How to size a batchrocket

[Deleted]

Hi everyone, I'd like to share with you a method I use to calculate the proper size for a batchbox :

The first step is to calculate the power of each batchbox, as a function of its internal diameter. By default, the power of a heater is defined as the mean power it delivers on a 24h period considering two fires a day. This power is calculated by considering an overall efficiency of 80%. With this efficiency, the combustion of a kilogram of air-dried wood will deliver 3.7 kWh of energy.

The bigger the internal diameter, the heavier the load of wood for each fire :

internal diameter (mm) vs. wood load (kg) vs. Mean power considering two fires a day (kW)

125 mm — 3.5 kg — 1.1 kW
140 mm — 4.9 kg — 1.5 kW
150 mm — 6.0 kg — 1.9 kW
175 mm — 9.5 kg — 2.9 kW
200 mm — 14.2 kg — 4.4 kW
230 mm — 21.6 kg — 6.7 kW
250 mm — 27.8 kg — 8.6 kW

The second step is to calculate the heat losses of the house (or the room) to be heated, which depends on three factors : the volume of the house, the insulation, and the required difference of temperature between the exterior and the interior.
So the formula is Q = G*V*DT with Q being the heat losses (W), G being the insulation factor, V the volume of the house (m3), and DT the required difference of temperature between the exterior and the interior (°C). G is estimated in comparison to a set of classical values :

- 1.8 for an old, leaky, stone and clay mortar house (classical french farmhouses)

- 1.6 for a house in bricks, stones or breeze blocks without insulation

- 1.4 for a house insulated with 4 cm of polystyrene

- 1.2 for a house insulated with 10 cm of polystyrene

- 0.8 for a recent house with 37 cm thick insulating clay bricks for example

- 0.5 for a strawbale house for example

The third step is the actual calculation. The idea is to calculate the heat losses of the house and then to choose a size of batchbox whose power is superior to the heat losses.
For example, let's take a 60 m2 stone and clay mortar house with 2.5 m under the ceiling. The volume of the house is then 150 m3. There is 20 cm of rockwool insulation under the roof. The joints have been done recently and the joinery is quite airtight. We can then take a coefficient G=1.6. During the winter the temperature floats around 0°C and can occasionnely drop to -5°C for a few days. The heat losses needed to maintain a temperature of 20°C inside the house during these cold periods is Q = 1.6x60x2.5x25 = 6 kW. The adapted batch is then a 230 mm with a power of 6.7 kW.

Of course it is a very simple approximation of the heating requirements of a house, but the advantage of mass heaters is that it is not a problem to oversize them. With cast iron stoves, it becomes much more of a problem because if you oversize your heater, then you will have to make slow, dirty fires in order not to overheat your house. With mass heaters the fire always stays hot and clean, and you just have to light it less often.

I have had coherent results using this method on 5 batch heaters I built last year that are in daily operation now. The very important point is to always OVERSIZE the heater. Of course, it is possible to make three or more fires a day with a mass heater that is not powerful enough, but this will lower the overall efficiency because the mass needs time to deliver the accumulated heat.

[pinhead]

This seems like it should be in the Reference Library.

[peterberg]

Could this be turned into a spreadsheet? That would be a great addition.

[Deleted]

I made it into a spreadsheet but I couldn't add it as an attachment because of the free space of the forum, so here are the links from my server :

in .ods format : webcloud.zaclys.com/index.php/s/xKIO9BI7CNvsNLa
in .xlsx format : webcloud.zaclys.com/index.php/s/Uo6o6kQojBVuxeG
in .pdf format : webcloud.zaclys.com/index.php/s/zVOJFt7MLIrVY6Q

Could it be added to the forum directly ? It would be better than the links.
Could the post be transfered to the reference library ?

thanks !

[ericaus]

That's great information yasintoda. Thanks for taking the time to compile and share it.
Eric

[peterberg]

Hi Yasin,
As far as I know it isn't possible anymore to upload files to the forum directly. At the time, when the other spreadsheet was made there was some space left.

I'd think the whole of this thread should be moved to the Reference Library.
Just to let you know I'll do that tomorrow so you'll know where it went.

[satamax]

Jan 15, 2017 13:14:24 GMT -8 peterberg said:

I'd think the whole of this thread should be moved to the Reference Library.

I would have done it earlier on, when it was first asked for. But i just have the superpowers in the library. Sorry.

