The image above shows the perfect example of why airflow gaps are needed with zero clearance fireplace doors.
Gaskets were added around the door frames, cutting off the airflow coming through the gaps between the doors and the main frame. And then as a further measurement, the gap between the firebox and door main frame was sealed with silicone. Both of these are very big NO-NOs.
Only approved, high-efficiency doors can be closed while a fire is burning in a zero clearance fireplace, and the one shown is NOT a high-efficiency fireplace door. So the doors should not have been closed while the fire was burning because the heat exceeded the thermal shock limits of the tempered glass, causing it to shatter. Sealing all the gaps in the doors probably accelerated this situation since there wasn't even any way for cooler air to circulate into the firebox when the doors were closed. There should not only be gaps around the frame but also between the glass panels.
Fireplace doors should NEVER be closed while there is a fire burning. The main purpose of fireplace doors is to reduce the loss of heat in your home when there is NO FIRE, blocking cold drafts and the warm air escaping from the chimney.
Another red flag in this image is the firewood being burned in this GAS fireplace. There are prefabricated fireboxes are made to burn wood, but this isn't one. The obvious clue to this gas-fueled fireplace is the gas valve key laying on the hearth so it's very likely a log lighter was installed. However, ONLY gas logs, the ones made of ceramic and cement material, should be burned in a prefab fireplace. Wood burning, masonry fireplaces can be converted over to gas, but the opposite is not recommended. Firewood burns much hotter than gas, and factory-made, gas fireboxes are not designed to withstand that much heat. The same goes for the tempered glass in the door.
With the airtight modifications made to this door, when the doors were closed while the fire was burning, there was practically no air circulating into the firebox to help keep it cool(er). With the hot firewood burning, it easily overheated and shattered the door glass. And broken glass is a minor consequence. The fireplace could have overheated to such a point that the firebox insulation was no longer was efficient to protect the surrounding structure and could have started a house fire.
Yes, there are louvers on the top and bottom of the gas fireplace, but the purpose of these vents are mainly to circulate air around the OUTSIDE of the firebox, keeping it cool. After all, if the fireplace is burned with the doors wide open like required, why would there need to be any vents for the INSIDE of the firebox?
We guess you could say that this picture shows just about everything you should NOT do with your pre-fabricated fireplace door, and there were many factors that contributed to the breakage of the door glass.