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McMullin & Co
Florence Diemont


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Heat exchanger: yes or no?

We're always inclined to caution people who want to install a heat exchanger in their wood stove, and because of this, on this point we're not especially popular with the enterprising do-it-yourself types. For years, furnace manufactures of every variety have been providing the wrong examples of wood-burning stoves that you can connect with a couple of cv-convectors! But it worked, right? Sure it did, you had to burn lots of extra wood, but indeed the furnace and convector did get hot! And as long as there was an open expansion vat, steam could be discharged without difficulty. So what are we complaining about, then?

Precisely this: Iron stoves with iron heat exchangers are made for burning coal, not for burning wood. If you burn wood in an iron stove, you take too much heat away from the wood fire, and consequently the wood burns at too low a temperature. The result: a lot of soot, a lot of smoke, and the increased risk of a chimney fire. And, moreover, the heating potential of the wood you're burning is less than half that of what is attainable when you burn properly dried wood – i.e. wood that has been dried in the open air for at least six months.

So, is it absolutely impossible to use a heat exchanger with an authentic wood stove or even with a stone oven or tile stove? We were able to find a solution, because over the years we've learned a lot about heat walls and the possibilities they offer. But, here, we are now talking about wood, as well as gas, fire, and we will be using our Finn-oven as an example.

Firstly, you must forget about cv-convectors as spreaders of heat! Acceptable models must – if they hope to completely heat the air – function with water temperatures ranging from 70° to 90° C. People who wish to use their tile stoves or brick ovens for conveying heat elsewhere must make do with lower (water) temperatures, of a maximum 40° C, and, thus, a heat wall of a few square meters (feet) is sufficient for heating a bedroom or study.
A simple heat exchanger is installed in a tile stove, such as a Finn-oven or standard tile stove, and is put in a place inside where there is no longer a fire and where the temperature of the smoke gases is only 200° to 300° C.



The illustrated design is based on our Finn-oven; however, the idea is so simple that it can be implemented using other sizes and (naturally, as far as possible outside the fire zone) it can be also be implemented in other tile stoves, and then, for instance, horizontally. To ensure good water circulation, we use a cv-pump. We can use an open expansion vat, or a closed vat with a safety valve.

For the heat exchanger, use a 15mm diameter copper pipe. It's recommended that you make the curved piping from one solid piece of pipe, and you must work with bricks that have pre-fabricated grooves in them. By doing so, you will then only need to bend the outer rings slightly apart from each other in order to put the two grooved bricks in between and to fit the pipes into the grooves.
If you prefer to work with individual pieces of curved pipe, then you must solder the joints with silver-solder and use stone blocks with drilled holes, or cast the two grooved bricks yourself, using fire-concrete. In the Finn-oven, conveyance through the rear wall is easiest (and also the least ugly) way of installing the heat exchanger.