[TheForge] Question about metallic heat shield for wood stoves

[Peter Fels & Phoebe Palmer]

The alternative is to build the system to comfortably absorb the heat from a chimney fire and not worry about it.
In the long run that's what i'd like to do here but don't know if i'll live long enough.
I've built a number of steel fireplaces with large surface area upper chambers and one with a junkyard heat exchanger in the stack.
My pines are dying from the bark beetle plague and are becoming a fire and disposal problem. Not much hardwood right here, so it's pine that i'm burning...sigh. Gotta go up and clean out the stack again, pretty quick..

On Oct 26, 2014, at 10:20 PM, Mike Spencer wrote:


Andy wrote:

On 10/26/14, 5:06 PM, Peter Fels & Phoebe Palmer wrote:

>> Dampers...good idea.
>> How do you clean that sticky crap out of a heat exchanger?
> 
> Beats the hell out of me.  By "easy" I meant access.  No idea how to
> remove creosote.

I have often thought about making a heat exchanger for our wood range
to extract more heat to the house.  But the basic notion of a heat
exchanger is surface area.

We have stainless stove pipe, ca. 3' vertical, elbow and 5'
horizontal.  When I clean it I find one of:

 + Crumbly soot.  Easily & quickly removed with a scraper.

 + Glassy, brittle black coating.  Removed with some exertion by
   reaching into the pipe with a needle scaler as far as possible,
   the hammering the unreachable part on the outside wit the needle
   scaler. Maybe some scraping needed.

 + Glassy/waxy, tarry black coating, the same as above but not
   brittle.  Needle scraper removes it with difficulty where I can
   reach inside.  Hammering the outside doesn't do it.  For this
   stuff, I prop the pipe at a 45 deg. angle and ignite some
   newspaper or a oil-soaked rag at the bottom.  The black crap
   ignites, cokes up into pipe-filling mass of sticky foam and burns
   furiously.  After capping off the pipe with a piece of tin to
   stop the flame, the foam becomes friable and scrapes out easily.
   Heavy gloves, fire extinguisher, bucket of water and/or Indian
   pump on hand for this.  This is exactly what happens in a chimney
   fire.  It's easily stopped by cutting off the air supply but
   leaves masses of glowing foam that're ready to re-ignite until it
   cools sufficiently.

+  Black glaze plus some black foam. The scary one because the
   foam-like mass means that I hadn't cleaned the pipes soon enough
   and we has the beginning of a stove pipe fire that was stopped by
   happenstance.

Having learned all that over the last 30 years or so, I just hate the
idea of having to do that with a large widget with lots of surface
area.  It would have to be openable so as to expose the entire
exchange surface to needle scaling.  It would have to be stainless
because creosote is corrosive and burning-out would damage mild steel.
It would have to be easily removable so as to be able to clean it out
of doors if necessary.

All just too complicated, a major project promising several revisions
before it worked as expected.  Test bed is *my house in winter* which
adds risk and bother.

Last time I visited Dimitri Gerakaris, he'd built one of those
European (Scandinavian?) ceramic stoves. Smallish fire box,
complicated internal smoke channels to act as a heat exchanger between
the smoke and the ceramic mass.  I asked him how he expected to clean
those inaccessible channels and he replied that you didn't need
to. The operation method was to build a roaring fire twice a day, heat
up the mass of the stove and let the fire go out each time.  The
notion was that the very hot fire and continuously warm ceramic
prevents creosote accumulation.

Some time later, I heard that they'd had a creosote fire in the smoke
channels of the stove, roaring flames, way too hot, scared the
britches off of them.  Huh.

So *an* answer might be: (1) All exchanger surfaces accessible from
access ports and (2) a needle scaler-type tool specially designed to
work inside the (6" or 8" or whatever) smoke galleries.  Even then,
when the creosote coating is glassy but also tarry and non-brittle,
you have to be able to burn it out.  No easy answer for a tidy domestic
device. 

(Yes, out firewood is hardwood, two years old, stored under cover, not
green and/or pitchy wood. And we don't seem to get much, if any, of
the dangerous glassy glaze in the outside flue.  It accumlates some
crummbly soot that is easily removed with a long-handled wire brush.)


FWIW,
- Mike

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                         /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^
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