David Date wrote:With a conventional RMH construction, I thought I'd have the radiator submerged in water, or nested into one side of the tub for heat exchange.
Is it possible to skip the riser and radiator all together and run a refractory tube through the floor or seat, and exhaust out the other end?
My understanding is a simple "No". Sorry to be a bummer.
Although it would seem more efficient to not lose heat to risers and bells and such, a key part of an RMH is that it achieves a high enough temperature in the riser and bell to completely burn all the combustion gases ... thus its clean and everything downstream of it is exposed to just hot air. The riser is very much a pump that drives the RMH and its height needs to be in a very tight range ... too long is too powerful and the gases move too quickly, too short and it doesn't have enough power to create draw and push exhaust through the system.
The other problem you run into is a simple one of temperatures - the insulated riser is critical because it is is insulated and thus it doesn't rob the RMH of the heat it needs to get going. If you just have a fire tube running through the water then you will probably never attain necessary temperatures as the water will soak up the heat. A criticism of the Snorkel style wood heater for tubs is that b/c the whole fire is surrounded by water, the firebox never gets hot enough for a clean burn. So you end up with smoke and a pipe that will accumulate creosote and slowly clog ... until the creosote catches fire!
So you need the riser. I don't think the bell is necessary here... and maybe what you need is a stratification chamber under the water. But it has to be absolutely water tight as any water that comes into contact with ~1000 degree air will flash to steam and bad stuff happens. And you need to have a pipe/chamber that transmits enough heat to the water (assuming you are treating the water as the mass. Might be interesting to consider a different mass that heats the water).
I'll second Tom's concerns about starting the thing. You might consider some of the design elements in the "Season Extender" that Uncle Mud built at Wheaton labs - and which Paul discusses in a recent podcast (#519 - RMH in the Tipi). I'm thinking of the boomerang path that has the final exhaust pipe adjacent to (even thermally connected to, or even inside) the bell, thus "priming" the pump and softening the cold air plug.