Date: Sun, 17 Aug 2008
From: "Lars Helbro" <lars(at)stenovne.dk>
To: <mheat(at)heatkit.com>
Subject: About cracking in the heaterface

Dear Norbert
 
We have been in contact before, but it was years ago.
 
I read with big interest the debate at MHA´s site about cracks.
Here in Denmark, we almost only build single-skin heaters, mainly because of the weather conditions.
 
Of course we experience cracks too, but through the years (about 20) we have found ways to minimize them,
that are not mentioned in the discussion, and maybe it could be useful to some.
 
The way we build our "traditional" heaters is sligtly different from Heikki´s version in the 80´s.
Most of it you can see in Bujnas report from our work-shop in Hungary this summer:
http://www.mha-net.org/docs/v8n2/docs/080704bujna.htm
(By the way - the "mysterious" holes in the bakeoven is explained here on my own site:
http://www.stenovne.dk/englishcentrifugalcarburator.html )
 
About the cracks.
Why they come: As one (i think Alex) said in the discussion, the brickwork is not heated up equal.
On a contraflow heater, the sidewalls will be heated up first, and from the inside.
On a single skin heater, this will happen quite rapidly.
When the inside is heated up faster, than the outside, the walls will try to bend, since the inside will expand faster than the outside.
The forces are away from the middle, and since there very often is less brickwork in the face than the back, the cracks will occur here.
(This is only a theory, but it corresponds with natural laws)
 
What we have experienced that help us:

We only use clay mortar, since the cracks will be very easy to repair for the customers, and we teach them how to do that.
I very much agree with Alex, that it is important to teach the customers what might happen, and how to maintain their stove. (A little water and a thumb will often be enough, when it is done in time).
Most important here though - the quality of the sand in the mortar has a great effect on how big, for how long,  the cracks will be. With very good quality sand, they might not come at all !
The importance of the quality is, how sharp the sand is and the mix of different sizes.
We use 0 - 4mm in a (if possible) reasonable equal amount of every size.
If the small sizes is missing (0-2mm) we have to use more clay, and be aware of the risk for drying cracks.
If the bigger sizes (2-4mm) is missing, the risk of crackings are bigger, and will come back more often after repair.
Round sand particles (e.g. from the beach), will make troubles for the customer for the life of the stove.
 
Many years ago i built a masonry heater for my grandaughter in Norway. The sand was sharp as broken glass, and my fingers where bleeding before i finished.
This stove has been used very heavily ever since, but has never shown any signs of cracks.
I think, that sharp sand has about the same effect, as metal reinforcement, but it will stay there forever.
 
About the castable quality:
As you maybe can see at the pictures of Bujnas, we have given up upon this product.
From our experience, it only makes trouble.
After we quit that (about 8 years ago) we could fire up every single stove we built immediately after it had been closed. This has given us much more experience with every single stove, and we can keep warm when we make the finish if it is in the winter time. (In the summer it can get quite hot, but we do it anyway - of course). The customers are very pleased with that, since there is no bigger trouble, than new toys you are not allowed to play with immediately.
 
I think, that this also has some importance for the decreasing of cracks.
When we fire up a new stove, the mortar is still very wet, so the stresses can much better be absorbed in this period, that often last for a couple of days with a reasonable fire each day.
Like children, they can learn more easily what its all about, when they are soft and easy to bend :-)
 
I have never experienced any trouble with the core, and i think that this correspond very well with Heikki´s experience with soapstone, castable and firebricks.
(4 glowing firebricks in 90ltr. of cold water in a mortar pan makes very nice nice bathing water in about 3 minutes :-) 
Nice to know when you are on a dirty job with no bathing facilities.
 

Best
Lars
 
www.stenovne.dk
Horskærvejen 4 Drejø
5700 Svendborg
tlf.: (+045) 23 23 93 39
 
email: lars@stenovne.dk