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Post by patamos on Jul 19, 2016 17:49:29 GMT -8
Hi Folks, Been away from the boards for a while, juggling numerous projects... Finally got around to firing up my second vortex heater. Same design as Trev's original. Made a slightly larger primary air port (+ %15). Runs through 6sq.m. of double bell ISA. Then up a 25' tall masonry chimney with uninsulated 6" flex pipe. I know i am asking a lot of the core, but have a bypass damper just 3' downstream of the throat. The problem after a few firings is that it pulses strongly in the 5 to 15 minute period of firing. Even wants to back draft a bit in the outward phase of the pulsing. My first thought is downstream restrictions in gas flow. One possibility is where the flex pipe bypass flue Ts into the main flue at a very tight 90 degrees. The flex pipe was installed after determining that the 6" x 8" clay flue liners needed replacing. The fellow who has been kind enough to WETT approve my heaters does a lot of woodstove installs and was very confident that i have enough vertical draft to pull things along. But then, he is not familiar with the 'all out burn' brinciples of these heaters… I know that 6" flex pipe has more intensive laminar flow than straight 6", which effectively reduces the CSA of the flue… But then, i see that Donkey has proven that reduced chimney CSA is fine on J feeds. I can move baffles around just downstream of the throat. Could also open my secondary air feed around the door somewhat. ANd - if i really have to - get into the bypass T and open up the corner somehow. Sure hope it is not the CSA of the pipe But first of all, putting it out there: Pulsing is caused by… .?
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Post by tallgrass on Jul 19, 2016 21:07:01 GMT -8
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Post by peterberg on Jul 20, 2016 0:49:47 GMT -8
But first of all, putting it out there: Pulsing is caused by...? Restriction. Where and why are completely different questions. I am not familiar with the vortex stove but I've seen this behaviour in a couple of other builds of very different construction. Since the pulse disappears after 15 minutes the problem lies in low chimney draft at the start of the burn. Is it a bell system or what?
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Post by matthewwalker on Jul 20, 2016 7:15:05 GMT -8
Pat, I recommend you don't go too deep right now. Tuning the stove to run well now will require you to do it again when it is burn season.
I know, I know.....I'm living in a shack and having fires daily too, but, it is "summer." Or so they tell me.
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Post by patamos on Jul 20, 2016 12:57:40 GMT -8
Thanks guys, The perspectives help... Ya Matt, i know burning in this summer heat is a worst case scenario. But i'd like to at least eliminate the possibility of reverse flow which it is at times close to doing. Given what Peter has seen in various designs, restriction is the main culprit - hopefully solvable by widening out the tight 90 bend at the bypass T. Before getting into that I can remove a baffle under the top cooking plate above the throat to give the gasses a more direct line to the bypass flue. Also, i forgot to mention that i experimented with an EGR port from the downdraft chamber into the ash pit under the grate. Wondering if this is a contributing factor to the smoke back. Easy enough to close off so we will see. I had photos and video of the heater under construction but then forgot my 'Ipod Touch' password. Didn't realize you only get a few wrong attempts before the thing is disabled and all info is lost. Back to borrowing my son's 'old school' digital camera. Perhaps we will trade
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eng
New Member
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Post by eng on Jul 20, 2016 22:16:02 GMT -8
Patmos, If your burner was an oil fired boiler I would say it is starving for air bu my non rocket stove burner pulses on light up when it has lots of kindling and small sized quick burning wood. My solution then is to cut the primary air back until the pulsing stops. The burner then burns cleanly and quietly. I used to make use of a draught meter when setting up a burner to make sure the flue was drawing well. My memory is dim and I cannot find my notes but I would say a flue drawing less than 6 mm is inadequate. (This is wrong! My memory failed me. 0.05 in [1.2 mm] water gauge is correct. The 6 mm w.g. was for a boiler I used to manage.) You can make a manometer easily and cheaply to give the info needed at the important parts of the burner.
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Post by patamos on Jul 21, 2016 8:50:41 GMT -8
Thanks for the suggestion eng.
