Weight constraints exerted by an RMH on the first floor

Hi Guys.

I'm just discovering the incredible potential of RMH by browsing this forum. Thank you for all the information you made available! I am planning to put one on my 140 sq meter 2 floors house.

Before digging into technical specs, I have a preliminary question about the weight that the RMH and its cob bench can exert on the concrete slab: my living room is on the first floor. However, a total RMH weight of 2-3 tons is often reported, for a floor area of ​​approximately 3m2 (32 sq ft). This is 850 kg/m2 (170 pounds per sqf), while the standard in Italy is to build concrete slabs that can theoretically support 250 kg/m2 (50 pounds per sqft).

While browsing the forum, I did not find any topic mentioning this constraint, except in the case of people who have a wooden floor, which is not my case.

Is it because this type of configuration (placing the RMH on the first floor) is automatically a bad idea and thus, the concern does not emerge in the questions, or is it because a concrete slab can in fact support much more than what is announced? In other words, is it a NO-GO for the RMH project in the configuration of my house or not?

I specify that I cannot put the RMH on the ground floor, because it is not a living room (it is a kind of basement with technical room, but not buried. Particular configuration probably due to our terraced land which pushed the previous owner to build the house in this way I guess). An image can help understanding:

Ground floor: tecnical room
First windows: living room + kitchen
Second floor: bedrooms

I know, there are some missing shutters. I'm in the process of rebuilding them


The entrance of the house is made laterally. On the left side, I guess the house is constructed on plain ground. On the right side, it is the living room.

Thank you all for your enlightenment!

All the best

Remy

PS: recently bought the farm. The entrance will evolve progressively, definitely too much concrete!

Remy,

It would be difficult for you to determine to what specification the ceiling/floor slab has been made. In masonry buildings a standard practice for solving the problem of placing a heavy heater in the corner of the room was inserting 2 or 3 heavy double T-bars in the walls adjacent to the corner. The bars would be parallel to the diagonal of the room. Rebar cage could be tied to the T-bars and a concrete slab poured. If I find more details in one of my books, I will share them with you.

Do not worry about the missing shutter (I noticed it). The house has a great potential and can be wonderfully finished.

After edit:

I have found it.
Diagonal beam is double T-bar, 10-16 cm high.
At least 27 cm length of this beam should be seated in the wall on the left and right side. The cavities should be filled with mortar - compatible with the wall mortar.
6 perpendicular beams are also single T-bars 10 cm high, placed upside down. The distance between them should correspond to the brick size that will be put on shiner (flat) on these bars, so in case of popular Italian size it would be 27 cm (1 cm steel thickness, 1 cm left for joints).
Then a layer of concrete is spread over the bricks to create the slab (dashed rectangle).

In case when the heater is located not in the corner of the room, then 2-3 beams would have to be positioned on entire span of the room - it would make it much more expensive.

Or build a wall in the lower room to provide the same support, or columns.

Hi guys and thank you for your ideas! This morning I checked the house configuration and yes, ground floord and first floor (where the RMH will be) match perfectly in terms of measures. so I think the easiest should be to build a few columns downstairs as suggested by John.

Edit: Sorry guys I'm a bit new to the forum, I have other questions related to a RMH-heated multi-floor house (not on weight stuff), can I ask here or it's better if I open a new topic? Thank you!

Column will give you more freedom in selecting heater location. I would build 4 of them, 40x40cm each using concrete blocks with vertical rebars and grout them solid. Most of Italy is seismic, but even if it was not - this is a good practice. Please make sure that they rest on some reinforced slab, not on the ground or not reinforced floor.
I just re-noticed where the chimneys are located, so probably this is the part of the house where you will be building.
You can ask more questions here, so everything related to your project will be located in one place.

Hi Remy;
Support columns are a good idea.
Have you chosen a build design yet?  
A J-Tube with piped mass?
A J-Tube into a stratification chamber? (Bell)
A Batchbox into a bell?
Single wall bell or double wall bell?
All these choices will weigh differently and spread the weight over different areas of size.

Yes, you could continue with this thread.
However, I would start a new thread for your next questions.

Here is why: Your current thread addresses a very common question from prospective builders.
Any new questions you ask may be just as important, but they will be buried under a thread header of weight restrictions.

