Flexible pipe

Hello, I have just found this site so this is my first post. I am building a rocket mass heater for the first time and was wondering if
it will be ok to use flexible flu pipe for the horizontal run inside the mass. Has anyone used this kind of pipe? It is used as a flu in gas
fire installations and is much cheaper than plain pipe of the same diameter. I would be grateful for any feedback. Thanks.

Too much drag.

Michael; Jay is correct, to much resistance with flex pipe. Use one piece of regular black stove pipe to leave your transition area, then switch to thin wall hvac pipe(5' sticks of 8" at H.D.for 11.00)I used a second stick of black heavy wall when I came back up out of the mass.Then back to thin wall to go thru the roof.

Michael Robertson : So make that 3 NOs in a row, as thomas rubino strongly implied, the size of your pipe and smooth interior wall construction is important !

We kinda need to know your location!, the minimum maximum number of square feet you wish to heat (to see if likely or possible) ceiling height,and insulation
values in walls and ceilings ! do not expect to heat multiple floors, while potentially possible you are retro-fitting an existing space, it just may not be feasible !

Have you been to rocketstoves.com to download your PDF Copy of the Brand new 3rd Edition of Ianto Evans' masterpiece Rocket Mass Heaters ?

This is ''The Book", With 100,000 + RMHs made world wide, most were made following "The Book'', and 95% of all the 1st time builds (that worked) were made
following 'The Book'!

With it you are prepared to come back to your Fellow Members knowing you will be using the same words to describe Your RMHs Size, Shape, Location, and the
Orientation of its parts to themselves and each other! Your comments and questions will show that you understand the basics and more redly help them to assist
you with the correct information so that you too can be a Rocketeer ! Hope this helps and is timely ! For the good of the Crafts ! Big AL

Fourth NO with a couple caveats

If your run is EXTREMELY short (like three feet or less) or you upsize the pipe a size or two it may work for an experienced RMH builder. But then it would still be suboptimal.

In answer to Allen Lumley - I am in the north east of England (we have a castle nearby called Lumley Castle!). I am building my rmh outside as an experiment. My aim is to heat a greenhouse which I have yet to build but which will be 16 feet by 10 feet and about 8 feet high on average. I will look into that site you recommended and consider getting the book. Thanks for your responses everyone.

I fired up my rmh for the first time yesterday and had mixed results. It is experimental at this stage and is in the open although there was no wind and the system is dry. I have used reclaimed bricks from a demolished house fire and chimney and stuck them together with cob mainly, apart from (expensive) fire cement in the burn tunnel. There is no pipework or mass yet as I was looking to see how it worked. At first it roared as it should and when I peeked down the heat riser the flames were coming half way up. After a while it slowed but still burned sideways with no smoke coming from the feed chamber. I narrowed the size of the opening to the feed chamber by placing a brick over the top and it seemed to improve the draw a little. It seemed just about ok but when I placed the drum over the heat riser the whole thing failed and the fire burned straight up out of the feed chamber. The space above the riser to the drum is two and a half inches. I will come back with measurements and pics but I have followed the advice I have taken from reading reams of posts from this site so I think the dimensions are in order. I have noticed on a lot of videos that people add height to the feed chamber - does this help the draw? Also the burn tunnel is currently not insulated - will a cob and perlite mix really make so much difference to the draw if I cover the burn tunnel. I'll do that anyway but any advice very welcome.

Michael R. : Fortunately the answer is simple, and proves a very simple point !

When the barrel goes over the top of the Heat Riser, it needs to be connected to a vertical chimney,(stovepipe) that is 4-5 feet taller than any nearby object/structure

This is one of those Automatic '' Everyone knows that'' kind of things that we sometimes fail to pass along because everyone had ought to know that ! The other little
secret fact that we fail to pass along Clearly is once you feel committed to building your own Rocket it is time to get the other 90% of your answers
From "The Book". You ,yourself have just proved that to be true !

