about 2 gallons so far, it smells just like the first batch I made, so I'm going with it. This time I boiled full on the entire way and was able to boil 30 gallons down to 2 in about 7 hours. Not bad, I'd say.

Yeah, I don't know a whole lot about doing walnut trees, but it was mentioned by some of the folks that led our PDC last summer. I'd love to hear about it if you give it a whirl, Marina.

The beer was racked off once last week and needs it again tonight. It smells super good, a mix of wintergreen and your typical birch beer soda flavor. I can't wait to toast some of this stuff!! Well, carry on.

I hear Birch syrup is excellent.  The sugar content of the sap is much lower than maple.  Takes 70-80 gallons of sap for a gallon of syrup.  Lots of boiling to do, good luck.

With maple syrup, continuously feeding the pot will result in a darker syrup.  Light amber is the commercially desired product, but there are plenty of people who like the darker syrup, has a different flavor.  I suspect birch syrup will also be darker with a continuous boil.

When  made maple syrup I tried both methods.  Continuous feed is difficult if the sap added is not preheated.  Each addition cools the pot, takes forever to bring it back to a boil.  I find the batch method works best for me with the equipment I had available.

Here in Florida, maple sugar is impossible.  I've been looking for alternatives, considering sorghum syrup.  Sweet sorghum is grown, the stalks are squeezed to extract the juice.  After that its the same process of boiling down as maple syrup.  Takes 8-10 gallons of juice to make a gallon of syrup.

Like I have time for it...

Hi,

I will also make this a new post, but hoped that it will email to those who have posted already. We are wanting to heat a low portable tunnel house (5ft tall and 4 ft long). This is a short-term thing until we can afford a permanent walk-in Greenhouse. We are wondering how it would work to build one of these and run a chimney pipe from the top through the length of our green house. (We would have the chimney pipe in a wooden box of stones, sand etc. to heat during the day). It would be quick and easy, the hottest part of the fire would be well away from the Plastic, we could put the feed tube outside the greenhouse, and it would be almost invisible from any council inspectors that might wonder

Because we a few miles from the beach, and on sandy soil, I will probably have to line my hole somehow. I thought of having a 6" tube to feed the fire, and also that the hole/ heat riser itself could be either clay lined (and use clay to seal in the connections between the riser and the feeder tube and exhaust tube) or it could be made out of piled up bricks.

Have you developed your use of this fire since posting. Any thoughts on how well it will draw through a chimney. Was it a problem keeping your fire dry?

Annie