So, any perspective builder will bypass this thread, looking for one that covers their question on, say, the transition area (another common worry).


 

Hi Cristobal,

Yes I'm in seismic zone. I take note for columns specs and measurements, thank you. Floor is reinforced already in technical room at ground level.

The 2 chimneys you see:
- On the left, is the chimney coming from the living room, where a conventional, small wood stove is connected to as of today. I will precisely use this one and replace the wood stove with the RMH there.
- On the right (small one): this one is the exhaust of a modern wood boiler located in the technical room, connected to a closed-circuit hydraulic heating system (2000L hot water tank, pump, expansion vessel, radiators in all rooms of the house) that ensure house heating + domestic hot water. The system is quite sophisticated and built with top quality materials, etc.,  but in the end, what is the result? 9 tons (I think approx 25 cords) of wood consumption/year for a 1500 sq. ft house in USDA zone 6a/b (poorly insulated I must admit), for a house that barely struggles to get to 63°F and that gets colder at night.

Hence my thoughts:
- 1. I need to insulate the house correctly, and the process has begun (but that will make another post in the appropriate section)
- 2. My so-called modern heating system, which surely pleases “conventional” plumbers, in fact generates no radiant heat and struggles to bring the house up to a stable temperature and dehumidify it (we get 80 inches of rain a year here). So RMH makes so much sense! But my house has constraints:
-> 2 floors
-> Quite small, separated spaces (5 bedroom, living room+ kitchen and 2 bathrooms).

What could be the weak points of RMH in my configuration? I noted:
- Difficulty heating upstairs and remote rooms
- How can I produce hot water? At present, my wood boiler system is energy-intensive but it works well. Electricity is expensive here, so switching to this for hot water once i have the RMH is not an option.

Browsing the forum, I think the ideal would be to opt, in my case, for a Batch RMH + satellite upstairs for heat, coupled with a water heating system on the RMH barrel, connected to my current circuit. I manage to find more information on RMH + hot water systems in the European forums. I was wondering whether in the USA hydraulic heating of rooms by radiators was a common thing? In fact, these systems can both produce domestic hot water and heat radiators in remote rooms from an RMH.
In Italy also, connecting a traditional wood stove to the radiator hydraulic system of the house is quite common.

Lots of questions, but one thing I'm sure of: my actual config is not energy efficient at all!

Remy,

I would recommend to start the journey with estimating the heat loss of your house:

https://kalk.pro/en/heating/heat-loss-calculator/

I like this calculator, because it has a list of various masonry building materials that can be assigned any thickness.
It would help if you provided a plan for each floor.
With masonry interior walls, the placement of the heater is crucial.

Remy Fils wrote:I manage to find more information on RMH + hot water systems in the European forums. I was wondering whether in the USA hydraulic heating of rooms by radiators was a common thing?

I think thin wood framed versus heavy masonry interior walls is the reason. Masonry wall will create a denser barrier for the heat (radiant or not). That's why houses in Europe have/had separate masonry heater per each room. Or recently they may use radiators with water connected to the masonry heater. I think that masonry heater that heats water is a more recent thing. Central heating systems that heat water are usually dedicated to heating water only and are placed in a basement.

As a Civil Engineer, I am confident 100mm steel columns will work.
Have a base plate at the bottom to spread the load around, say 300 x 300mm x 10mm thick.
Using 100mm diameter pipe will ensure the L/ R ratio will ensure the column is not too slender and would buckle.
A steel or timber beam at the top will spread the protection across the floor.
I consider Cristobal's suggestion of 4 / 400 mm brick columns overkill

Hi Cristobal,

Too bad I'm not there at the moment to take all measurements. I'll ask some people that are there now to help me on this. I keep you informed! Thank you for your help!!

John C Daley wrote:As a Civil Engineer, I am confident 100mm steel columns will work.
Have a base plate at the bottom to spread the load around, say 300 x 300mm x 10mm thick.
Using 100mm diameter pipe will ensure the L/ R ratio will ensure the column is not too slender.
A steel or timber beam at the top will spread the protection across the floor.
I consider Cristobal's suggestion of 4 / 400 mm brick columns overkill

Thank you for your input John, much appreciated!

