Proper Air Control & Throat Flue Damper Operation for an old Fisher Insert

[DrJabberwocky]

Hello!

New member / First time poster.

First off, thank you for your website - it is full of excellent advice and has been a tremendous resource to me!

I have a question about best practice / most efficient use of the air controls and flue dampers on my “old” Fisher wood stove insert.

But before I get to it - first, please know that I am a renter and my landlord won’t upgrade / replace my stove. I can make modifications/enhancements on my own but from my own pocket, so only low-cost things.

Furthermore, my apologies for the long post…I have a done a lot of internet research on this topic but have had a really hard time finding consistent, useful information as most websites and forum posters do not specify what type of wood stove they have, discuss their operating environment and/or goals, or define their terminology (people seem to use the word “damper” for everything and rarely give indication of what they’re referring to).

So, to avoid this confusion, some background information:

I have an “old” Fisher, wood-burning stove that has been inserted (I presume) into an old, brick fireplace/hearth with straight external chimney. The stove is a long rectangle that is 50/50 coming out of the old, fireplace box with the other half inside the old fireplace box. The old brick fireplace box has been framed/lined with metal inside the box - back, sides, top and bottom - with an ~2” air gap between the lining and the Fisher stove above and to the sides of the Fisher stove. Looking at the face/front of the hearth, it is framed with ~ 12” wide, metal sheet on both sides of the stove and above it, covering the old brick / surrounding the stove when looking at it (less the air gap on top).

The Fisher stove itself has two twist knob, air control “dampers” - one on each door (neither door has gaskets) - and a chain-pull, circular, throat flue damper that, when open, visibly swings down into the Fisher stove-box. This throat damper is connected to/part of a 2”, metal, cylinder (flue?) that connects the Fisher stove-box to the top metal fireplace lining, bridging the (visible/accessible) air gap that runs between the top of the Fisher stove and the metal lining inside the old fireplace.

Above the top metal lining where the throat damper opens into, there is a larger chamber that appears to be the beginning of the old chimney, as, after sticking my camera up there, I can see the old rectangular chimney flue damper (which is permanently stuck open with no user controls) and has been mostly lined with metal, except for the sides where I can see brick. Taking a 90 degree turn from the brick along the wall and heading back out towards the front of the old fireplace box, there is an air gap (maybe 2” wide) on the sides between the old fireplace brick and the metal lining that was inserted into the fireplace box. This second/“outer” side air gap is where the throat flue chain runs and also where the old (and now inaccessible) lever/arm is that used to control the rectangular damper on the old fireplace.

In case that’s hard to follow, if you move from inside the Fisher stove to either left/right (east/west) side, you get: empty space inside the Fisher stove-box, steel Fisher stove wall, 2” air gap, metal lining, another 2” air gap, original brick of old fireplace box. If you go up from the inside: empty space inside the Fisher stove-box, steel Fisher stove wall, either the first/“inner” top air gap or the throat flue damper/2” pipe, top metal lining, metal lined flue chamber, old fireplace flue damper that is always open into straight chimney to spark/ember arrester/grate and the outside world.

Looking at the front/face of the hearth, the metal framing is flush with the fisher stove on both sides (but not the top), blocking both “side” air gaps except for (1) the hole where the stove’s throat flue chain comes out and (2) two narrow but long gaps where the metal framing that runs along the top front of the hearth, above the stove, overhangs the front metal framing the runs along either side of the hearth as the top and side framing are each separate pieces of metal that are bolted together. At night, when the fire’a roaring, I can see light from the narrow gaps and throat flue chain hole coming from this second/outer air gap that is part of the “flue” chamber between the Fisher flue and the old fireplace flue/chimney.

This is all to say, the space/chamber between the Fished throat flue damper and the old fireplace flue damper/chimney is not airtight and not built-into/inside the Fisher stove fire-box…so, if I understand this all correctly, it is therefore not a “proper” smoke combustion chamber.

There is no catalytic converter.

