Dax Robbe-Grillet

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.

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.

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: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!

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.

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.

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.

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.

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

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...