Cristobal, so I understand that a good proportion of houses in the rural US have thin wood framed interior walls? That would explain a lot of things! On my side, when the small wood stove is fully heating the living-room, you can get a comfy 70°F but when you pass the living room door, it can easily go down to 60°F just in the room next door so what you say makes complete sense.
And what you are telling on historical evolution of Italian rural houses is true: my house had individual wood stoves until I think, 2012, when the old owner installed the centralized wood stove system in the basement, that heated hot water both for tap hot water and for newly installed radiators that ensured house heating. But what you can see is that this system is not adapted to my humid conditions and mold tends to develop quite fast on walls if you don't respect strict window opening rituals everyday. So I really think that RMH will change the house, not only for temperature, but also for air quality and interior humidity.

As you were saying, the RMH positioning will be key. Dunno if it helps, I tried to sketch the house interior. Only the livable parts. Under living room, as I was saying, there is the ground floor with technical room + cheese laboratory. Over the second floor (over the 4 rooms), there is lost roof space that I will insulate this summer. Please note that it's not at scale! Some specifications:
- All rooms are separated by 40 cm concrete walls
- A key element of the house are the stairs. It is an open point of connexion between floor 1 and floor 2. So there is not clear separation between the 2 floors, which can allow hot air to go upstairs, provided there is air circulation.
- All rooms and livable spaces are closed by doors (that we can open but not all the time for privacy reasons)
- I did put, in the sketch, the RMH in the living room. Maybe a work is to be done on satellite thermal masses to allow heat to go on other parts of the house.


As soon as I have the possibility, I make the measurements for the heat loss calculator.

As an image is worth a thousand words, here are some photos to help understanding the house:

What you see when you enter the house. Right door gives access to living-room + kitchen; Left to room 1 and bathroom. And the stairs could give the possibility to hot air to go up.


Some views of the kitchen + living room. The RMH would replace the small wood stove, extending its heat mass towards right.



What you see when you arrive upstairs. This corridor is separated to each room by a door. So the heat would eventually arrive upstairs but then wouldn't enter in each individual room + bathroom 2.


Our actual wood boiler + 2000 liter how water tank for house heating:

thomas rubino wrote:Hi Remy;
Support columns are a good idea.
Have you chosen a build design yet?  
A J-Tube with piped mass?
A J-Tube into a stratification chamber? (Bell)
A Batchbox into a bell?
Single wall bell or double wall bell?
All these choices will weigh differently and spread the weight over different areas of size.

Yes, you could continue with this thread.
However, I would start a new thread for your next questions.

Here is why: Your current thread addresses a very common question from prospective builders.
Any new questions you ask may be just as important, but they will be buried under a thread header of weight restrictions.

So, any perspective builder will bypass this thread, looking for one that covers their question on, say, the transition area (another common worry).


 

Hi Thomas!

Sorry I just saw I missed your answer. Meanwhile, I continued posting...I will rename the title, for it to include also the other thematics.

I still have to keep on doing research on RMHs, but from what I've read, batchbox is the better option for my house configuration. Single or double wall bell, I still don't know at the moment!

Cristobal Cristo wrote:Remy,

I would recommend to start the journey with estimating the heat loss of your house:

https://kalk.pro/en/heating/heat-loss-calculator/

I like this calculator, because it has a list of various masonry building materials that can be assigned any thickness.
It would help if you provided a plan for each floor.
With masonry interior walls, the placement of the heater is crucial.

Cristobal,

Just made the calculation with all the measures taken. We obtain this:


If needed, I can try to share the input data.

Please note that this summer I will insulate the ceiling and the floors, which should increase house thermic performance by 40%.

I would say there is no free dinner.

1. Your calculated heat loss of 31 kW is quite large.
To compensate it let's use 3 230 mm batch box heaters. Each of them can generate (with twice firing a day) 6.7 kW. 6.7 kW * 3 = 20.1 kW.
Assuming 50% power increase with additional firings in the coldest days we can get to 20.1 kW * 1.5 = 30.15 kW.
So we have total 3 * 2 = 6 firings per day which equals 21.6 kg * 6 = 130 kg of wood per day.
If you need to heat for 100 days it gives you the total amount of 12 tons of wood which is close to 9 tons you use currently. I'm assuming that in the calculator you selected greater temperature delta than you current situation, so this would be the reason for 12 vs 9 tons.

