A look at my masonry heater

[DennisCA]

Figured I should write a little about masonry heaters. The general principle I gather is well known though, big massive heating place absorbs warmth from fire.

Masonry heaters are very common in Finland and Sweden and other colder countries too, but IMO their development into modernity went furthest in Finland. They are an effective way of using firewood. The principle is simple, the smoke from the fire is led through channels and forced to take the long way around and the heat is absorbed by the mass of the heater.

Mine is a modern heater and was built in 2014 along with the house. It's 1400kg and requierd the floor slab under it to be thicker and reinforced. It's located in the center of the house as well. That is almost always the case here, placing it against an exterior wall would lead to excessive heat loss.

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. 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

As did the lower parts of the chimney

The chimney base was built first

And before I knew it they had slapped up the heater 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 few weeks and 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 I am happy we have it despite having a heat pump.

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 there but this is one of the more modern features of such a heater. It allows fresh air to flow up above the fire without having to pass through the flames which means it has more oxygen. When the fire gets going this means the flue gasses mix with the fresh air and are combusted again. Which equals more energy out of the wood and much less soot and pollution.

Looking into the firebox, the amount of soot here indicates the last fire wasn't optimal...

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.

This picture should illustrate how it works better than I can explain:

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, it's my own and it's free.

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. I close the chimney flue to half at this stage because I have such a bloody good draft in my chimney that if I don't I am loosing a lot of the heat. I don't like to adjust this bit on the intake because I want the fire to get the oxygen it needs. If you constrict the incoming air too much you will get incomplete combustion, resulting in soot and tar products and perhaps condensation in the chimney. Overall bad stuff.

I want a hot clean burn. Hence I adjust the chimney flue to reduce the draft, that way I know the fire gets as much oxygen as it needs. This takes testing and is a personal variable, 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 did two fires per day when it was really cold, now that's warm and hovering around 0C one per fire day is more than enough.

I think that is enough for now. I'd like to write about classical models later. I know mostly about finnish and swedish models.

 

[pedrobedro]

It looks stylish and effective and easy to assemble. Nice write up Dennis.

 

[reb78]

Agree! ^^ Nice looking useful bit of kit. We have loads of firewood. At the moment I have the logburners on and internal doors open and keep the oil boiler off (other than to heat water). My log burners were here when we moved in and are cheap Villager C style stoves so nothing fancy and I am sure using fire as a heat source there must be more efficient options like yours.

 

[gaz1]

actually there not well known

though yours uses a bypass valve to get it going properly

straight up the chimney as it needs to be heated first so it can properly draw before the bypass valve is closed for it to start pulling in the downdraft mode

and its this point yours fails at for firing

nice systems though

heres a few others

but downdraft fire is the better one

 

[DennisCA]

I've never had to use the bypass valve to get a fire going.

 

[DennisCA]

Swedish masonry heaters

Masonry heaters first came from germany to Sweden in the 1400s, these where then quite primitive devices, called "pot ovens". In the 1760s Sweden was having an energy crisis, firewood was getting scarce because swedish mining and industry was using up large amounts of forest in the form of charcoal. The government at the time assigned Carl Johan Cronstedt and Fabian Wrede. They came up with a refined version using channels to lead the smoke through. Over time the tile oven became popularized in sweden, particularly for the wealthy.

The round tile oven is the most recognizable shape today. Most modern ones still look the same, they might use grates and have ash dumps, but the traditional tile ovens did not often have grates, the fire was just put in the fireplace and the draft was regulated with both the flue and the hatches which had small adjustable openings. Andirons designed for tile ovens could be used though.

Most swedish tile ovens use what they call the "five-channel" system where there are five channels total for the smoke, the fireplace itself and the 4 side channels.

A photo of a tile oven under construction, the smoke will go down the two front channels and back up the two rear channels, the chimney exit is on the top. They still need to be closed up yet to get the proper circulation.

Rörspis or "pipe ovens". While tile ovens like the above where used by the upper classes, country folk and less well off used the pipe oven. It works on the same principles but uses no expensive tiles. I think they are quite good looking though.

They where often designed as open fire places because at the time, most people relied on an open fire to get lighting, oil lamps & candles where for the rich. I think if you want an open fireplace, consider the pipe oven and you'll get a lot more out of the firewood. Perhaps the best combo for ambiance and efficiency. I think it could fit nicely into a british home.

They are however commonly found as enclosed heaters as well:

Finnish masonry heaters

In Finland they seemed to develop the masonry heater further than in Sweden, I suspect this is because Sweden industrialized sooner and they moved over to central heating and boilers half a century earlier than in Finland. So the masonry heater was still developed in Finland in the 20th century as a serious heat source for the masses, after it had become seen as obsolete in Sweden.

The modern finnish masonry heater that was built in the largest numbers is known as the "pönttöuuni" or "plåtugn" in swedish, metal oven is the best translation I can come up with but it sounds wrong. It looks like the swedish round tile oven, but it's exterior is sheet metal. Here are a pair of 100 year old pönttöuunis with copper shells:

The idea of using metal instead of tile was first mentioned in 1878 in a swedish patent, but it never seems to have caught n in Sweden, but in Finland it became the norm. Copper was not the norm either, but galvanized sheet metal, often painted or left as is. Sheet metal is cheaper than tile and it does not leak or crack. It's thus a functionally superior material over tile. In addition the sheet metal rings where reusable so once the heater had been worn out (50 - 100> years depending on materials and usage) you would pour water into it to soften the clay mortar and disassemble it, then you could reassemble with new bricks and reuse the rings.

