hi all, im attempting to make a RMH for my greenhouse as its soon to be winter here in aus, its made on the cheap out of old bricks (which might make it a masonry stove but oh well)
the burn chamber is all made from brick aswell as the heatriser, but i have the exhaust coming out of some old 100mm chimney pipe i picked up at the tip, the problem im having is that the exhaust that is coming out is real smokey and comes out rather quick, also the exhaust pipe only feels a little warm after 30 mins brun time, i put this down to: A) the bricks still arent hot enough to create a proper burn of all the gases ?. and B) my exhaust is too small and forces the exhaust gases out to quick.
so what i propose to do is collect smaller barrels (like shown in picture) cut the small pipe near the riser and join it to a series of the 60L barrels (with the bottoms/tops cut to make 1 big barrel) and then feed it back out a bit of the smaller pipe and out the chimney - i am assuming the gases will linger in the new exhaust chamber longer to heat up more (i will be making a raised garden on top- so it will heat a mass, prop concrete, the soil on that)
i have also included a dodgy paint drawing outling what i aim for, if anyone with better knowledge of these things has any comments, i would be happy to take any help onboard - i tend to get excited and rush into things
What cross sections do you have in your feed tube, burn tunnel and heat riser? The feed looks distinctly larger than 4"/100mm. The system works best if it is constant size throughout, with a few typical bottleneck areas made wider. A smaller exhaust duct will slow the gas flow considerably and reduce the fast, turbulent draft that is part of the clean burn. What kind of insulation do you have around the combustion zone? Insufficient insulation will let the combustion zone cool off too much and reduce combustion efficiency.
The barrels you propose to use in place of the horizontal duct will most likely make a big difference. If you can replace the rest of the exhaust with duct or stovepipe of the same size as the combustion zone, that will help more.
How tall is the chimney? What is your climate, and what is the weather as you are testing the system?
posted 4 years ago
Hi Glenn, ive added another pic with some measurements, over and around my burn chamber i have white brickies sand, probably not the best hey, should have used cement but it was a quick way to seal the bottom of the barrel to stop the smoke.
so the burn chamber itself should be the same as the feed, if ive stacked the bricks right @ 150x80mm ( its a rectangular chamber in there aswell till it hits the heat riser hole which is square all the way down @ approx 130x130mm.
sorry im a little confused, "A smaller exhaust duct will slow the gas flow considerably and reduce the fast, turbulent draft that is part of the clean burn. If you can replace the rest of the exhaust with duct or stovepipe of the same size as the combustion zone, that will help more." - in essence the pipe is about the same size as the burn chamber, its just round not rectangular.
"The barrels you propose to use in place of the horizontal duct will most likely make a big difference." - A good difference or will it bugger it ? i see it similar as a bell setup?
do you think if i cut the 100mm pipe i have now back to about a foot from the base, then attatch the small barrels to nearly the back length of the greenhouse is a good idea?
also, i know im being a pain, could my burn chamber be too long (450mm) do i need to bring the feed closer by a brick or 2?
chimney feeds out the wall and will be mounted just above the roofline ....so it will prob be 2.5m tall. climate is temperate, it can snow here. days at the moment are about 25C night time lately 7C
posted 4 years ago
hopefully this helps too
so it will go from 100mm to what ever the barrel is - back to 100mm at the end to be fed out the window
A square of 130x130 mm isn't equavalent to a circle of 100 mm. In fact, it is over twice as large. In order to have the cross section area the same throughout, the exhaust pipe should be very close to 150 mm.
Apart from that, you appear to have used the bricks on flat without any insulation around it. Lots of mass and no means of keeping the heat in is a good recipe for incomplete combustion. Changing the horizontal exhaust to a barrel without changing the vertical stack won't do either, the gas speed will be way too slow for complete combustion.
My advice: break the whole thing down and start with the bricks on edge and apply insulation all around the feed, tunnel and riser. Keep the cross section area the same through the whole gas path. Keep the tunnel as short as practically possible and see that you have a proper vertical chimney pipe, preferably double walled when outside the greenhouse and ending above the highest point of the roof. Every turn of the gases need more space i.e. wider in order to avoid restrictions.
The setup you have now won't work as intended, ever. It isn't sensible to scale the whole thing down to the 100 mm chimney pipe because this small a rocket is very difficult to get right.
I concur with everything Peter has said. Please follow his advice - he is one of the most experienced RMH researchers in the world and knows what he is talking about.
Your barrel idea would be good in addition to Peter's advice. And to be more explicit about his mention of making the tunnel short, the burn tunnel in a small system will work better when it is as short as practical. Your thought about shortening it a brick or two was on track.
And inside of my fortune cookie was this tiny ad:
Devious Experiments for a Truly Passive Greenhouse!