thermal mass on pocket rocket

Hey guys, been lurking and learning.for a while and o try to find answers on old threads before asking but to.save time on this one I thought is just ask.
My fiance and I bought a 1920s built cottage this year with a.working open hearth fireplace, it does however have a damaged hearth (the still craddle in which the wood sits) and I.figure having a small pocket rocket sitting in the bottom and has its exhaust reaching up the actual house chimney. would work for a more fuel efficient burn... Then encasing the majority of the steel drum in a jacket of cob to catch and store some of the heat.
Please shoot me down if its hopeless let me know if to sound feasible and or advise me how to tweak things if I need to.
Additional details: fireplace is about 4 foot by 4 foot and 2.5 feet in depth to the back. The main room, other than the fireplace is on mudbrick steps, the exterior of the cottage is under local heritage listing. We live in am area that never gets below minus 3 celsius (sorry don't know F), I intend on having some way to clean the ash out of the bottom of the barrel every few weeks (probably using an old vacuum once the mass is completley cooled).
Thanks for the help in advance guys

Hi Myles; If your house chimney matches the rocket chimney in size then you could do this .It will work and be a more efficient burn than a regular fireplace but ... I think that you will not be very happy with the result. You will be adding wood ALOT and because your mass is so small i'm afraid that it will go cold unless somebody is up at night to relight it. The other problem i foresee is your old chimney may not be able to handle the temps that a pocket rocket shoots up a chimney ... so a good inspection of your chimney would be a necessity, possibly a reline would be required (expensive). A rocket mass heater would only have temps in the 130 degree range going up your 90 year old chimney and would not worry me at all. I would look long and hard at how to fit a RMH into your cottage, a nice cob / brick / wood / couch maybe ? Another option could be a dragon heater or a self built similar masonry style heater. Good luck, congrats on purchasing your cottage.

Myles, check this thread!

https://permies.com/forums/posts/list/40/31382#246585

Thanks guys, Thomas would I be able to us vermiculite to fill the mass of a couch over cob? I am worried about the weight on the timber floor over mudbrick steps which are just set on dirt, they are sturdy but I think a couple.of tonne of cob might be pushing it. I understsand ill need to replace the ducktong every few years Antone, ill check that thread after work. Everytime I check this place in the mornings I run 10 minutes late. Thanks again

would I be able to us vermiculite to fill the mass of a couch over cob?

Vermiculite is an insulator.

What you need in your bench or couch is something which conducts and stores heat. Water is the best at this, but it can only be heated to 200°F. For the weight, dense fireclay brick does better than cob and it has the best price/performance if you don't count "free". That is why masonry heaters and pizza ovens are built with it.

The blog site in my signature has a chart of potential thermal mass materials which you can use to confirm this analysis.

Myles; Cindy is correct ,with all she said. I'm wondering about adding some support under the floor ? I'm not familiar with your building style , but if there is no access could one be dug ? A few extra blocks / rocks / steel beam placed under your mass should hold the extra weight . Choosing the best location for your mass so the weight is spread over many floor joists is essential . There is a style of mass heaters that uses a "bell" they have less weight than a cob & rock mass. I have no experience with these but I know that others have. Try looking at past threads under bells. Tom