We had a log-burner installed last year. Our external chimney stack had previously been taken down, back into the loft floor. They dropped a flexible steel liner tube down our old chimney and bridged the gap between loft floor and the roof tiles with stainless steel "double wall" tube.
We have a Burley wood stove which are claimed to be around 89% efficient when up to temperature. Since their operating temperature is many hundreds of degrees I imagine the steel tube in the old chimney must be doing a good job of drying out any damp. 89% efficiency is up there with modern Gas boilers (though I expect there are all sorts of caveats if I expect to achieve that figure). They use secondary combustion to ignite unburnt gases. The way logs burn in an efficient sealed stove is very different to how they burn in an open fire.
Burley call it "the fireball effect". The logs aren't burning as you'd normally expect, they just get so hot that it "squeezes" the gas out of them and you see the gas burning.
I was so impressed I took a video of the effect...
Fireball 2ndary combustionThe log just slowly turns to dust over a period of about an hour and kicks out 5kW (I have a very small stove which can turn out north-facing front room into a sauna if we leave the door closed).
You also have a 30Kg lump of iron acting as a radiator in the morning, many hours after the actual fire has gone out.
I clean the ash out my my log burner about once a month, because there isn't any ash left to speak of, possibly a few tablespoons of it. It burns logs to dust. They are very efficient.
Open fires are extremely inefficient unless you view them as an efficient way of getting rid of surplus wood rather than an effective way of heating a room.
Open fires are about 10% efficient and a really bad one, with a poorly matched chimney, can actually draw more heat out of the room than it puts back.
Cleaning a steel tube will also be less hassle than cleaning an old chimney I expect.