3D CAD of My Rocket Mass Heater Design V0.9 What Do You Think?

After the past two Fimbulwinters in the Midwest, this former desert dweller is planning to build a Rocket Mass Heater. The RMH will be built in the basement. I know, I know, . . . the basement is where I will be spending my time during the depths of winter. After last winter, the first thing I did this past Spring was to insulate the basement walls. Due to this and other projects, as well as my belief that this past winter could NEVER be as cold as the prior one, the construction of the RMH was postponed. As it turned out, . . . well, you know.

The design was created in SolidWorks. When the final design is completed, the model(s) will be posted in various CAD libraries for common use. Before I complete the design, I would like to have your thoughts on the design of the RMH and the model. Thanks!

My thought at first sight: waaay too large!

But you aren't mentioning the system size, what is it, 6", 8", 10"? From the top of my head: an 8" system could drive a horizontal duct of 40', for each 90 degrees elbow deduct 4'. Your bench duct is already too long, I would say. Looks like it is already 8' longer than the 40' I am not sure about.

The tall thing aginst the wall, I would interpretate that as a bell. An 8" system could not service a larger bell than about 6 m2 internal surface area, or 64 sq ft. That is to say, without bench or barrel, just the bell. I don't see a way to calculate the inside of that bell so that need to be done yet. Note: all the inner surfaces of the bell do count, except the floor.

All together, I'd estimate your system is over 2.5 times as large as it could possibly serve with an 8" core. This is all an educated quess, I don't have all the relevant information neither the exact length of what the bench should be. I know Ernie Wisner provided some bench lenghts but I am uncertain where.

It looks like this RMH is intended to be built next to an existing fireplace, keeping the option for an open fire for cheeriness.
Yes, the duct with all its bends looks far too long. I would suggest making two of Matt Walker's "half-barrel bells" connected by short lengths of duct. This would reduce friction considerably. You would still need to calculate the ISA to extract enough but not too much heat.

Also, you show the duct exiting the bench and going up, over and up again to the chimney. Is there some reason you can't go directly over to the chimney flue at floor level? That would save a 90 degree elbow and help your flow, as well as reducing the clutter next to the chimney.

Hi Peter, thank you for your thoughts. This design is based on the standard 8-inch duct design.

The length of the bench duct is easily addressed by removing the 'S' loop. The integrated kotatsu decreases the thermal mass by about 25%. It is likely that a cob back rest will be included along the chimney-side of the bench.

The basement firebox and the 2-story chimney are represented in the model. The RMH exhaust will use the third flue. The chimney is the only suitable exhaust point at the front side of the house. There already exists a hole at the point where the exhaust duct will exit the basement wall.

Here is an additional angle:

Glenn Herbert wrote:Also, you show the duct exiting the bench and going up, over and up again to the chimney. Is there some reason you can't go directly over to the chimney flue at floor level? That would save a 90 degree elbow and help your flow, as well as reducing the clutter next to the chimney.

Thanks, Glenn. My earlier repsonse to Peter explains more about the chimney and its flues. Two 90-deg bends were dropped into the drawing because it was easy. In practice, the abrupt change in direction will be softened there. I'll change the model to reflect that aspect of the design.

Dax, is that an existing fireplace in the drawing?

Satamax Antone wrote:Dax, is that an existing fireplace in the drawing?

Yes. That firebox will have a second wood-burning insert: for both ambience, as Glenn observed, and as a redundancy. A wood-burning insert on the ground floor has heated the home for two years . . . but, only with too much effort, and the rapid dissipation of both firewood, and me.

Dax Robbe-Grillet wrote:

Satamax Antone wrote:Dax, is that an existing fireplace in the drawing?

Yes. That firebox will have a second wood-burning insert: for both ambience, as Glenn observed, and as a redundancy. A wood-burning insert on the ground floor has heated the home for two years . . . but, only with too much effort, and the rapid dissipation of both firewood, and me.