As for the bottom, if you look under the Fisher stove, there is a metal framed air gap - open on the front - that is divided into two channels (~1.5” tall that runs the width and depth of the stove). There is a small rectangular cutout towards the back of each channel - one on the left and one on the right - that opens into the “inner”/first air gap for convection heating out the top, open air gap.

As for environment, I live in the Colorado mountains - winters are below freezing - and we use the stove for home heating. I typically do a FULL load of tightly packed logs, east-west, in the hopes of getting a full 8 hour cycle down to coals; then I ash, rake, load and light the next one leaving 1” of ash. 2-3 fires a day all winter long using free wood of all sorts from neighbors’ Spring landscaping work.

Finally, my question (thanks for bearing with me!)….what is the right way to use the air control dampers and the throat, flue damper to maximize heating, efficiency/wood use, and fire duration for my setup?

I’ve read that, when starting a fire, everything open until the fire’s ripping, then close the doors but leave the air control and throat flue dampers fully open, then gradually twist the air control dampers down basically as far as you can without putting out the fire (no smoldering!) to extend the burn time as long as possible throughout the cycle but…never close the throat flu damper as this will lead to creosote build-up.

Is this correct?

I ask for a few reasons:

(1) the stove is so old (there are no door gaskets) that I can pretty much fully shut the air control dampers and the fire still rips, especially when it’s very cold outside. In these cases, should I slightly close the throat flu damper to further moderate / extend the Burn?

(2) since I don’t have a proper smoke combustion chamber, I was wondering if it made sense to use the throat flu damper to try to keep as much smoke inside the stove’s firebox without it pouring out into the house via the air control dampers, doors, and/or secondary air gap so that the smoke is more likely to combust. Would this be cleaner / more efficient? If so, how should I use my air control dampers in this scenario? Also, is this more likely to cause creosote? How far should I close the throat flue damper? Any dangers (CO, smoke, etc.)?

(3) I’ve read that throat flue dampers are made to let some gas out even when fully closed, does this mean I should fully close (or near fully close) my throat flue damper (and air control dampers) when I’m near the end of my cycle and down to just coals?

Thank you so very much!

And if you have time for one last question, any thoughts on magnetic/stove-top thermometers for my setup? Do you think these are useful for helping inform optimal fire conditions (air control and throat damper settings) for my type of stove as I have no glass to see what’s going on in there? Any product recommendations?

You guys rock!

 

[coaly]

Here is what a Fisher Fireplace Insert looks like. It has a firebox sticking out the front, with a second steel box around the back half inserted into hearth. It uses radiant energy radiated into the room from the front, and removes hot air by convection in the back.

It has a air intake opening under ash fender (shelf) and this extends up the back, and over the top around the flue exit pipe on top. Hot air rises out the top slot.



This is a later one with a handle for the flue damper control. The early Insert uses a chain control.

There should be a stainless steel pipe connected to the top, extending all they way up. This is a liner.

Originally they were installed into the hearth using the existing flue. This proved to allow flue gases to expand into the larger area, cooling, and forming creosote. That practice is no longe done or to code. If this was installed that way, it can can continue to be used, but not advised.

There should be no visible flame or glow anywhere around the Insert. The firebox is sealed with welded steel plate. You may have no liner directly connected seeing glow out the top, which is not good. It absolutely needs a liner. Preferably insulated.

This did not use door gasket material to seal doors on the solid door models. Only glass door models use gasket material.



This is a nickel plated door installed with faceplate to cover opening around Insert. This was also used to prevent indoor air from leaking up chimney around the Insert. Without a liner, indoor air can leak in, mixing with the rising flue gases. This is dilution air that cools the flue gases, again cooling them below condensing point forming creosote.

Operation is starting fire with flue damper open, and intake spin dampers open a few turns. As fire builds, close them down to about 1 1/2 turns. As it heats up close farther to about a turn. Then adjust for desired heat output. Leave flue damper at top open.

This is a chimney control that slows the velocity of rising gases up the flue. It may or may not need to be used with your installation. That we can advise after knowing how this is installed with what type liner.

If you can give us some pictures it would help greatly !