Open fireplaces are “lookers” not “heaters”. Approximately 80% - 90% of the heat from the wood that you are burning goes up and out your chimney.
Glass doors do not produce more heat, but they do prevent the heat loss. As your fire is dying out you close the glass doors to keep your house air in your house.
A wood insert, wood stove, gas insert or gas stove installed into your masonry fireplace will provide approximately 6-8 times more heat.