mashedpotatoes101

Holy shit, there's other people designing rocket mass heating systems out here? My dad builds them professionally, I'm currently studying chemistry, and have quite a bit of experience testing/working with docket mass heating systems. My home used to be heated by a rocket mass heater attached to a central heating systems! I'm not sure if you are familiar with Peter van Der Berg, bad his Rocket Batch Box, it has won some prizes for high efficiency I believe. I helped design a small part of that thing! (Preheat of secondary air by routing of the P-channel was my idea!)

Anyway, condensating wood stoves are extremely hard to design with any durability. As you said, rockets mostly output steam, but some volatile hydrocarbons and other junk is still in this steam. It's nearly nothing during the normal burn, but during startup.... it gets dirty. This means that the condensate gets polluted with all sord of nasty tar like acidic chemicals, which are really, really good at damaging internal structures, and are also combustible, which is of course an issue as you really don't want a fire in your chimney.

Some of the solutions we used are stainless steel chimneys with water drains. This works! However, it's nearly useless. If the rest of the stove isn't resistant to these chemicals (Wich it isn't, see below) , you can't condense there, which means that the heat extraction has to happen in the chimney, where storing any useful heat is hard, as the chimney needs to be well insulated for improved draft (Which is more important to high efficiency as the energy gain)

The only material that can withstand this stuff without being a fire hazard and operate at high heat I've found thus far is stainless steel or extremely expensive ceramic liner designed for steel furnace's.

One way we have figured out works somewhat is using a water-based heat storage, where you build a flame-pipe heat exchanger out of stainless steel. This way, only a small size area needs to be made out of the expensive stainless steel, as the heat exchanger and chimney are the only parts exposed to the condensate. The water van be stored in some sort of buffer vessel. A tank of 1000L of water is enough to store all the heat to heat a well insulated house all day and night, in the dutch climate drying winter, at least.

I'm super excited there's more people who like these stoves. Any idea where I could sign up to help spread the word and share all the data and designs I have?

Cool! I might join then!

Also,i was talking about using stainless for the second chimney, if you use stainless for the riser it will indeed melt, as I can personally attest to 😅, however, if you are running you don't need to use stainless at the hot side as condensation is a non issue there, you could use any ceramic there. The stainless would be used posey cool down. In the flame tube exchanger, it is actively cooled with loads of water, so I don't see it's melting point being a problem here either.

Peter has much experience with casting RBB's out of castable refractory cement, has he recently stopped using it?

Also, if the mass gets above the boiling point of water at any time, you might have a steam explosion at your hands.

Ask me how I know.

Ceramic fibers are suprizingly environmentally friendly! Just make sure to cover them in a sealant of some sort. My favourite method of building rocket cores is by casting them out of a castable refractory, as this is an easy way to prefabricate cores and have exact sizes. For insulation, have you looked into vermiculite? It's a puffed up mineral used as a ground replacement when growing plants. It's also cheap, and entirely natural (it's made out of puffed up rocks). And, best of all, it's a superior insulator to sand/ash. I've had the stuff glowing red hot without problem! Vermiculite can also be added to castable refractory to increase its insulating properties. I've also been looking into designing a clay based porous castable, Wich would mean cheap and acute cast rocket cores. (so just using river clay and some other material to create bubbles that last)

However, having/using a wood fired boiler is actually quite a good idea. It's basically just a Biomass heating system, and you can get quite efficient at residential scale.