Good luck, and come back here often, for the craft! BigAL

Well I was misled by this video where at about 2mins 30 sec they put the barrel on to test and say it worked! Anyway thanks for the advice. I measured up today and discovered that the burn tunnel was too large cross section area. I was aware that this has to be the tightest part of the whole run but somehow messed up. I intend to leave the heat riser in place and pull apart the feed tube to access the burn tunnel and reduce its cross section. I'll try again then. The rocket hasn't been very powerful so far so I will persevere. Once that is sorted I will move on to exit side pipework although getting the hvac pipe over here is a problem I am finding. My heat riser is 9 x 6 inches so a csa of 54 sq in. I take it that my burn tunnel should be about 7 x 7. Ordering a copy of the book today.
Thanks.

Michael : Repeat after me- '' There is a lot of crap on you-tube "Say it, Say it, SAY it again ! U-Tube only cares about content, is hungry for content

If someone will film it U-Tube will show it, let the Viewer beware ! !!! Big AL's personal soap box !

You said the burn tunnel is not insulated... is the heat riser insulated at all? What is it made of? The draw depends on a big temperature differential between the inside of the heat riser and the barrel space. If there is no insulation on the riser, it will heat up the barrel quickly and damp down the "pump" that drives the draft. The insulation on the burn tunnel and riser both allow the internal temperatures to get much higher, which intensifies the draft.

My experience with L-shaped fireboxes says that flames can easily come out of the top of a 3 or 4 foot high riser. A rocket firebox I built recently for an oven, after 10-15 minutes of warmup, had flames out the top of a zigzag 6" riser (3' or so total length) with a very small fire, on the first burn with wet cob construction. (It did have partial sacrificial wood forms which burnt out.)

To Glenn - The burn tunnel is made of solid yellow fire bricks, the old fashioned heavy kind and is not (yet) insulated. The heat riser is made of solid old type red bricks (without frogs) mortared with cob. They came from a chimney off a victorian house. I take it this kind of riser does not need insulation. I have room to apply a coating of perlite/cob mix as a render if you think it is worthwhile. I will be insulating the burn tunnel with the same mix when I rebuild it. Thanks for your replies.

Dense firebrick and antique "common brick" constructed heat risers will require (for best RMH performance) insulating the outside circumference. Perlite/clay mix will work, or even better use a one inch thickness of ceramic wool blanket banded in place with something like steel wire, steel wire mesh / hardware cloth, i.e. the stuff with 1/8" to 1/4" holes between the weaves.

Michael : Do a Search for the "Fake firebrick'' Thread in the Rocket Stoves Forum and scroll on down to Erica Wisner's Thread extension, this can be considered
to be the ultimate authority on what Firebricks are, where they can be used, and exactly what is a 'working substitute'…!

Short version on Old Red Bricks, that are acceptable. They are the dead soft brick that can be used like sidewalk chalk, and will leave a clear red mark, if they
are so hard they leave a scratch mark, these are not the acceptable type of ''old house brick''. the later type of brick should have been used for general house
building, as such they were acceptable to be the outside layer over large terra cotta Flue tile. The tile handles the heat and the outer brick is basically a retaining
form making up the flue tiles outside walls, it makes no difference if the chimney was on the house or in the house.

The dead soft brick can be laid up one Course at a time with the bricks placed on their sides to increase the amount of room you have for insulation which should
be very close to 3'' thick if using the clay slip / perlite mix as the Clay Slip while an excellent binder will slightly decrease the insulating value of the perlite . Yes
you need insulation on the outside of your Heat Riser. Ether Ceramic wool fiber blankets. or Rockwool will work as insulation The Ceramic wool is more efficient,
and the rock wool is cheaper! Ether wire form will last a couple of heating seasons minimum at this location ! For the Good of the Crafts ! Big AL

Thanks Allen - I will check the bricks. I sent for the book today but it will take from two to four weeks to get here and I want to keep working on my rmh so keep fielding my questions in the meantime please. I was able to access an old version of Ianto's book a year ago and remember he mentioned the horizontal run possibly being made of brickwork without the need for metal pipework. Since I have about 1000 bricks lying around (I was going to block pave my driveway but it can wait!) I thought since I am having trouble getting the hvac stuff I could try making a brick tunnel with a steel pipe vent at the end near the heat riser drum. What do you think? Lots of work but I have the time (retired) and the materials pretty much. Let me know your thoughts please - much appreciated.

