RMH materials question

New to the forum but have been doing much reading and research.

Here is the question....if money was not an object, what materials would you use to build your RMH?

After test builds in my pole barn, i plan on installing a RMH in the house but dont want a 55 gal drum or cob benches in my Arts & Crafts style house. I have a locally found antique copper and cast boilers that are similar in size to drum. I plan on covering the bench in 1/2" soapstone slabs for decoration and mass. I also have a great source for scrap aluminum and wanted to use that as the bulk of the thermal mass.

I guess the biggest concern would be the aluminum. Would this work?

What would your wishlist be for materials?

Randy, I spent 20 years working in an aluminum foundry. My gut reaction is that aluminum will not retain heat in the way you hope. It will heat quickly, but it will transfer that heat to the surrounding air/mass much quicker than an equal amount of iron/steel.

I don't know, but it sounds like your RMH will be beautiful! I hope you'll post pictures of it when it's finished!

Kathleen

The heat capacity of aluminum is .21 which is about the same as that of cob. Greater density could be achieved by using heavy rock such as granite There's an online resource called the engineering toolbox where you can search out heat capacities for various materials. All values are based on that of water which is 1.00 . Aluminum is very good at moving heat around as it is an excellent conductor. I plan to use a small amount of recycled aluminum extrusion for that purpose. I'll be moving heat to water tanks and hot tubs.

I'm not sure what you're trying to achieve with the use of so much aluminum. It would certainly move the heat to the soapstone quickly but at an exceptional materials costs.

Water can store much more heat for a given volume and you have the advantage of being able to place the containers empty and fill them later. You would also have the option of drawing off water for various purposes. Of course you need to ensure that the system never freezes. If a very large amount of aluminum is used it may move heat out of storage faster than you want it to move.

In some ofnthe reading i have done itnstatednthat aluminum had a specific heat of .90 and a heat capacity of 90. Granted i know very little about it, just seemed like the numbers supported using alum particularly for the weight savings.

Another reason for its use would be its heat sink capabilities in the summer, drawing heat out of the room.

I also thought that the aluminum would allow greater design flexibility (i.e. Raising the bench of the floor) and possibly reducing cubic footage needed compared to cob.

Currently we heat our 1900 sq ft house, solely with a relatively small soapstone box stove so i figured 5 ton of cob was a little overkill.

You're probably looking at numbers for speed of heat transference. 1 pound of cob will store just as much heat as 1 pound of aluminum will store. Aluminum and copper are right up at the top of materials which transfer heat well. That's why you see them in radiators.

Having most of your heater made of this will reduce storage time so that you'll be getting heat from storage immediately after or during a burn cycle. The whole idea of using cob is because it's nearly free and the heat will be delivered over a longer period as needed.

The specific heat of aluminum is definitely .21--------------------------------------------------------------------------------------------------------------------------------------------------------- I did a little math a few months ago concerning heat storage in cob and in water. Here it is------------------------------------------------------------------------------------------------------------------------------------ Here are some very useful figures for anyone who is trying to choose whether or not to include a water tank within an RMH. Cob weighs 95 pounds per cubic foot. Water weighs 62 pounds per cubic foot. The heat capacity of cob is .2 which is 1/5 that of water which has a capacity of 1.00 So supposing we want to build a RMH which occupies 100 ft.³ of space.

First the cob - 100x95 equals 9500 pounds. 100 ft.³ of cob will weigh 9500 pounds. 9500x.2 equals 1900. So our cob bench has the same heat capacity as 1900 pounds of water.

Water weighs 62 pounds per cubic foot, therefore the tank containing 100 ft.³ of water weighs 6200 pounds

6200 divided by 1900 equals 3.26

A given volume of water can store 3.26 times as much heat as the same volume of cob.

It is true that the cob bench could be heated to temperatures far beyond the boiling point of water. But in order to store the same amount of energy as water at 200°F, a cob bench would have to be heated to more than 650°F. This is not common practice and if it were it would result in lowered efficiency with higher exhaust temperatures and badly burned bums Water stores much more heat at temperatures which are practical and safe. And because heat transfers through a body of water through convection, heat being absorbed by the thermal mass will be available in short order. If some lag time is desired the tank could be cobbed over.

Some may be worried about the danger of steam explosion. A water tank which has an open vent to the exterior is no more dangerous than a rattlesnake on TV . It's easy enough to monitor the temperature and allow the fire to burn out before the boiling point is reached.

It would be a shame to have a giant water tank like this for thermal storage only. Water could be drawn off to heat a hot tub and for regular domestic uses. For those who don't want to do any fiddling a tank could be placed into a cob bench which would be a pre-heater on the way to the hot water tank. During the heating season your hot water tank would receive preheated water. During the summer when the heater is not in use cold well water would absorb heat from the thermal mass. This would have a mild air conditioning effect and the water would enter the hot water tank at a higher temperature.

Good info thank you.

Randy, if the scrap you have available is aluminum plate , you could fashion that into a storage tank for water. The aluminum will heat rapidly and transfer that heat to the water supply.

Dale, thanks for having run the numbers on cob vs. water. We are still planning for a build next year so new ideas still have time to percolate through.

Randy Acton wrote:New to the forum but have been doing much reading and research.

Here is the question....if money was not an object, what materials would you use to build your RMH?

If money and space were no object... I would build a masonry heater.... probably a Teplushka with built in cook top. (an oven is already there) Brick and soapstone seem to be the materials of choice. However, refractory concrete can make things go together much faster. Brick and cob and refractory mortar are in many ways all the same stuff... CEBs work quite well as mass too. There are all brick RMHs out there, both square and round... cob and barrels were from day one a cost cutting feature. Other pluses (and minuses) have shown up through use.