Can you clarify what you mean by '6mm' ? Suppose i could loo it up elsewhere, but am here for now… also, where might be a best place to start looking for info on building manometers?
thanks
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Post by patamos on Jul 22, 2016 10:02:09 GMT -8
Removed a baffle under the top plate and plugged the EGR this morning. Then fired up a 'worst case scenario' biggish load of kindling and small 2x4 off cuts on a hot windy day. The pulsing has diminished about 30%. With the odd puff and lick of flame from the primary air intake. Think i will still open the elbow on the bypass somewhat. Maybe the exit port in the bell as well.
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eng
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Post by eng on Jul 25, 2016 2:11:55 GMT -8
Patmos, See above for my correction. A piece of clear plastic tubing, about 6 mm diam, fastened to a board in a U shape with water in the U, one end open to atmosphere and the other to a small hole in the flue will do the trick. There needs to a short length of metal tube into the flue from the plastic tube. The legs of the U need only be about 150 mm long. If you make the leg that is open come off the bottom of the U at 45 deg it will show small differences in water height more clearly. A good reference book is The Woodburners Encyclopedia by Jay Sheldon, Vermont Crossroads. See chapter 5. Some of the diagrams of burners in this book and others like it are now outdated because of the work done the clever people sharing their expertise on rocket stoves. I am finding I have had to relearn a good deal. Your last post indicates to me that your burner is short of air. The hot gas inside the firebox is looking for air to burn. To me a good flue is essential for a good burner.
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Post by patamos on Jul 25, 2016 12:08:01 GMT -8
Thanks Eng i will look that up. Peter, i have a small video on the way to show the shape of things and flue-to-bell configuration. Yesterday i had a go at softening the 90 turn where the bypass meets the flue. Very awkward to access. So instead opened/raised the CSA of the exit port in the bell-to-flue transition. This improved flow somewhat. This morning i fired the heater up and found it still wanted to pulse and puff a bit in the first third of the burn, but the issue abated when i cut the primary air by 50%. If i opened the primary back up quickly it wanted to puff backdraft like a cranky steam engine. Not fun. The issues were much the same with the bypass damper open or closed, but generally less pronounced when closed All of this points towards Peter's assessment of low chimney draft in the early stages of the burn. More and more i am worried that the flex pipe flue liner is the heart of the problem. I have built less powerful 6.5" J-feed with as much bell ISA and similar height of uninsulated flue to the roof. No pulsing there, but a smaller fuel load trying to ramp up. I have also built this same vortex core with slightly less bell ISA and insulated pipe. No problem there either. I remember Ernie Wisner writing somewhere that using flex pipe (in place of straight pipe) in RMH flue runs meant having to reduce the run by about 40%. So this leaves me with a few options: 1 - tweak down the ISA of the initial flue run (in bench) towards the bell (in back rest) to speed the rate of flow into the bell. 2 - coach clients on reducing primary air sooner than later if they want to burn in Summer. And see how the system wants to perform once the weather cools. 3 - cut into the chimney now (before finish plastering) and replace the flex liner with straight pipe. My main question at this time is, given that i have a marginally functional heater in summer firing,.. how big of an improvement will the season shift likely make? All ideas welcome
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Post by satamax on Jul 26, 2016 0:08:00 GMT -8
Well, it should improve a lot. But you not necessarily heat only when it's cold. In spring, i heated up the workshop several times, when it was cooler than outside. And it was a chore to start.
So, best advice i can give. Dig that pipe, and change it.
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Post by patamos on Jul 27, 2016 19:54:18 GMT -8
Thanks Satamax,
always good to hear your perspective on things. it might come to that after all. But for now I think i have tamed it down enough. Widened the secondary air delivery around the door a bit. when primary is tuned down to half open the problem is gone. Therefore might wait and see how things change with the weather...
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Post by Vortex on Aug 11, 2016 1:36:10 GMT -8
Hi Pat, Sorry, I've only just seen your thread, I don't read the board much in the summer.
I've only ever built Vortex stoves on straight insulated chimneys. I wouldn't even try to make one run on un-insulated flexi pipe - the stuff is a nightmare.
If I light my stove with the bypass closed and use a lot of fast burning fuel like small pine sticks, it will pulse with the occasional puff back, closing the primary air slightly stops it. Once the stove is warmed up you can open it back up to full again.
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