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RMH: Specific heat of cobb vs water

Matt Hennek

Joined: Mar 24, 2011
Posts: 2
While I've seen some very ingenious hot water heaters heated by rocket mass stoves (i.e. Nick Ritar's), I haven't seen many people use water as a heat mass.  More people seem to tend to gravitate towards cobb as a heat mass.  I think waters high specific heat and ability to efficiently conduct heat would make it an excellent  heat mass for a rmh, but I'd be a little concerned about it's weight.

Since I'm quite a noob when it comes to RMH's, I have a few questions:

1. What's the specific heat of cobb?  Water has a very high specific heat of 4.18 J/g.  I know this will greatly depend on the clay source and ratio of straw to clay, but does anyone know a general estimate?

2. What's the density (gram per cu cm) of cobb?  I've heard Mr. Wheaton mention a few places that it's quite heavy. 

Joe Pacific

Joined: Mar 02, 2011
Posts: 13
Location: Washington
According to one website, the heat capacity of "sandy clay" is 1381 J/kgC, while "wet mud" is 2512. I doubt anyone has done a formal test for cob. Either way, it's a lot less than water. I can't imagine water would weigh much more than cob, but I wouldn't be the best one to ask. Water would be a superior thermal mass when looking at the numbers, but I imagine dealing with a liquid for a RMH has some heavy disadvantages too.

Kirk Mobert

Joined: Jan 07, 2011
Posts: 136
Location: Point Arena, Ca
Actually, water ain't that great as thermal mass.
It does have a high specific heat, which looks good at first..
The biggest problem with water is that the phase change temperature of water is comparatively VERY low.. You can't get much heat into it before it turns to steam, which is quite dangerous. Also, it tries to get out of whatever you put it in and given enough time, it WILL find a way.
'Course, the advantage is that it can be pumped around.

I don't know what the specific heat of cob is, probably not very high compared with water. The advantages of cob though completely outweigh this one disadvantage. Those being that it can be made into ANY shape, it's SUPER cheap, non flammable, provides decent thermal mass, is very easy to work with and more.

Build it yourself, make it small, occupy it.
Dale Hodgins

Joined: Jul 28, 2011
Posts: 5487
Location: Victoria British Columbia-Canada
I'm in the demolition business and large vessels capable of containing water are a dime a dozen. Big propane tanks, oil tanks and other vessels are commonly cut up for scrap. One of these items could be set in place at the end of a cob bench and  molded into the structure. Heat from the cob would transfer to the vessel without coils or other piping. An open vent to the exterior would deal with the unlikely event of reaching the boiling point. If a clean vessel is used water could be drawn off for bathing and other household uses. If an oil contaminated vessel is used a heat exchanger could warm up well water before it hits the hot water heater. Used oil tanks have little if any market value and would be ideal for this purpose.

Dale's picks - These are some of my favorite threads. Greed - My garden - ethics - Good wood bad wood Alder - Bees - Pulling nails -
Dale Hodgins

Joined: Jul 28, 2011
Posts: 5487
Location: Victoria British Columbia-Canada
    The heat capacity of cob is .2 and this is one fifth the heat capacity of water. Heat capacity is concerned strictly with the weight of a given material and not with its density. Any mix containing, common clays, sands, bricks, rocks and pebbles will come very close to this figure.

    In the discussion called"rocket stove efficiency/certification"I give the details of how this was worked out including the engineering tables. I've also worked out a very clear plan on how to arrive at a reliable efficiency rating based on burning a known weight of fuel inside a rocket stove of known weight. If you have a rocket stove and live on southern Vancouver Island or in Vancouver I would be interested in conducting this efficiency test and publishing the results. A properly conducted a test like this is what building authorities are going to want to see rather than huge amounts of anecdote.
I agree. Here's the link:
subject: RMH: Specific heat of cobb vs water