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New Member
Mar 31, 2016
Wellington, NZ
Hi. I posted briefly on another thread about my Fisher stove, in Wellington NZ. probably a Honey Bear - brass cathedral doors.

It never worked properly, and we were going to get rid of it in our renovation, but we have decided to keep it, and see if we can resurrect it. I found quite a large masonry fireplace behind the plasterboards, that was still the same as it had always been - a large open cavity behind and around the fireplace, with a bit of rubble thrown in.

My father in law, who knows more than I, has suggested falling up that huge cavity with sand, to provide some insulation. He says the fire was just heating the air inside the fireplace, and never getting hot enough to generate reasonable convection and heat the room itself.

I have a great deal of broken mortar from the removal of the 1970's feature wall that dominated the lounge, and can insert these broken pieces along with smaller bits, to fill up the area behind the fire insert. There would be no flammable material whatsoever in that area, just concrete and clay brick pieces.

Is this a good idea? Does there need to be a gap behind and around the insert? Is it safe to entirely fill up that area with stone, cement and sand?

A single wall stove and the front of yours heats by radiation. The firebox with air chamber and external shell heats by hot air convection. A blower should push air into the bottom, up the back and out the top. The most heat extracted is from around the exhaust area in the air chamber on the top. The old way of heating was the fire would radiate heat out the front of fireplace as well as absorb into the masonry. This also radiated in all directions including upwards with a lot of heat loss. So now the object is to heat the air from between the firebox and outer shell and get it into the room without heating the mass of masonry. The heat from outer shell radiating rearward should be minimal and is not the problem of low heat output.

** I suspect chimney flue size is your problem. It must be the same diameter as the outlet on appliance to the top. **

The purpose of the chimney flue is not only to get exhaust and smoke out of the building. The important job of the flue is what makes the stove work.
The chimney flue is the engine that drives the stove. Air going into the intakes mixes with flammable gasses coming out of the wood. The more air, the stronger the fire. Like a throttle on an engine. A larger size flue (built for fireplace) allows hot gasses to expand and cool. It is the temperature differential between inside and outside of the flue that causes gasses to rise. They must rise fast enough (called draft) to make a low pressure area inside the flue and firebox. This allows the higher pressure area in the home created by atmospheric air pressure to PUSH into the stove. That is what makes it go. An insulated metal liner inside a larger flue stays hotter inside (and must maintain 250*f. all the way to the top when smoke is present) to create the correct low pressure area (vacuum) and prevent creosote formation in the flue. Below 250*f water vapor from combustion allows smoke particles to stick forming creosote. That is the basics of a chimney and what makes a stove work.

The smaller the firebox and the more efficient the extraction of heat, the less heat you have to heat the chimney. (waste) So with your smaller firebox, the chimney flue size becomes more critical.
Tell me about your chimney. Diameter and height of flue inside. And construction material. I'm sure you'll find the problem is not the stove, it's probably the chimney.
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Thanks. The flue up the chimney appears to be exactly the same size as the outlet. The flue up the chimney looks like a tube made of thin metal , maybe tin? It extends up about 10-12 metres to the top of the chimney. I suspect we simply need a blower. We didn't have one and the deep fire box didn't radiate much heat.

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