not southern hearth.com. southern hearth was manufacturer of unit, made about 1995?. probly out of business. i need o.m. for 40,000 btu direct vent fireplace insert. thanxxxxxxxxxxxxxxxxxxxx

Try talking with your local hearth and grill shops. Maybe you will luck up.

I have a Marco (direct vent) gas fireplace. After running for a couple hrs Saturday evening I was in the other room and heard an explosion - and saw that the (sealed) glass had broken into a million pieces (shattered glass) the whole (albiet now shattered still in the frame. Since this is supposed to be tempered glass - what could the causes be?
I am sorry if i wasn't clear (trying to keep it brief). The "explosion" was the glass shattering. It is a sealed gas fireplace - direct vent. so no flue that I am aware of. No one sprayed anything I was home aloe on the fireplace had been on for a couple hours and I was in another room for a minute. While the glass shattered (and what I heard was like an explosion_ because it was tempered glass it did not spray the glass into the room.
Oh and it is about 13 years old; if gets used about 5 - 10 a year and has had several service calls but they have been related to the electric ignitor not working and darking or dirty "burn off' within the unit.

The answer is fairly simple: the glass was poorly tempered.

The tell-tale signs of a tempered glass failure is the lound noise and it breaking into many small pieces.

Glass is tempered to increase it's strength and is accomplished by heating the surface of a finished glass and cooling it, which because of thermal expansion and the different cooling rates from the outer surface to the inside of the glass puts a compressive stress on the glass surface. Glass can only fail in tension, which means it can only break when a crack forms and then "propogates" or expands through the glass. A compressive stress on the surface acts to close any cracks that form thus not allowing them to propogate.

This all sounds great but there is a problem with tempering glass and you have suffered the consequences. Because it is nearly impossible to evenly heat and cool the surface, there will be regions of uneven cooling across the surface, which creates regions of uneven stress, or as we say "stress concentrations". These regions put the glass in an unstable condition where even under it's own internal stresses the glass can violently shatter. These tempered glasses often fail when heated because the thermal energy along with the expansion from heating causes these stress concentrations to reach a level where bond breaking can occur. Once the bonds break in one of these regions a chain reaction occurs as other bonds no longer have the strength to hold the glass together.

In essence tempering a glass strengthens it, but the glass itself wants to relieve that stress on the surface by breaking. It does this through two mechanisms when the glass fails. First, the glass break creates a compression wave (loud noise) and second the glass forms as much new surface area as is needed to relieve the internal stresses - this results in the glass breaking into many small blunt (sometimes almost spherical) pieces.

Many glass manufacturers are moving away from tempering because of this inherant instability to strengthening processes such as ion exchange but that doesn't help you and your now shattered fireplace.