2. Heat is transferred in 3 ways:
-conduction: hot object is touching a cold object - it usually does not happen with masonry heaters, unless it touches the wall or has a bench and a person sits on this bench
-convection: rising hot air, it usually ends up by the ceiling and would need fans to dissipate it
-radiation: the main one and easily blocked by solid barriers - especially by 40 cm thick masonry walls

The house in which I lived had solid masonry 60 cm outer walls and 40 and thicker for interior. We had five heaters/stoves: three for heating and two for cooking. Coal fired, kachel covered. When the living room heater was hot it made the room very warm, but the hallway stayed cold and the rooms on the other side of the hallway got nothing. We also had central heating with radiators. No heat ever penetrated the second floor if the first floor was warm. It's possible that eventually some heat would trickle to other rooms if heater was running every day, but we never tried it.

3. Insulation would lower your heat loss and this is what I would start with. It's quite possible that your heater is very efficient - all devices in Europe have to be very efficient, because Europe has to import hydrocarbons. It may turn out that you will not save much wood at all after significant material and labor investment.
I would recommend to insulate the house first, no matter if you will use the current system or masonry heaters. Then you would see how much wood was saved.

Some correction for US readers:
You mentioned 9 tons. 1 cord is 3.62 m3. Assuming that it's dried oak with average specific gravity of 0.7 it would amount to 2.5 tons if packed with no voids. If voids were 30% then it would be 1.9 tons so your yearly use is 9/1.9 = 4.7 cords.

I would start with insulating. The loft/attic first. Exterior walls will be more difficult to insulate. I would also measure your central heater's exhaust gas temperature at the chimney inlet. If after determining that the temperature is low enough for good efficiency and after insulating you could be much happier temperature-wise and use less fuel.

If the efficiency is not satisfying, you could:

-still build a heater in the most used kitchen/living room and turn off the radiators there
-you could build this heater with taller bell that would penetrate the ceiling and heat kitchen/living room AND bathroom 2/room 4 and then you could turn off the radiators in bathroom 2 and room 4
-you could build a second heater on the other side of the house either in bathroom 1 (penetrating to room 3) - to have toasty baths or in the room 1 crossing with the bell/bench to bathroom 1 and still penetrating the ceiling to heat room 3

I don't know if you central heater could be run at half or less power and still be efficient. If this was the case then closing some radiators in the rooms where masonry heater was used could result in higher total fuel use (less efficient operation of the heater + masonry heater).

Cristobal,

you said:

"Your calculated heat loss of 31 kW is quite large"

I agree.
I plugged in reasonable values for my house, heated entirely by electric heat pump, and got impossible numbers. Way higher than physically possible, given the capacity of my heat pump; and far higher than the observed usage.

I suspect user error, or a calculator that is not suited to USA building styles, or just a poor model of heat loss. I wouldn't rely solely on that heat loss calculator to design a system.

B Beeson,

For my adobe house with delta 20 C I'm getting 4 kW which is in the same ballpark as the results from other calculators I have used. I like this calculator, because it list a lot of masonry materials and any thickness can be selected so it's good for custom builds.
However if I add the floor with not precise "floor over unheated room" I'm getting 11 kW and I don't know if the calculator assumes that the "unheated room" under the floor has the same minimum temperature as the outside air (I selected 0 C). I have a slab on grade and the soil temperature at this depth would be around 10 C (50 F).
I agree that 31 kW is very high.

Remy,

Could you provide the input data that you have used for the calculator?

It definitely treats the unheated space under the floor as if it is equal to outside temp. I adjusted by calculating separately for the actual crawlspace temp to get the "heat loss through floor", then the true outside temp for the walls, ceiling, windows, etc.

Another limitation is that it treats horizontal, vertical up, and vertical down heat losses exactly the same, but they are very different. Heat rises, its really, really good at going up, so attic insulation and sealing are most important for winter heat loss.

Within walls, preventing horizontal convective loops and sealing gaps are more important than straight R-values. Obviously doesn't apply to thick adobe or masonry walls as you and Remy have.

These are good points. I just used another calculator and got around 5 kW. If Remy provides the input data we will see what could have gone wrong.

Hi guys!

Here it is: https://jmp.sh/s/HvO2r8Y4CnpNZOjsnOVf

It's even worse, last calculations give a 35 kW result.

And yeah I did put "unheated" both for the room up the ceiling and under the main floor.

