Improving the efficiency of a finnish contraflow

Hello, this is my first post here on the forum, after lurking for quite some time.

Back in 2022 we had this masonry heater built in our home, (it was built just a month before the energy crisis debacle we had here, so yay for that).
We have been reasonably happy with it so far, I have however been thinking that it probably isn't quite as efficient as it could be.
My observation is as follows: in order to be able to have a clean burn, I need to burn a full chamber of wood, this results in a nice clean hot burn, that mostly burns it free of soot.
However, with such a large amount of wood being burnt over a short time (1-1,5 hours), it seems that the mass simply isn't able to absorb the heat fast enough, we have a cleanout on our insulated chimney about 4 meters up the chimney, and when we do a full burn, the cast iron door gets too hot to touch.

So, why not do a smaller burn? I have tried this many times, but I cannot get it to burn clean, I suspect this is due to the relatively cold mass around the fire absorbing the heat from the small fire, leaving it to burn cold and sooty.

My idea for improvements are either a.:
Add a small heat riser core to the masonry heater.
I have added a drawing of the internal dimensions of the heater, I have no idea if it would be possible to fit anything heatrisery in here, maybe something with a small horizontal riser over the firebox?

Or b.:
Insulate the firebox much like a modern wood stove.

Last winter we used about 12m2 of pine/spruce heating a 120ish m2 house

I would love to hear if anybody has any ideas for improvements. (Preferably ideas that don't require tearing down the heater )

Front view on the left, and side view on the right

I forgot to add the dimensions are in centimeters

Hi Peder;
Welcome to Permies!
Your Finnish heater is very nice looking!
If your chimney cleanout is getting that hot then you are losing too much heat up the chimney.
A properly sized Batchbox with a bell will have exhaust temps of 150F- 250F  (65C-120C)
How warm does your bench get?
Do you know the path that the hot gasses travel inside your stove?
Is your bench hollow like a bell, or does it have channels for the gasses to follow?

Peter van den Berg has developed a new Batchbox core named the Shorty core, the shortest Batchbox currently designed.
With Peter's guidance, I built the first one in the US this spring and we are installing it next month in my home.
Here are several threads, two from me on my build and one long thread from Peter on the development process.
It might fit inside your current heater.

https://permies.com/t/254292/Airframe-Construction-Shorty-Core
https://permies.com/t/254283/Shorty-Core-Montana-Version-burn
https://permies.com/t/234638/Development-compact-batchrocket-core

thomas rubino wrote:Hi Peder;
Welcome to Permies!
Your Finnish heater is very nice looking!
If your chimney cleanout is getting that hot then you are losing too much heat up the chimney.
A properly sized Batchbox with a bell will have exhaust temps of 150F- 250F  (65C-120C)
How warm does your bench get?
Do you know the path that the hot gasses travel inside your stove?
Is your bench hollow like a bell, or does it have channels for the gasses to follow?

Peter van den Burg has developed a new Batchbox core named the Shorty core, the shortest Batchbox currently designed.
With Peter's guidance, I built the first one in the US this spring and we are installing it next month in my home.
Here are several threads, two from me on my build and one long thread from Peter on the development process.
It might fit inside your current heater.

https://permies.com/t/254292/Airframe-Construction-Shorty-Core
https://permies.com/t/254283/Shorty-Core-Montana-Version-burn
https://permies.com/t/234638/Development-compact-batchrocket-core

Thanks for the welcome :) .
I think I have too much heat going up the chimney.
My bench gets warm to the touch but not too hot, the bench is a poorly designed bell that has the intake and outlet right next to each other at the same height, so it has never been the most effective.
The gasses of the stove go up through the bake oven, into a chamber over the oven where it splits in 2 channels that run down the side of the core, down at floor level they reconnect and then go into the bench.

I had been looking at Peters Shorty core when I thought of the idea to retrofit my heater, so I will look through his dimensions to see if it will fit.
I will also be sure to read through your threads.

I have attached a photo of the bench during construction

Hi Peder;
After looking at your drawings and converting them to inches, it appears, you need more room in your bell.
Consider giving up your bench and increasing its height to a traditional bell size.
This way you will capture much more heat from your current stove.
You can still contemplate swapping cores, but it could be in the future rather than as soon as possible.

thomas rubino wrote:Hi Peder;
After looking at your drawings and converting them to inches, it appears, you need more room in your bell.
Consider giving up your bench and increasing its height to a traditional bell size.
This way you will capture much more heat from your current stove.
You can still contemplate swapping cores, but it could be in the future rather than as soon as possible.

Hi Thomas.
I did actually consider removing the bench and building a bell the same height as the heater, but for now we would like to keep the reading nook.
I also need to get permission from our local chimney sweep to rebuild it, so for now I will try to improve efficiency with smaller fires.

I did a quick drawing in solidworks this morning, and since I am already ordering some insulating firebrick for a forge project, I may as well experiment with them in my heater.

The plan is to use firebrick to Insulate the firebox, with an insulating silica board on top of 3 rows of firebrick.  
Here I will leave a gap at the front for the gasses to pas through, where it then passes through a channel that has firebrick on the sides and silica board on top.

This experiment is virtually free since I need them for the forge.

Anyway, it probably won't act like a rocket in any way, but I hope it would raise the temperature of my fire a bit.

The progress photo of the bench looks like the exit is right at the top of the bell cavity. This would allow hot gases to exit immediately instead of lingering and giving up their heat to the mass. If this is true, and you can add baffles to ensure that the bell exit comes from the bottom of the cavity, that could increase your heat capture.

Glenn Herbert wrote:The progress photo of the bench looks like the exit is right at the top of the bell cavity. This would allow hot gases to exit immediately instead of lingering and giving up their heat to the mass. If this is true, and you can add baffles to ensure that the bell exit comes from the bottom of the cavity, that could increase your heat capture.

I think rebuilding the bench would definitely be a good idea, I have been thinking about making it taller and deeper, this would greatly increase the internal volume, and then of course be sure to make the exit lower than the entry.
I think, however, that I will wait with the bench rebuild till next summer, as I am too busy at the moment, and I don't want to tackle this project during the heating season.