Who said rocket insert?

http://donkey32.proboards.com/post/16159/thread

And fireplace rocket retrofit?

http://donkey32.proboards.com/post/13115/thread

In regard to duct runs, the usual statement for an 8" system is that it can handle 50' of horizontal duct, minus 5' per 90 degree bend. I don't know that it has ever been rigorously studied, but between Peter's statement above and this one, you can draw a good idea of what is feasible. Making the actual duct length much shorter than 50' will reduce the amount of heat transferred, so with a number of bends as your layout requires, the half-barrel bells may get the heat exchange surface you need without too much drag. A basement location with a good chimney may have enough draft to overcome more duct length/drag.

A thought with regard to living with this....

You show a table. Do you really want to have to crawl over the ducting/bench to get to the table? Seems inconvenient to me. Why not change the routing of the ducts to allow people to walk directly to the table?

Erik Weaver wrote:A thought with regard to living with this....

You show a table. Do you really want to have to crawl over the ducting/bench to get to the table? Seems inconvenient to me. Why not change the routing of the ducts to allow people to walk directly to the table?

True, that might provide better access. It certainly would increase the external surface area. However, we would lose some of the intimacy of the shared kotatsu within (and with) a warm body. We would also lose a warm, enclosed container for other possible uses. The table top is meant also to be lowered to become a cover for the 'kotatsu pit', and then become a single, large, warm, comfortable surface. You know, so you could do something like this...

Also, the 'kotatsu pit' might provide some control of the rate of heat transfer.

Glenn Herbert wrote:A basement location with a good chimney may have enough draft to overcome more duct length/drag.

What defines "a good chimney"? I have a 2-story, 3-flue, brick chimney on an external (east) wall. 7" x 9" tile forms the flues.

One of my unanswered questions is if, and how best to line the chimney for an 8-inch duct RMH.

The ground floor flue with my wood-burning insert has a 14-foot, stainless steel liner. This liner is insulated at both the bottom and top with rock wool (Roxul). The 6-in liner allows little clearance for an insulated flue liner. The walls of the first floor firebox are insulated with roxul behind a stainless steel shield.

A fully internal chimney would be better, but as long as it is in good condition, the chimney you describe sounds fine. It's not too large or too small to give good draft. It's not a separate unit built onto the outside of the wall.

Hey Dax-

I'm actually planning a very similar system in a small place to be build this summer/fall. Ideally i would use a 6" system... I think that an 8" system would generate to much heat for the small, (350 sqft) well insulated cabin that is planned. I currently have ~24' of horizontal duct planned with 5 near 90º bends, which would dictate an 8" system be used. Your comment about the SS flue liner attached to your existing firebox, made me start to think about using a flexible duct for the run through the mass. By increasing the radii of bends you should produce less drag in the ducting. I can get my horizontal run down to ~20' with minimum bend at 2' radius, save one 90 near the chimney. That might be getting closer to a exhaust system that could be driven by a 6" system.

Anyone have any thoughts or experience regarding drag reduction using a system like this?


Also curious what you were planning to use for a mechanism to raise and lower you table?

Dale Walker wrote:Your comment about the SS flue liner attached to your existing firebox, made me start to think about using a flexible duct for the run through the mass.

The flexible stainless steel flue liner for my existing fireplace insert has spiral ridges on the exteror and interior surfaces: Not too good for reducing drag. This flue liner is designed to handle expansion and contraction cycles. In my installation, the flue liner is connected firmly at the insert's exhaust, and loosely constrained by the cap on the chimney's top. The rock wool insulation allows for some movement.

I ask the forum: How does fixing a flexible flue liner within a mass effect the liner's performance and durability?

Dale Walker wrote:Also curious what you were planning to use for a mechanism to raise and lower you table?

Undetermined. It could be as easy as removing the center post. Of course, this answers the eternal question: "Why are manhole covers round?"

I was imagining some mechanism for the continuous adjustment of height.

A quick search turn up this ... http://www.dryer-ell.com/welcome_dryerell.htm. They are 4", but I'm sure 6" could be found.

Certainly better than a segmented 90. I'll be interested to hear what other folks have to say!


As to the table... I was considering trying to find a hydraulic cylinder with a hand pump, probably overkill, but it would be cool!

