A DIY Rocket Stove Smoke Hood

 

Eco-Zoom Rocket Stove with DIY Smoke Hood

Somebody who claims to speak for the ‘hood don’t need no private jet.

– Chuck D

A DIY Rocket Stove Smoke Hood

About half of the energy in wood literally goes up in smoke, and with it, a good deal of our own! If the smoke can be burned, efficiency nearly doubles. The corollary is that only half as much fuel produces a given amount of heat.

Rocket Stoves were developed as a low-tech solution. At their heart is an L shaped, insulated tunnel, wide open at both ends. Fuel is fed horizontally, oxygenation and combustion burn hot at the crux, and what little smoke is left is pushed vertically, up and out the top. Typically, a pot directly atop the stove absorbs heat from the hot, gaseous outflow for cooking.

We’ve been using an EcoZoom Rocket Stove for years as a nearly smoke-free, otherwise open deck fire for outdoor cooking in hot weather, an evening blaze in nice weather and to heat metals for various projects (it easily takes ferrous metals to red-hot). It runs on limbwood from about thumb to wrist size, the former for cooking and the latter for longer, more subdued heat. In short, perfect for easy wood-gathering, especially as we age..

In planning MUSTELID, we hoped to bring this marvel indoors for cooking and heating. It requires far less wood to gather, dry and store within her small volume. Being insulated, wood can be dried directly alongside the stove, warm but nowhere near wood’s flashpoint (we can touch the stove sides without burning!).

To bring it indoors, we must gather and direct combustion gasses outboard, like any other onboard stove. The solution is a smoke hood fit snugly over the stovetop with a stack, which has the extra perk of increasing the cooking surface area. We wanted one more feature… we wanted to easily install and remove the stove for outdoor use on deck or ashore. So we went for a swing-up, cantileverd variation.

This called for some head scratching…

Grate spacer for airflow under,

Ash tray…

Whole thing slides aft behind bulkhead lip

And between heat / splatter shields.

Removed...
Note black fire-retardant pad
protecting carpet from sparks

Swing hood upward to remove

A look under the hood...
a vertical ring fits closely over upper plate
for extra fume collection

*****

Smoke Hood Plan
Not to Scale

Smoke Hood Elements

At it’s simplest, the hood is a flattish box made from sheet metal (in our case, stainless steel from and old stove’s grease trap… a heavier top plate has better thermal mass for cooking, if available. A large hole in the bottom fits closely over the Rocket Stove top, while a smaller hole in the top has stovepipe fit into it, which leads to a standard deck-jack and smokehead. If you leave the stove in pace, this is all you need.

For a swing-up top – allowing removal of the stove – we broke the hood into two boxes, each with an open end:

1. The far box is fixed (in our case to bulkhead and flanking heatshields), with the flue let into its upper surface. Its near end is open, with rounded shoulders and an under-lip. This part is female, slightly larger than the second part.
   
   

2. The near box is cantilevered (stops may be additionally fitted to help support heavy pots). Its far end is open, with a partial rounded shoulder fairing to 45deg… extending an over-lip, while the angled section limits upward swing. This part is male, slightly smaller than the first part. Insert its open end into the female’s until their respective hinge-pin holes align.

Other parts:

• The long hinge pin (alternatively, short, opposing pins) are let through paired holes centered on the shoulders. If it binds while swinging upward, grind away from the lower male shoulders till clear.
  
  

• A vertical smoke collar can be used around the stove-top cut–out (not sure it’s necessary). We used a strap of sheet metal, its edges doubled over just short of its outer mid-line (result about one inch wide) and tabbed around its outer mid-line (tabs bent alternately, slightly away from cut-out). When pressed into place, it ‘snaps’ in to lock. It works best to cut the tabs before bending.
  
  

• Rims, rails, bars and what-have-you may be added to secure pots while cooking underway. In our case, we added a low front rim and let that and tall heat / splatter shields along each side contain pots. Watch for swing-up clearance.

Notes:

• Any gaps allow air to be drawn into the hood. The closer the fits, the more energy is available for cooking and heating. Once the stove is drawing (seconds) there appears to be no leakage, even from wider gaps. We didn’t use gaskets, but they can’t hurt.
  
  

• We sized our male part to be 1/16in smaller than the female’s inside dimensions. Given our crude techniques, we managed between 3/32in and 1/8in. So long as it slides in without undue forcing and the gaps are small, it should work fine. Thermal expansion binding isn’t a problem as we only ever move it out once cool.
  
  

• In both cases, the lips should overlap the opposite part to help close the gap and contain ash. Small tabs may optionally be bent along single edges to help minimize warping with heat.
  
  

• Sizing can be more or less according to your available space. We sized around our frypan plus our small teapot, to cook and heat water at the same time.
  
  

• The smokehead can be lower to the deck than is usual. Although it does induce some draft, Rocket Stove’s generate expansion in their combustion chamber, and actually push gas about 40ft. Still, an efficient draft smokehead hedges our bets in some gnarly conditions.
  
  

• We didn’t have a brake (sheet metal bender), so clamped sheets over a piece of sharp-edged hardwood, then hammered it over. Cutting was with a thin-wheel grinder and touched up with a mill file. We used bent tabs with (copper/stainless) pop-rivets to join. Wear gloves, eye, ear protection and work safe!

Looking Ahead

One of the intriguing possibilities with Rocket Stoves is to make our own from a substance that insulates, absorbs and radiates thermal energy. In other words, which acts as a thermal mass or ‘sink’. Such are easily made from brick, firebrick, concrete, clay, cob and other materials. 

Perlite (pumice) is a light, inexpensive insulator, and can be mixed with concrete for a lightweight aggregate with decent thermal mass. The combustion chamber can be lined with refractory clay. A thick, steel cook surface is easy to install over a lipped smoke chamber for a full sized stovetop / smokehood. Sand makes a good gasket between plate and stove. Alas, this doesn’t sound very portable!

But we keep on thinkin’.