Having built a few (!!) drip burners I would say with certainty that heating the air ( or fuel) is not going to make any worthwhile difference to the way the thing burns except for a possible brief period on startup.
The burner should run with an internal temp of over 1000o C. Heating the air even 2-300o is going to do nothing for the oil that is burning from well under 300. There is such an excess of heat in the burner while it's operating and the things will run so far above the boiling temp of the oil, any preheated air ( or fuel) is a moot point.
The other thing may be that you are taking heat from the burner which will be pumped out the flue instead of into the room.
Yesterday I found some motivation and a rush of blood to the head and came up with a new and very simple original design for a burner. I was going to go with another setup but I like to keep my designs as simple as possible which also makes them easy to fabricate.
It's a horizontal design ( not that it matters which way you turn it) and the air tube runs the length of the burner. The air tube presently seals against the end of the extinguisher bottle internally and I cut a notch for the air to escape directed at the bottom of the bottle. The fuel has to be heated and vaporised and come into contact with hot metal to do so. The idea of the burner was that the air and fuel would travel to the far end of the burner with the tube passing through the flame front and vaporising the fuel in the regenerative cycle needed to change its state from a liquid to a gas as needed.
I did stand there looking at the thing before I welded it up trying to run simulations of the thing running to work out where the flame front would travel and the path it would take and if the in rushing air would cool the central pipe too much to allow vaporisation of the oil or whether it didn't matter and it would burn in the chamber section anyway.
After much head scratching and subsequent splinter removal from my fingers, I wasn't sure what would happen but I was pretty sure that it would happen. As the design is so damn simple anyway, I pulled out the plasma cutter, cut the extinguisher bottle with a slot and the hole for the inlet needed, tacked it up with the MIG and gave teh thing a run.
Wow! Talk about easy to light. I get a perverted satisfaction when I dream something up, cobble it together and the thing exceeds my hopes and expectations. I could hear the veg oil I was using was a bit wet by the crackling which is always hugely detrimental to easy startups even if it makes little difference to running but the thing was taking off faster than I would have expected.
I ran it a good while with errant flames happily coming out where the thing wasn't sealed and it worked like a charm. I ran it up to full power on the castle blower and with the PWM speed controller ran it low as well. Didn't matter, didn't care, whatever output you want. That's why I love drip burners. You can go from a candle to an inferno all with the same setup.
The biggest surprise was confirmation I thought I saw when it was running but confirmed when I shut it down. I turned off the fuel, waited till the flame lessened and turned off the blower. Sure enough, I could then see the air inlet tube still glowing quite well.
I wouldn't have thought it would get that hot with the amount of air going through it but I was satisfied that my design theory worked much better than I expected and was sound. I wasn't so much trying to preheat the air but rather get the fuel to boiling point which it does soooo easily.
As far as your air preheat goes, I would suggest thinking about going the other way. Post heat the air from the chimney and direct that into the room where you have the heater. There is a shipload of heat we don't recover from the Flue and even though we are using free fuel, it's just good engineering to make things as efficient as you can with simple modifications.
The only thing preheat can do is help briefly with cold start up. once the burner is up and running, there is more than enough heat to vaporize the oil and keep the thing going. :0)
Alternatively, you could heat water instead of air using the half pipes welded to the Flue.
