What Are The Best Materials To Use As Thermal Mass In Your Rocket Mass Heater?

Hi everyone, I'm Liam and I'm nearly ready to start my RMH build but want to make sure I get the most out of my build. I'll be heating my bungalow style house with the RMH located in the centre of the building. Id like the mass to absorb as much heat as safely possible and store it for as long as possible!

At the moment I'm thinking of using 25L water containers side by side for the majority of the mass,filling gaps and finishing with cob. Does this sound like a good idea to you experienced folk?

Kindest regards

Liam

Welcome to Permies!

I am not one of our RMH tinkerers here on the site but we have quite a few!

I hope these links might help you get some ideas that might help you out.

https://permies.com/t/224692/heating-plan

This is a link to our Thermal Mass Forum - https://permies.com/f/504/

Hope to see you in the threads!

Hi Liam;
Welcome to Permies!
Well, water indeed is one of the best at heat-holding.
However, it has some drawbacks.
The most well-known is it turning to steam and exploding.
In your case, I think you might have more issues with evaporation and or contaminated stinky water after a while.

So some questions?
What are your thoughts on how you were planning to use those water tanks?
Are they vertical and refillable? Or were you thinking of laying them flat?
What style and size RMH are you planning?  A  J-Tube with a piped mass? Or a batchbox with a bell?

Difficult question without a lot more info, are you planning a massive 12” batch box or a 6” J tube?
Have you considered a bell or are you thinking about a piped bench?
Water has some potential but dense brick or vibrated concrete will possibly be an easier option.
Can you tell us more about your ideas…..

I'm based in Portugal and here you can get 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...



Or this...



It's generally cheaper than graded stuff, and packs down beautifully. I think it will work better than sand or gravel as it has the mass of the solid stone but with sandy stuff filling in the gaps. It would also be easier to remove than cob if any maintenance needed to happen. I'm not sure what it's called in other countries or if anyone else has tried it and could report.

Hi Burra;
That is a wonderful backfill but less than optimum as a mass.
The larger the stones the better and the sand gravel needs to be left out as they are an insulator.
It sure would work if that was all you have but I would not use it in a mass myself.
Large dense stones with soft clay mud packed into any air pocket make a much better heat holder.

Liam Alderdice wrote:Hi everyone, I'm Liam and I'm nearly ready to start my RMH build but want to make sure I get the most out of my build. I'll be heating my bungalow style house with the RMH located in the centre of the building. Id like the mass to absorb as much heat as safely possible and store it for as long as possible!

At the moment I'm thinking of using 25L water containers side by side for the majority of the mass,filling gaps and finishing with cob. Does this sound like a good idea to you experienced folk?

Kindest regards

Liam

Besides the safety factor, 25L of water will only hold/store 6875 BTU's if you using a base temp of 75 with 200 degree top  I did not do the corresponding  in C but it is not much.  3-4 pounds of wood or less than 2 K  So if your house is of any size...??  So the "as much" and "for as long" will be disappointing. The money used for the containers, could buy bricks?   And you have fire bricks in your budget too?

Fox James wrote:Difficult question without a lot more info, are you planning a massive 12” batch box or a 6” J tube?
Have you considered a bell or are you thinking about a piped bench?
Water has some potential but dense brick or vibrated concrete will possibly be an easier option.
Can you tell us more about your ideas…..

Hey Fox,

I'll be building a 6inch piped bench system. It will be located in my lounge/dining area 6.2m x 5.8m ceilings are 2.4m all walls have 100mm rockwool insulation and 12mm plasterboard.

I'd like to be able to heat 2 bedrooms and kitchen (all in eye sight of the RMH) I'd like add radiators to these rooms that will be heated from a water tank placed on top of the barrel.

The bench/sofa will be an L shape 2m seat and 3m seat.

I will take some photos tomorrow to give y'all more insight into my vision 🤙💚

thomas rubino wrote:Hi Liam;
Welcome to Permies!
Well, water indeed is one of the best at heat-holding.
However, it has some drawbacks.
The most well-known is it turning to steam and exploding.
In your case, I think you might have more issues with evaporation and or contaminated stinky water after a while.

So some questions?
What are your thoughts on how you were planning to use those water tanks?
Are they vertical and refillable? Or were you thinking of laying them flat?
What style and size RMH are you planning?  A  J-Tube with a piped mass? Or a batchbox with a bell?