Experts have said that the flexible (ribbed) metal duct has a lot more drag per foot than smooth duct. I wouldn't use it at all unless you have a very short system.

A better alternative to your 24' of duct with 5 elbows would be to use a couple/few of Matt Walker's "half-barrel bells". These allow hot air to rise to the top and slowly give up heat; as the air cools, it falls and moves out the bell exit located at the bottom. Drag is significantly reduced.

Based on previous comments/suggestions, here are some changes to the design of my basement RMH. I've simplified the duct path and eliminated a few 90-degree bends. I've also rotated the combustion unit to account for an error in the placement of the firebox in my original layout. Now, the location of the cleanout may need modification.

Reviewing earlier posts, I want to reiterate that making an 8" system with only 24' or so of duct will not likely give up anywhere near as much of the heat to the bench as you could do. The gases will still be quite hot going up the chimney, and this chimney looks like it doesn't need the boost from hotter exhaust. The bells would allow this heat to be harvested while reducing drag and spreading the heating surface wider than the 8" duct does.

Glenn Herbert wrote:Reviewing earlier posts, I want to reiterate that making an 8" system with only 24' or so of duct will not likely give up anywhere near as much of the heat to the bench as you could.

Thank you for your input, Glenn. It seems that the effective length of this second design is well under 20 feet. I think that I may have misinterpreted some earlier comments. I'm back at the drawing board.

I would prefer to keep the mass within the current boundaries. The walking paths in the basement should remain. The total mass in the design is just over 4 tons; without any additional cob detailing (back rests, steps, etc.). Additionally, the concrete floor will provide thermal mass.

Peter van den Berg wrote:From the top of my head: an 8" system could drive a horizontal duct of 40', for each 90 degrees elbow deduct 4'.

Does a 180-degree turn equate to two 90-degree turns that are separated by a straight run of at least several feet? How does the effect of a sharp turn change with its proximity to either the burn unit or the vertical stack?

Thanks!

Dax Robbe-Grillet wrote:The integrated kotatsu decreases the thermal mass by about 25%.

Oops! Correction: More careful calculations show that the kotatsu decreases the total thermal mass by about 12%.

(I originally omitted a square in the units conversion of the circular area.)

Peter van den Berg wrote:for each 90 degrees elbow deduct 4'

How does the bend radius effect these calcs? This design uses the standard 2/3 ratio of duct diameter to bend radius (bend radius = inside diameter * 1.5). To be explicit, the 8-inch duct 'elbow' has a 12-inch bend radius.

I haven't seen precise drag calculations for different degrees or radii of elbows. I used to have a Ductulator that had that kind of scale built in, but haven't seen it in years.

The effective length is not at issue; it is a matter of the actual amount of duct surface that can transfer heat. As short as 20' will not extract all the heat, but putting a couple of half-barrel bells in place of some duct will give more area. It will not affect the footprint of the bench at all.

Glenn Herbert wrote:The effective length is not at issue; it is a matter of the actual amount of duct surface that can transfer heat. As short as 20' will not extract all the heat, but putting a couple of half-barrel bells in place of some duct will give more area. It will not affect the footprint of the bench at all.

Thanks again, Glenn. After reviewing this conversation thread, it appears that a distinction should be made between the maximum (effective) length allowed by the driving forces, and the length of duct required for maximum heat extraction. It seems to me, at initial glance, that a bend reduces the length that can be driven, while increasing the heat extraction; but not in an equivalent, nor even in an inversely linear relationship.

I see now that the bells are the solution to these often conflicting limits when the shape of the thermal mass is constrained. I will review the body of knowledge on the placement of the bells. For this design, I am first considering whether they could be located along the final length of horizontal duct that runs along the wall.

Another factor in the standard elbow loss quote is that there is generally only one style of elbow easily available for ducts, which is much sharper than the radius you are talking about.

My feeling would be to put a pair of half-barrels in place of the first big loop (the bench is wide enough to fit two 23" x 35" barrels side by side), and one at the last part where you initially had the S-shaped duct segment.

Bump. Did this get build? It seems like a very cosy design and would love to see pictures