What is the best material for a heat bank?

Hi Guys, I keep hearing that water is the best material for storing heat and for using as a heat bank but I don't know why. Can anyone shed any light on this? What would be the difference between heating a ton of cob as opposed to a ton of water? There is a table close to the bottom of this Wikipedia page, but I don't have a clue as to how to interpret the data. Can anyone help with this also?
http://en.wikipedia.org/wiki/Heat_capacity
Thanks.

Eric K. : A late Welcome to Permies.com, our sister site Richsoil.com, and a Big welcome to the Rocket and Wood stoves Forum threads !

This is a very good question which can be divided into many subsections !

There is a reason that the study of heat Capacity (and the separate item heat transfer ) Is generally called Thermo- god damn- ics !

The chart listed gives water a score of ! (and a bit ) and every thing with a lower #, holds less heat-
every thing with a higher # holds more heat !

'Pure' cob is a mixture of sand and clay, and is actually insulating, as we want to store heat, we use dense, heavy materials from our
environment, field stones, or chunks of urbanite ( locally recycled chunks of slab concrete ) buried close to the Core of the Horizontal
Chimney within your thermal mass !

Water is a very good Heat Exchanger, as well as having a great capacity to hold heat ! Therefor it is a nearly perfect Material to use to
store and transfer heat ! HOWEVER

A well made Rocket Mass Heater will easily produce a stream of hot exhaust gases with Temperatures of 1800 degrees F. This is well
above the point that liquid Water flashes to steam 212 dF ! Here at Permies we Call this BANG - SQISH ! Think Boston Marathon Bombing
with more causualties and more 2nd and 3rd degree full body Burns !

Even if you Had a Plumber Mason corectly make/instail a Rocket Mass Heater RMH, for you, to safely operate an RMH with Water for a
Thermal mass, You will need to educate yourself to at least the equivalent of a Associates degree in H.E.V.A.C. Technology, to Know that
you can opperate this combination of heating Heat Storage Tehnologies!

This is not a system that you go a way for a weekend and leave My brother-in-law incharge of !

There are much easier systems to run, please consider going to rocketstoves.com , and download your PDF Copy of Rocket Mass
Heaters , after you have read thru this book you can come back here and be sure that you are using a commom core of Words to
discuss the Size, Shape and Orientation of the RMHs parts to each other, and themselves !

If there is an RMH in your future, we will help you find it ! For the good of the Craft ! Big AL

water stores the most heat per lb or volume at human household temperatures and is cheap. This is why it is used in greenhouses and high-mass passive solar houses. The problem is it does't work at high temperatures. Rocket stoves use cob and rock/rubble because it is cheap, handles high temps, and is slow to release the heat (time-release).

allen lumley wrote:Eric K. : A late Welcome to Permies.com, our sister site Richsoil.com, and a Big welcome to the Rocket and Wood stoves Forum threads !

This is a very good question which can be divided into many subsections !

There is a reason that the study of heat Capacity (and the separate item heat transfer ) Is generally called Thermo- god damn- ics !

The chart listed gives water a score of ! (and a bit ) and every thing with a lower #, holds less heat-
every thing with a higher # holds more heat !

'Pure' cob is a mixture of sand and clay, and is actually insulating, as we want to store heat, we use dense, heavy materials from our
environment, field stones, or chunks of urbanite ( locally recycled chunks of slab concrete ) buried close to the Core of the Horizontal
Chimney within your thermal mass !

Water is a very good Heat Exchanger, as well as having a great capacity to hold heat ! Therefor it is a nearly perfect Material to use to
store and transfer heat ! HOWEVER

A well made Rocket Mass Heater will easily produce a stream of hot exhaust gases with Temperatures of 1800 degrees F. This is well
above the point that liquid Water flashes to steam 212 dF ! Here at Permies we Call this BANG - SQISH ! Think Boston Marathon Bombing
with more causualties and more 2nd and 3rd degree full body Burns !

Even if you Had a Plumber Mason corectly make/instail a Rocket Mass Heater RMH, for you, to safely operate an RMH with Water for a
Thermal mass, You will need to educate yourself to at least the equivalent of a Associates degree in H.E.V.A.C. Technology, to Know that
you can opperate this combination of heating Heat Storage Tehnologies!

This is not a system that you go a way for a weekend and leave My brother-in-law incharge of !

There are much easier systems to run, please consider going to rocketstoves.com , and download your PDF Copy of Rocket Mass
Heaters , after you have read thru this book you can come back here and be sure that you are using a commom core of Words to
discuss the Size, Shape and Orientation of the RMHs parts to each other, and themselves !

If there is an RMH in your future, we will help you find it ! For the good of the Craft ! Big AL

Hello Allen, Thanks for the welcome and also for the detailed reply. I understand the concerns with using water for the heat bank, but there must be relatively simple maths to work out what volume of water would be needed (given a certain size RMH and also the type of wood being burnt) to estimate the maximum achievable water temperature. Take the example of a burning candle. The flame from this may well bring a thimble full of water to the boil, but will not be able to boil a cup of water. There must be some sort of equation to relate a RMH output to the volume and surface area of a water storage vessel. This of course would be a vented system with safety pressure and temperature relief valves in place. Probably a sacrificial anode as well. I do still like this idea. What do you think? I'll have to Google H.E.V.A.C. technology. I have no idea what that is. In fact I'll do it right now!!
thanks again.

R Scott wrote:water stores the most heat per lb or volume at human household temperatures and is cheap. This is why it is used in greenhouses and high-mass passive solar houses. The problem is it does't work at high temperatures. Rocket stoves use cob and rock/rubble because it is cheap, handles high temps, and is slow to release the heat (time-release).

Hello R Scott, thanks for the reply. Do you know if the heat capacity is by volume or by mass. I think cob would have about three times the mass of water, so would this mean to achieve the same results as using cob, we would need to use three times the volume of water to make up the equivalent mass? Is it all about density?
thanks again.

Hi Guys, I just found this chart at dragon heaters all about heat storage of various materials. Seems water is the top of the list for both volume and mass.

Here's the link to the page on Dragon Heaters' website which has that chart - Burning Wood – Thermal Mass Material Selection.

We like to ensure that we always provide proper attribution.

Here is the thread from last year with better explanations than I can articulate: https://permies.com/t/18864/wood-burning-stoves/thermal-mass-materials

So, as a great conductor, aluminum is too expensive to use as a large thermal mass. It sparked an idea for RMHs that use aggregate around the exhaust tubes ... maybe this is already being done. But, wouldn't fins on the exhaust tubes running the length of the tubes and extending out a couple of inches into the aggregate be an enhancement of the conduction? Would they be even better if the tubes and/or fins were aluminum?

The best compromise between price and performance on that chart is fireclay brick. The type being referred to in the chart is a dense fireclay brick weighing 7-9 pounds for a standard size brick. Or, you can acquire some castable refractory which has the same characteristics as dense fireclay brick. With fireclay brick, you don't have to worry about exploding water or spalling concrete.

If money is not a concern for you, choose soapstone; it is better looking.