I have a wood burning fireplace insert with glass doors and was told i can not keep the glass doors closed when I have a fire going. Something about shattering the glass doors. Any truth to that?
Do you have an insert wood stove or just some kind of enclosure with glass doors?
If it is a woodstove, you go by the listing of the mfr. If it is unlisted, then you should be able to burn either open or closed.
If you have gas logs, whether vented or ventfree, you want to burn with the doors wide open. Tempered glass is only rated for about 550F. Closing the doors on gas logs drives the temps way up but it also typically causes the flame to impinge on the glass leading to breakage.
Keep the fire inside the fireplace.
I also find that pilot safety or other gas control valves tend to get "cooked" when gas log sets are regularly operated with glass doors closed.
I often find pilot safety valves with a sticker on them limiting their use to temperatures below 170 degrees F. If they or other gas control valves are operated at the high temperatures often found when glass doors are closed, the high temperatures tend to cause the grease in the valve to evaporate, causing the valve to leak or seize up.