Bricks have a much rougher surface than ductwork, so you would need a distinctly larger cross section for a flue built that way. You might also consider alternate heat exchange/storage methods like "bells" with that material on hand. It would depend on the space and configuration available, and how you want to use the heat (is a big warm bench important, or is floor space more important?)

A larger csa in the ductwork is not a problem and I can line the inside floor and sides with smooth cob to help flow. My greenhouse will be about 16 feet by 10 feet and I hope to have a heated mass bench about 10 feet long (with a return so 20 feet of ducting in all) and about 3 feet wide overall. The main aim is to ensure a frost free environment. We get temperatures down to about minus 10 degrees centigrade sometimes, although not often, but sub zero temperatures are common from November to April. I think the big warm bench is the way to go. I don't know what "Bells" means so I am off to look in the forum to find out! Thanks for the advice about making the brick duct large - I wasn't sure about that. Could you tell me should I insulate the inside of the burn tunnel or just the outside or both. I hope to start re-building it this weekend.

Regarding Bells:

The blog in my signature has an article discussing the contrast between the way the exhaust is stratified in a bell and flows all together in a flue, bells vs. flues. Bells have some advantages, although they are not part of a traditional rocket mass heater as described in Ianto Evans and Leslie Jackson's book.

Cindy - Thanks for responding. I think the mass heated bench is most appropriate for a greenhouse, as a warm bench giving bottom heat for seedlings and cuttings.

Allen - I checked a few of the bricks and they all left a lovely red chalk mark so they must be acceptable.

I demolished the heat riser this afternoon and altered the dimensions of the burn tunnel. It is now as short as I can get it which is 8 inches long. It is 6 inches high and 7 and a half inches wide. I narrowed it using cob - is this ok, will it last? Can I now make the heat riser 10 inches by 8 or is that too big? I am limited by the size of the bricks unless I cut them. Is it an advantage to keep it close to the burn tunnel csa? Sorry for all the questions but the book won't get here for weeks. I sent for a 100 litre bag of perlite today for the insulation. One last question (for now) what is the ratio of perlite to cob to make the insulation?

Thanks

I found the answer to the cob perlite ratio. You make clay slip and mix perlite until it binds and when squeezed it doesn't exude water but still clings together. Ok now another question. Does the cob/perlite insulation on the heat riser have to be contained with something or will it stick of its own accord? Still waiting for the book to arrive! One more thing, I have some sheets of corrugated asbestos left here by a previous owner and they are difficult to dispose of. If I need to encase the perlite/cob insulation could I use the asbestos sheet?

M.R. : You have 90% of it ! The ball when squeezed in your fist should stay intact but when squeezed between thumb and fingers
should easily pop apart !

Because the Clay slip reduces the insulation value of the perlite slightly it is important to use enough at least 2 inches, 3'' would be
better!

If your C.S./P is at least 2'' thick you should be able to wrap it in hardware cloth or stuff the C.S./P. between the heat riser and a
large piece of H.E.V.A.C. cold air retune pipe, here we are not worried about its eventual failure directly under the drum, but just
supporting the C.S./P.

This is where we both are going to get excoriated by our Fellow Members!

The current building codes do not allow for any use of asbestos, but does allow for the encapsulation of it where it is found,(I Think)
usually when this is found in public buildings it is just ripped out and hauled off to a remote $ Location $

We are trying to prevent inhalation of the fibers, drinking or bathing it are not the problem, a neighboring town has asbestus pipe
for its municipal water supply, not ideal, but not deadly !

I am stymied on visualizing what you have, perhaps if you had a retaining box with some sand in it you could carefully bed the
asbestus sheets on the sand and cover with more sand this should improve the protection at floor level, otherwise the golden rule
should be to handle it as little as possible ! and use a mask and wash your work clothes immediately after as a separate wash !

