I am renovating a house, the previous owners used the fireplace often. The mortar between some of the bricks has broken away. Is this a simple repair, and what type of mortar should be used.

Is the firebox lined with refractory brick or common brick? Refractory brick is much flatter, smoother larger and alomost no mortar line at all.

I am renovating a house, the previous owners used the fireplace often. The mortar between some of the bricks has broken away. Is this a simple repair, and what type of mortar should be used.

I believe the brick is common brick and not refactory brick.

If the box is made of common brick your problems will continue but a refractory mortar will help. If there is room in the box you could line it with refractory brick with almost no mortar line.