[Artur Milicki]

Hi Yasintoda, Peter and others

Since Peter asked me to translate this chapter into Polish i would like to clarify few things and add some contribution from my own experience of building and projecting stoves for heating homes in Poland. Winters in Poland are much colder than in France or Netherlands and we in Poland and Russia have quite good access to technical literature that nicely documents tradition and methodology of building and calculating stoves that are mass heaters meant for heating the houses. Actually since four years now I am building and projecting different kinds of stoves and for me the issue of choosing the proper size of batch box, ISA of the bell and the entire volume of the accumulating mass is the key factor especially when stove is going to be the only heating source of the house.

So I would like to explain the methodology I am using.

STEP ONE
First we need to know the MAXIMAL heat losses of the house or room or rooms that are going to be heated by the stove. In this case average heat losses are not as important as MAXIMAL possible heat losses during the coldest periods of winter. Actually these maximal heat losses occur only once or twice during the winter season and they last from few days up to one or two weeks but this periods are the most important periods when the real exam of the stove takes place, and the stove must be properly projected to heat nicely during those periods. In some regions of Poland minimal winter temperature easily goes down to -30Celcius. In most cases when average heat losses are 2 - 3kW maximal heat loses are 6 - 9 kW. This makes huge difference.

So, how do we calculate MAXIMAL and average (just for comparison) heat losses?

In most cases there is nothing to calculate because usually architectural design of the house contains the chapter called “heat ballance” and both average and maximal heat losses during the winter season are already calculated there. Not only the heat losses of the entire house but heat losses of each and every room in that house are calculated there.

Sometimes it may happen that architectural design of the house does not contain “heat ballance” chapter (some architectural designs of existing old houses may not have it) or there is no architectural design at all. In such cases I always recommend to carefully calculate heat losses with the use of the professional software. There is huge variety of such software's. In Poland there is even one that is open source, available for free, and works online. You can check it here: cieplowlasciwie.pl

Placing all the required data to this software (such like measurements of the house, materials the external walls, floor and roof are build of, thickens of the walls and insulations, amount and sizes of the windows, doors, etc) takes about 15 minutes and after that we get very reliable result that we can work with.

I do not know if similar open source software is available in other languages for other countries, so you have to check it yourself. In case you do not have easy access to such software I strongly recommend to find some architect in your local area who uses such software on daily basics (any architect should use it) and do the proper calculation even if it costs some money.

I definitely do not recommend using simplified methods of calculating heat loses presented in first post because too many important factors are neglected by this method and in many cases it can produce the result that is far away from the truth. Roof insulation maters, floor insulation maters, single, double or triple windows matters, kind of ventilation matters... etc... and all those factors are neglected by this simplified method.

STEP 2
I will post it tomorrow.
I want to do it nicely and writing in English is quite time consuming for me.

[Jura]

Feb 14, 2017 5:05:12 GMT -8 Artur Milicki said:

Hi Yasintoda, Peter and others

Since Peter asked me to translate this chapter into Polish i would like to clarify few things and add some contribution from my own experience of building and projecting stoves for heating homes in Poland. Winters in Poland are much colder than in France or Netherlands and we in Poland and Russia have quite good access to technical literature that nicely documents tradition and methodology of building and calculating stoves that are mass heaters meant for heating the houses. Actually since four years now I am building and projecting different kinds of stoves and for me the issue of choosing the proper size of batch box, ISA of the bell and the entire volume of the accumulating mass is the key factor especially when stove is going to be the only heating source of the house.

Hi Artur! As I also gave a hand in the process of creating PL translation I allow myself to drop my 3 pences:

As an engineer mind I have an inborn propensity to maintain high level of exactnes & paying attention to details, but... the level U propose is IMHO art for the art sake. :-)


If we tend to follow this this way we shall also take too many upredicatable variables of heat source inputs under considerance.

For example the energy inputs source of different human activities.
(activities siting, standing, with one , two hand palm usage that's mostly waht the table presents :-) (No guys! sex in not mentioned in it :-))


 

While most of stoves pundits use the belows one for most of the tasks.




I'm sorry for the tables being placed in PL lng. Short of time today.

[peterberg]

Hmmm... Maybe it's best to let Artur tell his story. Since the online calculator is in Polish only and specificly tailored to Poland and nothing else, only Polish people could benefit from it. What I am curious about is whether the results of both methods are worlds apart or not. Moreover, what the differences might be.

And yes, those tables are pretty worthless on a forum which is exclusively in the English language. I'm afraid it doesn't contribute to making your point any clearer to 99.9% of the people here.

