Hello Greg, and welcome to the forums!
Watching the fire is definitely one of the pleasures of heating with
wood.
However we (and the building inspectors) don't like to see two combustion set-ups connected to one flue.
Although as you say, it sounds fairly straightforward to toggle between them, there is a nonzero probability that at some point either exhaust will leak back down the unused fork of the chimney, or the toggle will be set wrong when lighting the operational stove, or someone will try to light both at once. It is not logically possible for the chimney to be the right size for each stove, as well as the right size if both of them are operating together, and any of these situations could cause exhaust to backdraft into the room in the wrong conditions.
Since the
rocket mass heater's exhaust will often be relatively cool, in a joined exhaust it could potentially get chilled and fall back down, re-entering the room through the wood stove. And it's clean enough that it could be invisible and hard to detect, aside from a slight smoky smell. But not quite clean enough to be healthy indoor breathing air. The times the RMH exhaust is most likely to have lower heat are during a cold start, or when the stove is not running full on, and those are also the points in the burn cycle where there is a higher chance of CO and other incomplete combustion products/poisons in the exhaust.
Other ways you could do the flame-viewing thing:
- Get a rack of candles (Ianto's suggestion)
- Have a separate stove or fireplace with a separate chimney. Through-roof kits for chimneys are commonly available, much more so than custom-cut ceramic glass.
- Place a reflective metal panel or convex mirror above the
feed, to reflect the flickering light into the room
- Place a comfortable chair near the feed, or build the bench and firebox in such a way that the feed is easily observed from a comfortable spot on the heated bench, to allow the lady of the house to sit and gaze into its hidden depths
- Consider a more technically challenging, but equally clean burning, batch-box style firebox, and get a custom door made with ceramic glass that can withstand the (slightly less intense and more evenly distributed) heat of the initial-stage batch fire. You won't be able to see the double-rams-head vortex in the secondary burn chamber, but not many ever do.
- Consider a small, careful window of ceramic glass in the coolest point in the feed, maybe 9" by 9" replacing a couple of bricks in the feed of the J-style firebox. Maybe double pane, to reduce the heat shock on each individual pane. You will need to do some careful detailing, bedding the glass with fiberglass gasket, or getting it set in a steel frame with tabs and gasket to be inset into the brickwork.
The insulation matters least on the short end of the J, most around the heat riser; a small window at the first corner of the J could be done without compromising the heater's operation too much. Matt Walker did a few stoves with windows in various places, and the ceramic glass holds up well. Workmanship is key to making it work, rather than have a leaky firebox.
- One of our
local boilermakers wants to try ceramic glass directly above the heat riser, in place of the barrel top. I don't know how well it will hold up to the asymmetrical heat source blasting its center, but the light show on the ceiling could be phenomenal. This is an "at your own risk" suggestion as I do not yet know of a successful prototype that has tried it.
I might suggest searching the forums using the "search" button near the upper right, and entering "rocket" and "watch the fire."
Or do a similar Google search that will let you put those quotes around "see the fire", like this: (copy and paste what's below into a Google search bar)
rocket heater ("see the fire" OR "see the flames" OR "watch the fire")
You may find more recent projects with flame-viewing windows that I haven't spotted yet, and you can also ask them about how clean the thing is burning as a result. Get them to inspect the stovepipe and get it on camera, or something.
Yours,
Erica W
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