Blacksmith Artisan and Inset Stove Design Part 1

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How an Inset Stove

Should Work - part 1

Some concern about the functioning of a friends Blacksmith Artisan inset stove has sparked this series of posts concerning the design, efficiency, and function of inset stoves generally.

Above is a diagram of a typical inset stove. An inset stove is essentially two boxes - one inside the other. 

TWO BOXES

(1) THE INNER OR FIRE BOX

The inner box is the "Fire Box" in which the fire burns. It usually has two air supplies, a primary air supply which allows air to come in underneath the fire, and a secondary air supply which allows air in above the fire. In some designs the secondary air is arranged so as it acts as an air wash to keep the door glass cleaner by wafting the combustion fumes away from the glass.

(2) THE OUTER BOX

The outer casing forms the "Convection Heating System". It is absolutely vital that the air in the "Convection System"  DOES NOT MIX IN ANY WAY with the air used for combustion. If it does mix, at the least it will cause the stove to be very inefficient, and at the worst, it may allow noxious fumes into the room. 

Possible causes which might allow the two air flows to mix are:

(1) a crack in the casing, or 

(2) poorly sealed joints, or 

(3) bad design.

WARNING 

COMBUSTION AIR AND CONVECTION AIR 

SHOULD NOT MIX

The Vital and Distinguishing Part of an Inset Stove

The distinguishing and vital part of an inset stove - and clearly different from a free-standing stove, is the outer "Box" or "convection airway" which causes air to circulate in and around the back, sides, and top of the inner box or "Fire Box" - thus carrying the heat from these surfaces out into the room.. 

Without a properly functioning convection system, an inset stove would be no better than a simple door on a fireplace, as the heat from the back, sides and top of the stove would simply be lost up the chimney..

TWO SEPERATE AIR SYSTEMS

How should the air flow systems work in an "Inset Stove"?  

First point to remember is that there are -  2 -  TWO DISTINCT AIR FLOW SYSTEMS to consider. The FIRST rule is that these two air flows should not mix.

I will write further on this very soon. Look out for the next post on this subject.

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Blacksmith Stove Review Part 4 - Alert -

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Possible Problems with the

Blacksmith Artisan

I have been doing some close up checking of an installed Blacksmith “Artisan” inset stove. I recon that there are fundamental problems with the level of heat output, and with the hot air convection from the back and sides of the stove

There would appear to be no convection of heat from anything other than the exposed front of the stove. How do I come to that conclusion?

In a normal freestanding stove, heat is dispersed from the back, the sides, and the top of the stove. In the Artisan inset, there are no sides, back or top. The only exposed surface is the front.

If the heat from the sides, back and top of the stove is to be used in the house and not dispersed up the chimney, it would have to be gathered, by air convection, from the hot surfaces of the fire box.

One of the Convection Air intake vents seen on the right  side

Looking at the stove, it would appear to have the necessary features for this convection air flow to happen. It has two air intake slots at floor level either side of the frame which roughly 8 x 4 cm.

Hot Air exit grill on top?? - The Air-Wash damper knob on the right?

It has a large grill, in the frame above the door, which runs the full width of the stove. I would assume that this grill is an exit grill for the hot convection air to get into the room.

Here is the dilemma: 

Air is drawn into both of the intake slots - I checked this with a sheet of paper held near the slots, and sure enough, there is a good flow of air which sucks the paper onto the opening.

In the course of things, you would expect this air flow to be heated around the sides and back of the stove and then to exit as hot air through the slotted vent at the top, wouldn’t you?

But here lies the core of the problem. Instead of an outflow of hot air from the upper grills, air is being sucked in causing a cooling effect. I checked this simply by placing a length of tissue across the top of the grill - it is clearly being sucked in - instead of being gently blown out by the hot air convection.

The airwash damper does not affect the air flow - nor should it - it should have a seperate air way.  So where is all of this air from the room going? What is this telling us?

What appears to be happening is that air is being sucked in both at the top grill, where hot air should be coming out,  and at the bottom air intake slots. This air is effectively cooling the front of the stove and reducing the heat from the one radiant surface.

Poor Heat Output from this stove

The manufacturer rates the stove output at 5 Kw. I have no means of measuring this in any scientific way - but let me say this much, it is well under 3 Kw of heat when compared to a 3 Kw electric heater.

Now, I am hoping that this case might be just a one off problem, and hopefully it does not represent a problem of poor design.

Contacted Blacksmith Stoves

I contacted Blacksmith and had a callback from one of their senior reps. however, he could not directly answer the technical questions, did not seem to understand the principle of the convection process, or of the need for the combustion air and convection air to be completely separate. I was left no wiser by the call.

I have asked for some technical diagrams of the Artisan stove which might clearly show the layout of the airways. I am hoping to have these early next week, and that they will clarify things a bit.

If the convection airways are incorrectly designed and unable to gather heat properly from the back and sides of the firebox, the efficiency of this stove could be no greater than 30% to 40% rather than the 70% claimed. The heat output to the room based on what I experienced is no-where remotely close to the 5 Kw quoted.

I will also request a copy of the independent labs test results, especially the details of the criteria used to determining mean output and fuel to room transfer efficiency.

Best Wait and See

We, that is myself and the owners of the Artisan stoves, will await some technical clarification and assistance, and detailed diagrams and instructions, and I will inform this blog of the outcome.

In the meantime - Caveat Emptor (buyer beware) applies. Personally, if I were considering a purchase or installation of the Blacksmith “Artisan” stove, I would postpone until the design, heat output, and efficiency can be fully assessed.

I will post information as it becomes available.


ADDENDUM:
Added 11th January 2012

I have since received a copy of the standards certification cert of the Artisan stove along with a set of technical drawings showing an exploded view of the stove from the people at Blacksmith. It all looks 100% kosher. As far as I can tell from the drawings, all the bits are there and in the right places.

I acknowledge that I have only closely observed one installation and that any inefficiencies may well be associated only with this one stove and installation.


The bottom line however, as far as this one installation goes, is that the output of this stove is substantially less than a similar size of stove, the Blacksmith  "Anvil", which I have no hesitation in recommending.  I would however put the output of heat from the inset "Artisan" as somewhat disappointing.

I would also point out that I have not so far been able to compare the Artisan inset against similar stoves by different manufacturers.

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