Smaller mass using sand?

Hi everyone - fist post here

So I have converted a camping stove to an indoor stove by beefing it up and improving efficiency. Now theres a ton of energy going out the pipe, and once the fire is out, it becomes cold damn fast. The room is 6 sqm and in Sweden, so it's damn cold outside.

If I let the pipes instead go through 20kg of sand, how long would that sand radiate and how much energy would he stored? Like would 20kg of mass of sand be enough for the night? Sand from what I have learnt is a bit special in that 1kg if sand will go from 40 to 20C over five hours at room temperature. So it's supposed to be excellent at storing heat for longer periods.

But I can't find any calculations on what adding 20kg of sand would practically store. I saw one guy on YT adding like 100kg sand in a steel canister and he managed to take diesel heater gasses from super hot to almost cool when it exited his house. Thus at least making the heater way more efficient, since the heat stays in the room.

Since you guys are doing rocket mass heaters... How would you calculate the need? Will 20 or 40kg make any difference in a small 6 sqm room over a night?

My plan was just getting a big steel bucket and run the pipe straight through it, hopefully storing a lot of that exhausted heat in the sand. But the dream would be to be able to keep the room cozy over the night when sleeping.

Cheers

the problem with sand is that there’s a lot of air in between all the sand grains, which acts as an insulator. there’s mass there but it’s little bits, hardly touching each other. something more one-piece is a much better mass.

This post suggests a type of unsorted gravel mix:

un-sorted gravel and stones and sand mix called tout venant, which is French for all that comes. It usually gets shortened to something like tuvna here. It's quite variable, but it generally looks something like this...

https://permies.com/t/235474/Materials-Thermal-Mass-Rocket-Mass#2156653

So basically I'm looking more for a mix of stones, preferably se many big as possible and then some sand in between to fill it out?

Still wonder if it would last more than an hour or two.

Maikel,

Instead of doing any calculations I would do the following.
Take 20 l PLASTIC bucket (not metal, to retard the heat transfer), fill with boiling water a and close it tightly. 20 kg of water will have around 4 times more energy stored than the same weight of sand heated to the same temperature. If this keeps you warm over the night then you will have a starting point for experiments with sand.

For quick and very rough calculation (without considering building type, room height, insulation, temperature required, etc): if the 150 mm batchbox masonry heater with a bell that weighs 1000 kg may heat, let's say 40 m2 room then 1000 kg/40 m2 = 25 kg/m2, so in case of your 6 m2 room, the required weight would be 150 kg of masonry material.