Yes, this is what I understood when I first got the house - open the
door, start the fire, wait for it to get going a little, then shut them.
Else it sucks in air from inside the house, causing the house to suck in
ouside air, cooling off every single room except maybe the one the fire
is in. My fireplace is specifically designed to work this way. It has
a closeable vent to the outside so that it can draw outside air into
the fireplace instead of heated, indoor air.
In any case, I guess it's just my bad luck that the thing broke. I do
believe fires burn hotter when the doors are closed and the overall
efficiency is better.
One more question. If I could heat up tempered glass slowly to a
ridiculous temperature, what would eventually happen? Would the
glass liquify, or shatter first?
It would anneal. Well, actually, it would reduce the stresses in the
tempered glass and would end up annealed if you then cooled it slowly.
If you just kept heating it, it would melt.
It will only shatter if it has a good reason. There has to be a reason for a
crack to form and/or propagate. If it already has a crack (or scratch) as a
stress riser _and_ the heating causes sufficient differential expansion that
the stress riser goes into tension, the crack will grow rapidly and the glass
will shatter. If the tempering process has created sufficiently high internal
stresses that heating one side causes the expansion (or even annealing) on
that side to result in serious tension stresses, it might crack as a result of
the high stresses without a precursor stress riser.
Personally, I take claims of tempered glass shattering "with no reason" with
the same enthusiasm as claims of people spontaneously combusting.
Ok, I do remember a few weeks ago, I had piled 5 or 6 pieces of wood
inside the fire place, and as the fire burned, the log stack became
unstable and the top-most one fell forward and smacked into the inside
surface of the glass. I guess that must have started something that
two weeks later resulted in the shattering. The glass is attached to
the frames rather loosely (to allow for expansion I guess) and one
theory may be that the glass was forcefully scraped across the metal
pressure tabs holding it in place when the log hit, starting a tiny
One other thing, the glass door is actually 4 separate pieces of glass
with 1/4" or so of space between each piece. Maybe when the fire gets
real hot, the differential heating caused by the sucking in of room air
through that crack causes local low temperatures near the spaces and
thermally stresses the glass too much?
well, yes, but tempered glass takes stress of heat differentials much better
than untempered. there is a limit to this stress though, and any small chip
or scratch makes that limit a WHOLE lot less.
sorry, it wouldn't anneal.
it would become untempered once you get it over bending temperature, which
for ordinary and normally found clear float glass, is around 1030F. on
cooling, it would no longer be tempered but regular window glass.
annealing is the process of reducing temperatures slowly enough so that
stress is evened out across the entire sheet of glass. if the temperature on
one part of the glass is different enough than another, when cooled, it
would set up uneven stresses, leading to eventual cracking and shattering.
in order to temper glass, it is heated up in special tempering ovens to
around 1200F (which takes about 5 minutes), then cooled to around 700F in
another 5 minutes. the tempering oven i'm familiar with has a 60 hp blower
to remove that amount of heat quickly enough. the equipment to do so starts
around $2million. it's not possible to temper glass without use of this type
there are other glasses, like pyrex or borosilicate, which can be tempered
the same way, but just at different temperatures. pyrex usually isn't
tempered at all.
I've had this happen. I did some searching and there is a ceramic
product available. DAGS but be careful, there is some material that
is designed to limit infrared transmission (like for viewing windows
in furnaces, etc) but you want it to pass IR.
I can't use the stuff in my fireplace because it's too thick for my
doors but it sounds like yours might.
Wood won't even start to burn until it's about 300 deg. F. A typical fire is
well over 1000 deg. F, and can easily go above 1500 deg. F. You got the fire
too hot, plain and simple. Don't remove the doors and put up a screen as
some suggest and don't bother complaining to the company. The reason that
glass is tempered, is because you're not the first person this has happened
to. Once is an accident, twice is stupidity.
Well tempered or heat treated glass is by it's nature under tension.
The internal stress is what causes it to shatter into small cubes
instead of long shards. Still sharp and potentially dangerous but not a
10" shard stuck in your chest. You can this stress in the side and
especially rear windows of your car. It's very obvious when viewed
under polarized light, normally occurring or through sunglasses. See:
So any imperfections, contamination in manufacture, external stress,
framing errors that cause undue stress or frame to glass contact ( as
noted above), damage, scratches, thermal cycling, large thermal
gradients can cause the glass to fail and fail rather dramatically as
you experienced. So I wouldn't worry about it any more than you would
that your auto glass would suddenly fail. Which (as above) it does do.
I just had some FP doors shatter recently, after getting my money back,
I plan to replce the shattered glass with something called "NeoCeram".
Some kind of transparent ceramic stuff that good for 3x the heat( saw a
video on the net one time where they poor ice water on a sheet while a
propane torch is firing up at the bottom) A bit pricey at $39 sq ft.
Let me know what you find out. Mine did the same thing!
Regards in fullest Anthony
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