The move to sheet metal shells alone is a genuine improvement over the tile oven but finnish pönttöuunis are also different on the inside. If you note the photos I showed earlier of the tile oven you can see the inside of the fireplace is directly connected to the very exterior of the tile ovens walls, it's a 1-piece design. This is why you cannot fire a tile oven too hard and fast, the thermal expansion and contraction wears on the oven and can cause the ovens tiles to crack and the oven to leak smoke as mentioned earlier.

It was also in 1878 that swedish designer E.A. Wiman, same person that suggested sheet metal instead of tile came up with two changes to the design.

1. The inside of the fireplace is not connected to the outer section except at the bottom. This means it can expand and contract at different rates without causing stress i the structure.

2. The chimney exit is located at the bottom, now the whole structure is free standing and not connected to the wall. It can now move without placing stress at the point where it is connected to the wall.

Finnish masonry heater designer G.E. Asp in the 1940s also made this, which is quite similar to Wimans design

He later updated the design, adding a grate and ash dump as well as a rear air channel for secondary combustion. This is the template for most finnish heaters. In this schematic one can see some bricks are in a different orientation to increase surface area, this was never put into practice that I know of, maybe to a lesser extent, as it made it difficult to sweep.

There's still quite a lot of variation in how they are built. A lot of them still have the chimney exit at the top for instance, despite the drawbacks it also has advantages, since the smoke has a longer path through the heater then.

Since the fireplace was internally free-standing from the exterior and the whole heater could be free standing, the pönttöuuni could be fired a lot harder than a swedish tile oven could, harder and more often.

 

[Rannsachair]

Really interesting, I can feel most of the heat from my woodburner goes up the chimney.

 

[Blobber]

Nice, everyday is a school day! can you get into the various chambers for cleaning or do they not need it?

 

[DennisCA]

There are usually hatches located at particular points so you can clean it out with a flexible sweep.

I'm interested in building something like this later (years away) to put in a greenhouse/orangery we want to build someday.

 

[gaz1]

https://en.wikipedia.org/wiki/Masonry_heater

masonary heaters are really rocket stoves

its been an interesting read

 

[gaz1]

Really interesting, I can feel most of the heat from my woodburner goes up the chimney.

on my rocket stove only the heat needed goes up the flue just to keep the flow of hot gasses moving up the chimney

the heat is directed into the room where you want it not up the chimney which means you burn less wood

with the use of secondary air you burn less wood than a normal rocket stove and a masonary heater

both burn at low flames and burn the gases

 

[gaz1]

There are usually hatches located at particular points so you can clean it out with a flexible sweep.

I'm interested in building something like this later (years away) to put in a greenhouse/orangery we want to build someday.

2 parts here

can you show all the wood you burn by photos for the day burn

for greenhouse reference

but you are far better putting it into water

 

[Gritineye]

One of my wealthier but less practical clients naively imported an old tile stove on a pallet from a relatives house in Italy.

It was in a very sorry state when it got here, mostly fallen apart, it was ever repaired, just sits in a barn.

IMHO chimney liners are the worst thing to happen to wood stoves. We had one fitted after 30+ yrs due to a chimney fire, little heat absorbed by the masonry now and the difference is quite noticeable.

 

[tom2207]

sort of like a wood powered economy 7 heater ,, the 45 gallon barrel heater works the same way , by adding the extra barrels to lengthen the chimney to get extra heat exchange , but the down draft bit is really clever .
Very interesting thread ,,, as Ive a new chimney to make for a heater in a farm workshop , It will now have a u bend in it .

 

[Dev]

Interesting stuff.
When you think of it having the chimney on an external wall really is a bad deign but is so common in the UK.

 

[Lewis_RX8]

Interesting stuff.
When you think of it having the chimney on an external wall really is a bad deign but is so common in the UK.

And simillar idea with radiators if possible keeping them on inside makes it far more efficient

 

[OJCar]

Interesting stuff.
When you think of it having the chimney on an external wall really is a bad deign but is so common in the UK.

Lots of the UK Local Authority housing built from about 1920 till the end of the 1950s addressed that issue.
The 1925 built ex LA semi-dat that I live in has our four chimneys on the party wall between the two semis.
The various LA houses I grew up in all had their chimneys on internal walls.
(They all had parts that they could be improved on mind!)

 

[DennisCA]

with the use of secondary air you burn less wood than a normal rocket stove and a masonary heater

Lots of masonry heaters do have secondary air, depends on the manufacturer. But G.E. Asp's schematic from 1948 shows it using secondary air, as do mine.

 

[DennisCA]

2 parts here

can you show all the wood you burn by photos for the day burn

for greenhouse reference

but you are far better putting it into water

I don't understand what you mean, do you mean show all the firewood used for a day?

If so then this is about how much I would use on a day like this, is it maybe 4-5 kilos of pine wood, if it gets colder like below -10C, I will burn two piles a day like this:

Water is no good if you want to let the system sit over winter.

 

[Barking Mat]

Brought back memories of East Berlin in the early nighties.

Massive big masonry heaters, then it was -20 outside.

Good write up, thanks.