[Jura]

Feb 14, 2017 8:35:58 GMT -8 peterberg said:

Hmmm... Maybe it's best to let Artur tell his story.

Definitely! That's what the forum is meant for
But I like playing a devil's advocate.

Artur knows that . Brain storm begets new ideas

And I must admit we had an opinion echange about the topic before so to be honest I know the story.

Feb 14, 2017 8:35:58 GMT -8 peterberg said:

Since the online calculator is in Polish only and specificly tailored to Poland and nothing else, only Polish people could benefit from it. What I am curious about is whether the results of both methods are worlds apart or not. Moreover, what the differences might be.

This site uses an algorithm that requires fulfilling of ALL of the od parmeters described in the the polish norm of 78 pages
It is tedious and differences are of range of 2-5 % while differences caused by using unstandarised fuel may be of 10%.
The humidity and CO2 content level in the house are of much more of importance in a real life conditions.

Feb 14, 2017 8:35:58 GMT -8 peterberg said:

And yes, those tables are pretty worthless on a forum which is exclusively in the English language. I'm afraid it doesn't contribute to making your point any clearer to 99.9% of the people here.

I will translate them into english to clarify my point of view.

I'd just like to  underline that my opinion about the issue is closer to the OP stance.

And by personall experience of two casses I may confirm the valoue of 0,5 for a strawbale house

[Artur Milicki]

Feb 14, 2017 7:29:24 GMT -8 Jura said:

For example the energy inputs source of different human activities.
(activities siting, standing, with one , two hand palm usage that's mostly waht the table presents :-) (No guys! sex in not mentioned in it :-))

Looks like in case of passive houses there is no need of any mass heaters other than human bodies. In case of coldest winter periods you can always invite few friends and make one week long dancing party to cover the extensive heat losses

Feb 14, 2017 8:35:58 GMT -8 peterberg said:

What I am curious about is whether the results of both methods are worlds apart or not. Moreover, what the differences might be.

OK, so let us make simple exemplary calculation of heat losses using both polish online tool and the simple formula presented in first post:

- external size of our exemplary hous is 6m x 8m
- Walls are made of bricks and are insulated with 10cm polystyrene
- Total thickness of external walls including plasters and polystyrene is 40 cm
- surface of inside area is (6m - 0,8m) x (8m - 0,8m) = 37,44m2
- height of inside rooms is 2,60m
- inside volume is 97,34m3
- let us locate our house in Warsaw area wchich means the projected outside temperature (or average coldest temperature) is -20Celcius
- desired or projected inside temperature is +20Celcius

Now we have all data required to calculate the heat loses with the simple formula Q = G*V*DT

The result is: 1,2*97,34*40 = 4,7kW (maximal possible heat loses during winter season)

Now, let us collect additional data required to get result from polish online calculator:

- Roof is flat - single sloped (or pent roof) insulated also with 10cm polystyrene
- floor insulation is also 10cm polystyrene
- the house has 3 windows 140x180cm (old style double glass)
- house has one brand new metal door
- ventilation is natural

The result is 3,8kW (maximal possible heat loses during winter season)
You can check that resut here: cieplowlasciwie.pl/wynik/27s4

Now let us play little bit more and do tiny changes to some additional data and see what hapens:

change 1) Let us add 10 cm polystyrene to the roof insulation so now we have 20 cm polyestyrene instead of 10
The result now is 3,5kW

change 2) Now let us change all 3 windows from old style double glass to brand new triple glas:
The result now is 3.0kW

Change 3) Now let us go to other direction and take off the entire roof insulation at all.
The result is: 5.7kW

change 4) Now let us take off also the entire floor insulation
The result is: 5,9kW

change 5) Now let us change back new triple glass windows to old style doube glass windows.
The result is: 6,4kW

Now we can clearly see how important are all those so called „details” of heat losses that are neglected by the simple metod of calculation. This is just a tiny house example. The bigger the house the bigger the gaps and changes in resuts may be. Detail in this case really matter.

So I will stress this point again. Proper calculation of heat loses is VERY IMPORTANT factor because heat loses play the most important role in sizing proces of the masonry heaters. I advice anyone who is seriously approaching to this subject to do it carefully with the use of a proper tool.

We do not want neither to overly oversize nor undersize the stove. We want to built the stove that is perfectly fitted to the specific conditions of the location within some specific limits. If the stove is overly oversized it will burn much more wood during the entire winter season than its perfectly fitted brother. If the stove is undersized our house may not be worm enough during the coldest periods of the winter. I will try to clearly explain that later while describing next steps of the methodology.

