James Gardiner

Another thought Glenn.

As I understand it, from ZeroFossil, The dimension ratios are critical and presumably the smaller the system the tighter the tolerances - Is this correct?

Reason for asking is could I increase the size of the feed tube, from 3.25x3.5" to say 5x3.5", without increasing the riser tube? or will this total kill any draw I have.

Thanks Glenn,
I have not looked at the batch box systems so something new to investigate.

Currently I'm getting about 45min to 1 hr before refueling, I'm burning 2-3' lengths of birch lumber, but as you say, if I'm not burning at optimum efficiency right now I imagine that time will reduce.

You have given me lots of food for thought,
However - if I understand you correctly
Even if I were to increase insulation around the riser, add some insulation around the burn tube and feed tube and insulate the outside chimney I'm probably still not going to get enough heat for the mass?

Currently the barrel is in "open air" i.e. it does not have any mass built around it, I did this to allow easy access should I need to take it off (kinda lucky by the sounds of it!!!). If I am to continue down this route would it be better to encase most of it in mass or to leave it in the open air?

Again, by using the sand as mass, I have the opportunity to reduce the length of ductwork, which may help a little, as well as increasing the height of the external chimney.

Once again, thanks for the input.

Hi everyone,
So I'm new to these forums, although I have been checking out this and other websites for a while.
My wife and I have built, this summer, a small green house/well house. its only 15'x10' (100sqft of greenhouse, 50sqft of well house) and have incorporated a rocket mass heater to extend our grow season.
primary construction is 2x4" timber and plywood with batting insulation in the walls. 3 sides are double paned windows and the roof is double layered poly on outside and single layered poly on underside. All of this has been done a SMALL budget (total cost of approx. $500 for greenhouse and RMH) nearly all materials are recycled, begged, borrowed or stolen, well not stolen just bought really cheap!!!

main reason for my post is I'm having issues with the RMH. when first lit it really roars for probably the first 10 - 15 minutes and it gains temp quickly. but soon after it seems to dampen down and unassisted will go out. I've gone through the heater again looking for air leaks, done some leak tests, found a couple of small ones and have rectified but to no avail!!! So I'm now concerned ive got a fundamental issue with my design.
As a temporary solution I have added a duct fan at the end of the system to increase the draw, which causes 2 issues.
#1 kinda not the point of the system in the first place!!! I'm trying to reduce electricity consumption in the well-house with supplemental heat not add to it.
#2 the fan actually draws too much air through not allowing the gases to burn off properly.

At first I was impressed with the temp figures I was getting, compared to our in-house wood stove. (over 400F at the top of the outer tank, and close to 200F at the start of the exhaust) but on replection this is not so good compared to other RMH, this is also using the fan to get super draw.

So here's some details of the heater. mass is just sand, may switch to cob if we get running right.
I used plans from Zerofossilfuels to build the unit, primarily as the materials I had to hand fitted his design and the size of our greenhouse didn't need anything bigger (I hope not anyways!)
attached is scale drawing, courtesy of ZeroFossilFuel.
The exhaust is 4" singlewall duct and is a total of 20' horizontal run then exiting to a 6' high chimney. there are 6 90 degree bends in total. see attached pics  

From doing some more reading I have 2 concerns, are my gases cooling enough to get draw and reburn. and is the gap between the inner wall and outer wall sufficient.

Sorry this is all a bit garbled, just wondering if anyone has any ideas