Procrastinating? Don't... Here's why you NEED to build a batch rocket heater!

Scott, Cristobal, great inputs. I will have to review my initial idea and make the bottom of the riser in dense firebrick. I guess my riser will become a bit higher as I already have the IFB.

A few days ago I got a training on refractory from a colleague and he was saying you have to make a compromise between temperature resistance - determined by the alumina and alkali attack resistance - determined by the silica. So more silica = better to resist alkali, but can handle only a lower service temperature. This appears contrary to what is claimed by Forscythe on the proboards forum thread.
Found a paper online in german saying the same thing.
Following this scheme, the lower alumina bricks would be better for the firebox, but not sufficient for the bottom of the riser.

Julian,

If that was the case then industry would not have a need to use high alumina bricks.
I have read a paper about the need to replace super duty (>40% Al2O3) with high alumina (>70% Al2O3) bricks in biomass incinerators due to increased slag fluxing caused by higher contents of alkali oxides in bark and roots.
Alumina is considered very refractory and the SiO2 melting temperature can be quite easily lowered by adding fluxes like it's done in ceramic glazes. Al203 in glazes will increase mattness level, because it does not want to melt even in kiln's 1300C temperature applied for several hours - conditions that will not be met in a masonry heater.

To sum it up: for firebox that uses wood you need high alumina and porosity in hard bricks. Porosity will allow expansion of particles that would normally result in development of microcracks in less porous material. That's why hand made bricks, that are more porous, are recommended as acceptable material for building fireboxes and good material for heater skin.

Another important factor is expansion ratio (reversible and not reversible), because if a heated brick expands too much it will crack itself if temperature delta is too high and the joints (lowering the tightness of the firebox).

Also a good indicator is a spalling test: heating the brick to I think 1000 C and dropping in water and measuring mass loss. The more trials it can go through, the better - it will be definitely more resistant to constant temperature shocks in heater's firebox (albeit not so extreme as water immersion).

All bricks with alumina >40% will have all these parameters in good range and be quality material for firebox and will last 20-30 years minimum.
I saw some analysis of the firebox damage in 20 year old heaters and the one built of Super Duty firebricks was in a very good shape. The other one built with lower duty bricks needed some repairs. The heaters were in some cold state of US or Canada so obviously they were used more than heaters in warmer climates.

I would not get to concerned, just look for a fire brick, made with 40-45% Alumina content.
They are  designed to endure extreme conditions up to 1430° and will last for many years.
It seems to me you can read anything about anything if you look long enough ie you can find completely conflicting info about virtually object on the face of the earth!

I second what Fox said.
Also it's most important to build. it's better to build a heater from low duty bricks and gain experience, enjoy the heat, even if the material is not the best than not build from the best materials :)
The best it to build and from good materials.

Julian Adam wrote:Scott, Cristobal, great inputs. I will have to review my initial idea and make the bottom of the riser in dense firebrick. I guess my riser will beco me a bit higher as I already have the IFB.
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I am thinking, build to proven dimensions and if you have a dozen IFB's left over... so much the better for the next stove.

If your doing a brick bell, a taller riser will change calculations for Interior sq ft. of the bell,  if you have to increase your bell height to get above the taller riser, and on it goes... Nothing wrong with calculations, but just like buildings, things get more complicated when the heights go up.

Julian, echoing what Scott said... which is actually a quote from Peter van den Berg: "Follow a proven plan and get proven results". There's a lot of performance data on his published dimensions, which I'm believe you have reviewed.

Forgetting the risk of negatively affecting the performance, keep in mind that if you extend the riser height you will require a lot more brick in your bell, more mortar, additional labor, taller height overall. It would seem that having some left over IFB's would be the best plan.

Again great inputs! In any case, i have new 40% alumina dense firebricks which I'm using for the firebox and the bottom of the riser. Some of you may have seen my build thread. Unfortunately there are some other priorities in my home renovation right now, so it will take a while before I can continue building. Enough questions asked for now and Glenn, sorry for asking so many in your thread!

I understand your concerns about the riser height, but my bell will be quite tall - about 1 m 90 top of mind, and I had ordered IFBs for 8 times base dimension, while I believe Peter said the riser wil need between 8-10B. In any case I WILL respect Peter's dimensions and instructions, also top gap clearance which I believe was 1 ft/30 cm minimum.

Julian Adam wrote:...Enough questions asked for now and Glenn, sorry for asking so many in your thread!...

No apology needed... the purpose of my thread was to provide my experience. encourage questions and motivate people to overcome uncertainty so they can begin the process of building a RMH of their own. The more Q&A from the community the more the interest grows and the more RMH's are built.

For the folks that like data to support your decisions. The best reason to build a batch rocket heater... because it works... really really well.

It's presently a chilly -12F (-24C) outside at 5:00 AM. The last fire in the batch rocket was at 7:00 PM last night, 10 hours ago and the bell skin temperature was 176F (80C). My 2,000 sq ft shop with only 2" of fiberglass blanket insulation is 57F (14C) for a 69F (38C) differential and the bell skin temp is now 143F (62C).