Finnish masonry heater of modern design

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New Member
Jan 3, 2022
Figured I should write a little about my "masonry" heater. The general principle I think is known though, big massive heating place absorbs warmth from fire. They're very common where I live and have been for centuries.

Mine is a modern heater and was built in 2014 along with the house, it's 1400kg. It's located in the center of the house to avoid heat loss. That's typical for houses here, fireplaces are located near the center most often.

It is not actually constructed from masonry though, it's made from pre-cast parts that is made from crushed olivine rock and high temperature mortar (and perhaps other things too). It's a very heavy and heat resistant element compared to bricks. Still I think the classic brick heaters are still good and fine.

Construction, 2014

It arrived in parts on several pallets. I had to carry all the parts inside, some pieces were too heavy for me to lift.

The lower parts of the chimney also arrived in pieces, it's a double pipe chimney. On the other side it will connect to another stove in the sauna.

The chimney base was built first (the top part is steel):

And before I knew it the masons had slapped up the heater while I was at work and I wasn't able to take pictures of it in progress. I would've liked to have seen it go up in parts.


Anyway after that it was allowed to sit for a couple of weeks to dry out before I made the first fire, which was small.

Since then we've used it every winter to get additional heat for the house. And with todays electricity prices in europe I am happy we have it. Our main heat source is a ground loop heat pump and hydronic floor heating.

After 7 years of use

Now here's a closer look after years of regular usage

Note the "slit" in front of the grate, it's a bit clogged with ash but I cleaned it out afterwards. It allows fresh air to flow up infront of the fire without having to pass through the flames, this allows for secondary combustion of flue gasses. I always considered this to be a modern improvement, but I've found mentions of it back to the 1930s in finnish stoves, such as the Porin-Matti which in the 1940s boasted an 86% efficiency rating.


Looking into the firebox, the amount of soot here indicates the last fire wasn't optimal, probably too small a load, usually the walls are mostly clean after a fire, since the soot burns off once it gets hot enough.


Looking upwards, there is a bit of a constriction to help the air mix with the flue gasses better. The flue gasses then pass down on the sides of the heater and then back up another passage to the chimney. Typical 5-channel layout that's been standard in swedish designs since the 19th century.


Here's a useful schematic if my explanation wasn't so good:

How I fire the heater

This here is a typical load of firewood, about what I can carry in one arm. It's just pine and fir now, it's from my own property and it's free. I have some birch firewood too that I bought, it's a lot nicer and gives a lot more heat.


All the wood is oriented the same way and the fire is lit from above:

After 10-15 minutes the fire is going full bore with this dry wood. I close the chimney flue to half at this stage because I have such a good draft in my chimney that if I don't the fire burns too fast and too much is lost out the chimney. This takes testing and is a personal variable that has a lot to do with your houses peculiarities, but shutting it to half is a good rule for my house. It'll burn down in an hour like this, I shut the flue to 2/3rds when it's just coals and then close it when there are no more embers.


After that the heater will take 2-3 hours to get to maximum temperature and it will be warm all night and into the next day. I do two fires per day when it's cold.



Minister of Fire
Nov 18, 2018
Downeast Maine
What a beautiful and efficient fireplace! That is one of the more "compact" masonry/mass heaters I've seen, despite the considerable 1400 kg weight.


Minister of Fire
Nov 19, 2005
NW Wisconsin
Nice! How large of home are you heating with this? What kind of room temps do you see?


New Member
Jan 3, 2022
Nice! How large of home are you heating with this? What kind of room temps do you see?

Our home is 137m2 (actual living space 121m2), that's 1474 / 1302 square feet.

Last night indoor temps varied from 18.5 C in the morning to 19.7 C in the evening. I had one fire going last night and it was still warm this morning. The living room was noticeably warmer than the bedroom thanks to it.
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Burning Hunk
Oct 28, 2020
Central VA
Thanks for this post. If I ever do a major remodel, I am going to see if I can get a masonry heater.
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Staff member
I'd love 1 too! That is definately one of the smaller ones I've seen! You said birch gives out more heat. So, you feel more heat or less heat with individual fuel charges? I wouldn't have expected that due to all of the thermal mass and relatively small fires.
Is your heater on a slab on grade , or did you need to construct footings. 1400 kg is on the lighter side for a masonry heater but still, that’s well, 1400kg.
I like the clean look of it. Looks like a quality product.