In any case, what Cristobal said is right. Before building the RMH, i may insulate correctly the house and see what happens in terms of wood consumption with my actual system. Oh, i forgot to mention that I have a solar panel for hot water that covers our hot water needs for whole summer and for sunny  spring days.  Question about main floor insulation: is it a good idea to put thermal insulation + wood floor on living room, knowing that I will probably destroy part of it to put my RMH there?

Remy,

For the area have you used the area of exterior walls?
Are the exterior walls over a meter thick (42")?
Are the concrete or hollow blocks or bricks?
You put 35" for the ceiling thickness. Is it some conversion error? The same for the floor.
Please provide house dimensions: length, width and height, and I will play with it myself.

I can see the reason for the high heat loss. If I type the data with roof insulation of 250 mm roof for my house I'm getting 4.1 kW but if I remove the insulation it jumps to 32 kW. Other calculator that I'm using increases the heat loss two times with no insulation. If you provide the data I will plug it in and check.

Cristobal Cristo wrote:Remy,
For the area have you used the area of exterior walls?

Yes

Cristobal Cristo wrote:Remy,
Are the exterior walls over a meter thick (42")?

Conversion error...Used metric system. Let me change it

Cristobal Cristo wrote:
Are the concrete or hollow blocks or bricks?

I guess hollow blocks. No bricks for sure.

Cristobal Cristo wrote:
You put 35" for the ceiling thickness. Is it some conversion error? The same for the floor.

My mistake

Cristobal Cristo wrote:
Please provide house dimensions: length, width and height, and I will play with it myself.

Ok let me get back to you shortly. Thank you!

Edit: correcting metric system error gave me even worse numbers: 70 kW heat loss!

For insulation, maybe I should put in the calculator what my intentions are (on roof and main floor short term), instead of how is the house today (uninsulated)..?

Dimensions:
Floor surface: 737 square feet
Height: 19 ft
I know floor surface, but not exact length and width (I'm not there at the moment to take the measures). So to calculate wall surface, I took square (737)*19*3+square (737)*19*0.3. I didn't multiply *4 walls because the fourth wall is adjacent to another wall (barn's wall), so it's not fully exposed to outside temp. To take in account this, I considered 1/3 of the heat loss of a normal wall instead of 1.

Thank you again for your kind help.

Remy, your inputs helped to clarify some major problems with this calculator.

I put in your numbers and got similar results (combining my house dimensions with your materials and thicknesses).

I added a second layer of 8 inch styrofoam on all surfaces, just to see how much that would improve things.... NO improvement!

The second layer option is completely non functional. This explains why I had such unrealistic results the first time. I had 1/2" sheathing plus standard 3.5" mineral wool in the wall cavities, and 1/2" drywall plus 8" mineral wool fill in the attic, but the calculator ignores the insulation layers. Reversing the order, insulation in the first layer, I got very good results. The 2nd layer doesn't matter at all. Replacing the 1/2" drywall with another 36" of insulation = identical numbers.

...........

A separate issue...  in your inputs, you have 35" of concrete in the attic, and 40" of concrete in the lower floor. Is this accurate? You have zero insulation above your top floor ceiling? If so, that is your first priority. Also, in a seismic zone, concrete ceilings, unless massively reinforced and engineered for shearing forces, are a death trap, so I agree with Cristobal that we must have some communication/conversion issues. I'd be happy to switch to metric if that helps.

..........

I see that Cristobal has typed faster than me, but the point about the non-functional 2nd layer is important.

Remy,

I have plugged it to a more serious tool:

RemysHouse

I did not know exact location so I just clicked somewhere north of Firenze.
I have assumed the house perimeter to be 10x7 m and the exterior walls to be 40 cm. Room heights 2.6 m. I have used 18 C as desired inside temperature. Average heat loss 11.1 kW with the daily average of -1.8 C and max heat loss 19.3 kW if the outside was -14.7 C.
I can make corrections if needed.

In this case two 230 mm BBR heaters would provide 13 kW with two firings and it would raise to around 20 kW with more frequent fires in coldest winter days. The wood usage would be 21.6 kg * 4 = 86 kg/day so in 100 days it would amount to almost 9 tons. If you insulated the attic with 25 cm mineral wool, the values would drop to 9.2/15.9 kW.