Check for the county agency that deals with 'Solid Waste', usually a given region has one day a year where you can turn in hazardous
waste without penalty ! Give them a call and pump them for information ! AND Share what you learned ! Big AL

On reflection I think that some microscopic asbestos might get into the flu gasses and come out through the exit chimney making it hazardous. I will drop that idea and think of some other way to contain the perlite mix. I have some corrugated tin roofing sheets which I can probably use if I fix two together and make into a circle. I rang my local authority and they will allow me to dispose of the asbestos at a particular site they run as long as I double bag it and tell them when I am coming and how many bags I have. Thanks for your help Allen but one more question, will the cob I used to narrow the burn tunnel stand up to the heat?

Michael R. : I do not know, The thinest places will probably crack 1st, I have always tried to make my cob mortar as thin as possible with as clean a Combustion Zone
as possible, in spite of the fact that we accept the idea that what we need there is Time, Temperature, and Turbulence to assure good mixing of wood gases and air
(Oxygen)! Good Luck ! By all means, let us know how You made out ! For the Crafts ! Big AL

The book arrived today so maybe I will have fewer questions from now on. I have re-built and now the dimensions are

Feed tube 7" by 6" = 42 csa (still not worked out how deep to make it yet.)

burn tunnel height 5" width 6" length 12" = 30 csa

Heat riser opening 7" by 6" = 42 csa The height will be 31" by using a thin walled pipe of 7" diameter giving a csa of about 38 sq inches with another steel pipe of 11" diameter meaning I will have a 2" gap for perlite/slip clay insulation. Do I take it that the insulation needs a burn before it sets hard or will it set naturally given time? The gap between the riser and the wall of the drum will be approx 5" all round - is this too big? I could make a bigger outer pipe by making one from sheet so giving more insulation and less "tumble" space. Please advise what is the better option. Oh and I have never used perlite but have just bought 100 litres - does it expand when mixed with the slip clay? Many thanks,

allen lumley wrote:M.R. : You have 90% of it ! The ball when squeezed in your fist should stay intact but when squeezed between thumb and fingers
should easily pop apart !

Because the Clay slip reduces the insulation value of the perlite slightly it is important to use enough at least 2 inches, 3'' would be
better!

If your C.S./P is at least 2'' thick you should be able to wrap it in hardware cloth or stuff the C.S./P. between the heat riser and a
large piece of H.E.V.A.C. cold air retune pipe, here we are not worried about its eventual failure directly under the drum, but just
supporting the C.S./P.

This is where we both are going to get excoriated by our Fellow Members!

The current building codes do not allow for any use of asbestos, but does allow for the encapsulation of it where it is found,(I Think)
usually when this is found in public buildings it is just ripped out and hauled off to a remote $ Location $

We are trying to prevent inhalation of the fibers, drinking or bathing it are not the problem, a neighboring town has asbestus pipe
for its municipal water supply, not ideal, but not deadly !

I am stymied on visualizing what you have, perhaps if you had a retaining box with some sand in it you could carefully bed the
asbestus sheets on the sand and cover with more sand this should improve the protection at floor level, otherwise the golden rule
should be to handle it as little as possible ! and use a mask and wash your work clothes immediately after as a separate wash !

Check for the county agency that deals with 'Solid Waste', usually a given region has one day a year where you can turn in hazardous
waste without penalty ! Give them a call and pump them for information ! AND Share what you learned ! Big AL

Hi again Big AL, looking for info on how to pack the insulation around the riser. Am I packing it on and just using the hardware cloth at the end of getting it on there? Or am I making a cylinder out of the cloth and then packing in the CS/P mix? Seems like the former would be easier (reaching down into the cylinder to pack the CS/P mix would be tough...

Also, what about turbulence from the rough CS/P surface? Does that impede flow?

You want to have the inner and outer forms in place and pack the clay slip/perlite between them. You could use a 3' piece of 2x4 or 2x2 or a 2" diameter stick with a flat end for packing - just gently tamp all around in lifts of a couple inches or so.

The inner surface should be smooth; using a section of sonotube (cardboard form for concrete piers) would be ideal. When that burns out, the remaining material will be smooth. The outside surface does not matter since the cross sectional area will be bigger and flow less intense.