You can do a hybrid approach with the hot metal burn chamber for wood gas and earthen materials to slowly radiate the heat. This is called a rocket mass heater. The exhaust temperature is usually below 100C and often smokeless (a complete burn). Using a fan to extract heat from the final vertical stack (pipe around the exhaust with air running by) can give you an exhaust temperature of around 40C. Here's a quick 3 minute video that explains the concept; there's other YouTube videos that go into greater details if you wish to research this topic more. https://youtu.be/fwCz8Ris79g