My time limit of English writing is over today. I will come back here tomorrow. Have a nice day guys.

[Artur Milicki]

STEP TWO

When we have our heat loses calculated nicely, now we can determine the design heating power (DHP) that needs to be emitted by the outer surface of the stove walls.

The formula is as follows: DHP = WCR * HL
WCR - winter condition ratio
HL - maximal heat loses

In order to make our stove nicely tailored for specific location we need to have some basic understanding of the winter conditions our stove will be dealing with.

In most regions of Poland heating seasons lasts from 180 to 200 days from October up to end of April. The average temperature during that season is ranging from -1 to +6 Celcius.

The minimal average temperatures are ranging from -16 to -24 Celcius. And those drops to minimal temperatures appear only sometimes in December, January and February and may last from few days up to one or two weeks.

Since this dramatic drops to minimal winter temperature appear only sometimes and they last just for few days there is absolutely no need to project our stove in such a way that its designed regular heating power (DHP) is equal to or bigger than heat loses. We deal with the lowest temperature just during the 20 days out of 180 or 200 (this is just 10% or 15% of entire heating seson)

As we know both from experience and professional literature masonry heaters are worming the house with the heat stored in their walls and their DHP can be easily increased up to 50% simply by increasing the number of firings in a row or by making the firings more frequent than it is projected.

So if maximal heat loses of our exemplary house are equal to 6kW we can safely make the projected heat emission (DHP) of our stove equal to 4kW. (4kW + 50% = 6kW)

I do not know much about characteristics of heating season in France or Netherlands. I only know that in those parts of Europe you have much warmer winters and probably you do not have such dramatic drops of temperature as we have. If you have smaller difference between average winter temperature and minimal winter temperature and if you have periods of minimal winter temperature much longer than we have (lets say 30 or 40% of entire heating season) than it is reasonable to make the DHP of the stove bit closer to maximal heat loses of the house. But definitely there is no need to make it equal to maximal heat loses.

To make our calculation simpler and uniformed we can use the variable called Winter Condition Ratio (WCR)

If you live in a place of the World where short periods of drastic drops of temperature during the winter season appear occasionally you can set the WCR to 0,67 (example is Poland: WCR = 6kW * 0,67 = 4kW)

If you live in a place without short drastic drops of temperature during the winter season you can set your WCR to 0,84 (example is France: WCR= 6kW * 0,84 = 5kW)

Does it make sens for you?
Did I properly describe the winter characteristics of France or Netherland?

To be continued...

[Deleted]

Hi Artur Milicki ,

Thanks for your input, I value it and understand it perfectly. In fact, I started to calculate the heat losses by a calculation method that was close to yours, but I changed to the simpler method that I proposed here for a few reasons :

- Most of the rocket stoves and the few other batchrockets I saw were built nearly without consideration to the heat losses of the house to be heated, which led to undersized heaters. So I think the new method should not be too complicated or time consuming, else it will simply be ignored by most people.

- I realized that people LIE about their heating requirements -- saying they want 18°C while 22°C is closer to the truth -- and/or that it changes with time, age, children, mood and so on.

- I saw that the load of wood varies a lot depending on the user and the size, the quality (crooked or not), the density of wood that is used.

- Also undersized heaters that are fired more frequently to compensate are less efficient because the mass needs time to release the accumulated heat.

- Last but not least I gave workshop to teach people to build their own batchrocket mass heater. The workshops last 5 days and yet it is never enough. To build a batchrocket is much more than just building the firebox -- the design of the bell and of the heater, the location, the foundations, the masonry, the chimney, the plaster work, the fabrication of the doors.. all this requires a great deal of thoughts, careful planning and technical knowledge. This is a LOT and I think it should not be underestimated.

I think that in-depth knowledge like the one you're sharing is a very precious here on this forum, but on the batchrocket.eu website I think that there is a need for concision and simplicity, because mass heaters are complex enough already for the people who just want to build one for their home.

[esbjornaneer]

Feb 16, 2017 11:48:40 GMT -8 Artur Milicki said:

To be continued...

Hello Arthur, (I hope someone can reach you if you don't see this post)
I want to express how greatly I appreciate your posts on this subject! You are very clear in the way you impart your knowledge. I can understand why you did not continue your explaination but I find it a shame that you didn't! I hope you will take the time to continue at some point. Though this detail and precision may not be as 'sexy' as the pulsing burn of a rocket to most readers of this forum I have to express that it would be great to have your knowledge expressed here.
Hoping to hear more, Esbjorn