Stove may be producing enough heat in firebox and flue pipe. But then exhausts into a larger, (chimney) colder environment before exiting building. If the stove were fitted with a double walled metal flue liner, the heated gases moving through the chimney, would just be heating that 4" or 5" or 6" dia. liner. As is, it's presently losing heat to quadruple the surface area and volume of air.
Not directly connected I know. Think of a bicycle's hand pump. As one pumps, the union of the pump against the flexible connector / pump against the valve gets warm; may even get too hot for comfort. This is due to increased friction, as the air is compressed through a construction.
Now imagine the process in reverse. Compressed air, (the heated gases from the firebox) being allowed into a larger space. The gases rapidly cool and can now no longer prevent deposits forming on the inner surfaces of the chimney. By fitting a liner, you help the escaping gases to remain slight compressed. And provide a reduced surface area in which to lose heat. This continued restriction and partial compression, also allows the stove to work more efficiently. As the hot gases remain in the stove for a little longer, leading to a greater combustion of fuel.
(If one opens a saw's muffler, by reducing the number of exhaust baffles and or drilling a bigger hole for the escaping gases. Those gases not only clear the space more quickly but help cool the exhaust port/s. But make the exhaust too large and back-pressure is lost. Resulting in an inconsistent combustion of fuel and loss of heat in an uncontrolled manner.)
In an engine, the combusted fuel produces movement via expanding gases and heat. Apart from keeping the cylinder & pot hot, so less useful energy is lost to absorption of heat from the exploding fuel. The excess heat lost in the exhaust gases, are just wasted energy.
•The function of an engine, is to produce/transfer movement, with as little lost energy as is possible. Any by product of heat is waste.
•The function of a stove, is to produce as much heat as possible, without explosive movement. How the heat is transferred, via the passage of heated gases - heated air - controlled loss of heat from a solid surface, is just as important, as the manner in which the heat was produced.