Hey Thomas,

I will be building a 6-in batch box tubed bench design. I was thinking of using the water containers upright so they are strong and flat on the ground. I did think of making it so I could refill the water but that might lead to more hassle in the long run.

The only reason I had the idea of using water is because I read about well it conducts and stores heat, I wanted to put it past you guys before stormed ahead. I'm open to any advice you may have!

It will be located in my lounge/dining area 6.2m x 5.8m ceilings are 2.4m all walls have 100mm rockwool insulation and 12mm plasterboard.

I'd like to be able to heat 2 bedrooms and kitchen (all in eye sight of the RMH) I'd like add radiators to these rooms that will be heated from a water tank placed on top of the barrel.

The bench/sofa will be an L shape 2m seat and 3m seat.

I will take some photos tomorrow to give y'all more insight into my vision 🤙💚

Liam Alderdice wrote:
Hey Thomas,

I will be building a 6-in batch box tubed bench design. I was thinking of using the water containers upright so they are strong and flat on the ground. I did think of making it so I could refill the water but that might lead to more hassle in the long run.

💚

I presume you know that for safety sakes, your water set up would have to be open for steam prevention reasons. And if your system is open, you will always be faced with evaporation, And with a 6" system, depending on your water location, (steam would be evident, right quick---  so refilling will be a must,  not sure if you meant using water will be more hassle, or refilling the water???

I have used water in various amounts, and I would not consider it a slow release of heat, but that depends a bit on the radiator system.  You never said, again if the 25L was a typo, or was your actual intended amount.  25L is not much for heat storage.

Ok, so I think your idea would be to use 25l plastic containers alongside the piped bench or somehow incorporated  in the bench?
Personally  I would study how a bell works rather than a piped option and depending on the look of your house a bell may look a lot better than a barrel.
 

The mass of the bench itself in a ducted cob bench stores the heat pretty well for most situations, I believe. You don't say what climate you are in, but take a look at the second video in Paul's post here: https://permies.com/t/231359/evidence-rocket-mass-heaters-efficient#2012171. Andrew is based in Caithness, Scotland and has a relatively small bench in his living room that is also exposed into the bathroom, but he claims it heats his house comfortably now. He also has a great build video that is worth a watch if you are thinking of doing something similar. In short, I don't think you need extra mass if you have access to good quality clay.
As regards heating water on the barrel and circulating it round the bedrooms. Please do take care! It is easy to have a serious explosion when water and heat mix! In a bungalow you don't have the option of gravity circulating, so will be reliant more on electric pump to move the water in the circuit. I have seen something done in Devon - Sagara was setting up a radiant floor system on one of his Rocket mass heaters ( https://www.youtube.com/watch?v=DR5sv0r8-lw), but it all looked very complicated to me! One of Paul's videos has an underfloor ducted system, which looked interesting, (https://www.youtube.com/watch?v=6lUCOowOmJ8) the flue ducting and firebox is set into the floor rather than a bench. Unfortunately I can't find a follow up video to see how it worked - the downside again is that involves digging up your whole floor, which may not be an option!
I think a combination of radiant heat (situating the heater in the wall between rooms like Andrew) and air circulation (ducting or fans to circulate heated air around) would be simpler and a lot less expensive. In Erica and Ernies book (which I recommend if you haven't got a copy) there are several discussions of layout that may be of help.

Fox James wrote:Ok, so I think your idea would be to use 25l plastic containers alongside the piped bench or somehow incorporated  in the bench?
Personally  I would study how a bell works rather than a piped option and depending on the look of your house a bell may look a lot better than a barrel.
 

If I was to use the water containers they would just be used as mass next to the flu that's been covered with cob. After hearing everyone's advice, I think I'll use other conductive materials for the bench. Do you recommend anything other than cob?

I have already purchased the flue so It's a bit late now and I also love the barrel aesthetic.

