What is the reason for the low efficiency of coal stoves?

There are many reasons for this situation, which can basically be found in the following aspects:

1- high exhaust temperature;

2- the excess air coefficient is large;

3- Large heat loss on the furnace surface;

4- insufficient coal combustion leads to large heat loss;

5- The heat loss of incomplete combustion of gas is large;

The physical heat loss of 6- ash is large;