Fireplace rocket mass heaters

...I want to know if anyone has put a rocket mass heater into an existing fireplace...is it possible, where can I purchase plans? Ta

It is possible to use a fireplace chimney but I think you will need to give a bit more info on exactly what you have in mind?

There are some threads here with people who have done it. We do need more information from you before we can give useful advice. What is your house and room like, clearances, dimensions of the chimney, your climate, how much of your heat you hope to get from this RMH...