Nancy Reading wrote:The mass of the bench itself in a ducted cob bench stores the heat pretty well for most situations, I believe. You don't say what climate you are in, but take a look at the second video in Paul's post here: https://permies.com/t/231359/evidence-rocket-mass-heaters-efficient#2012171. Andrew is based in Caithness, Scotland and has a relatively small bench in his living room that is also exposed into the bathroom, but he claims it heats his house comfortably now. He also has a great build video that is worth a watch if you are thinking of doing something similar. In short, I don't think you need extra mass if you have access to good quality clay.
As regards heating water on the barrel and circulating it round the bedrooms. Please do take care! It is easy to have a serious explosion when water and heat mix! In a bungalow you don't have the option of gravity circulating, so will be reliant more on electric pump to move the water in the circuit. I have seen something done in Devon - Sagara was setting up a radiant floor system on one of his Rocket mass heaters ( https://www.youtube.com/watch?v=DR5sv0r8-lw), but it all looked very complicated to me! One of Paul's videos has an underfloor ducted system, which looked interesting, (https://www.youtube.com/watch?v=6lUCOowOmJ8) the flue ducting and firebox is set into the floor rather than a bench. Unfortunately I can't find a follow up video to see how it worked - the downside again is that involves digging up your whole floor, which may not be an option!
I think a combination of radiant heat (situating the heater in the wall between rooms like Andrew) and air circulation (ducting or fans to circulate heated air around) would be simpler and a lot less expensive. In Erica and Ernies book (which I recommend if you haven't got a copy) there are several discussions of layout that may be of help.

Hey Nancy, I actually went to visit Sagara and he gave me the idea! I'll be building this with my dad who is a qualified plumber. He visited Sagara too and thinks we can create the system safely.

Thanks for the other links. They've been helpful

Scott Weinberg wrote:

Liam Alderdice wrote:
Hey Thomas,

I will be building a 6-in batch box tubed bench design. I was thinking of using the water containers upright so they are strong and flat on the ground. I did think of making it so I could refill the water but that might lead to more hassle in the long run.

💚

I presume you know that for safety sakes, your water set up would have to be open for steam prevention reasons. And if your system is open, you will always be faced with evaporation, And with a 6" system, depending on your water location, (steam would be evident, right quick---  so refilling will be a must,  not sure if you meant using water will be more hassle, or refilling the water???

I have used water in various amounts, and I would not consider it a slow release of heat, but that depends a bit on the radiator system.  You never said, again if the 25L was a typo, or was your actual intended amount.  25L is not much for heat storage.

Apologies for not being clear. No, I was thinking about using the 25 l water containers side by side throughout the bench coming to the total of 22 containers (550L).

Because I am on a suspended floor, I'd like to keep the weight down if possible (I will be adding extra support posts where the heater will be). If this is going to jeopardize the performance of the RMH I will build a retaining wall and pour a concrete base underneath the floor boards.

I am open to suggestions about what mass to use. Water was just the first thing that came to my head 👍

You might want to look at the pebble style rocket mass heater which I believe was designed to go on a suspended floor and therefore be lower weight. Rex Holiday posted some pictures of his build of it here

Nancy Reading wrote:You might want to look at the pebble style rocket mass heater which I believe was designed to go on a suspended floor and therefore be lower weight. Rex Holiday posted some pictures of his build of it here

I don't want to beat a dead horse, but.........if you have less mass, you will have LESS heat holding ability.  With the exception of soap stone (source and cost issues)

I did originally read the 25 L part wrong, as I did not read it as multiples of containers, but the figure would remain on BTU's stored per container  

But back to pebbles (if they weigh  less, they will hold less, not much getting around that,

I have about 4500 lbs in a 4' x 5' single bell, And I am adding to that all the time.  If I store more, I can heat more.   Some of the mass is well above 200 degrees, at base it is about 30-40 degrees less, from those that I have talked to, this would be about normal.  I can make it hotter yet at the cost of hotter flue temps.

Here are a few photos of the room and ceiling. I have laid the flu out in the rough shape I'd like the bench to be so you can get an idea.

Liam Alderdice wrote:Here are a few photos of the room and ceiling. I have laid the flu out in the rough shape I'd like the bench to be so you can get an idea.

On the other side of the unfinished stud wall are 2 bedrooms I plan to heat with radiators heated by the RMH.

Id like to heat the kitchen and living room with the heat stored in the mass of the bench and heat radiated from the barrel.

😂 I AM AWARE THAT BARREL IS PLASTIC, JUST ONE I HAD TO HAND TO GUAGE THE SIZE 😂