The siphon nozzle is the way to go, no doubt about it after all my experimenting with a standard nozzle setup under all sorts of conditions, temperatures, etc. There are just too many members here on the forum running their siphon nozzle burners successfully now that support this. Using a single dual use (air/oil) preheater as close to the siphon as possible seems to be the best setup. If your preheater is any further back, then you are going to want to think about keeping the nozzle warm somehow, maybe with a cheap, Carlin slip-on nozzle heater. A well wrapped preheater is also going to cut way down on the amount of electricity used, much less than is required of the standard nozzle pre-heater setup.

One drawback is that a source of pre-heated, compressed air is required, however, and mother-in-laws don’t count: at least 8-12 psi capability with a cfm that matches the size nozzle you decide on. Some members talk about a setting of 3-4 psi, others are running around 8-10 psi, but since there is no way to verify one pressure gauge against another, I would have make sure that I was pumping at least 10-12 psi with a strong cfm, to be able to dial it down.

I have to step in and say I could never get the siphon working right. My problem was smoking and a wetting of the chamber walls. No matter what I did, I couldn’t get rid of it. Using and old retention head helped over the kagi but it was still not burning right. I had the preheater as high as 200* celsius which is even hotter than ron is running. Under firing the boiler and over firing a nozzle helped as well but with a .75gph nozzle at 12-15psi it couldn’t keep up with the hot water. I had a heater on the nozzle also thinking that was the problem. It seems to me that the shearing effect of the air just wasn’t good until at least 10psi. Anything under that and it smoked worse. Hope i’m not discouraging anyone, it just didn’t work for me.

I found that too much air from the fan can be a problem, almost blowing out the flame cone. In just about every successful siphon nozzle burner I’ve seen working, the vanes are shut almost all the way down, so this tells you something. The oil has to hot, 275 to 300 degrees when it reaches nozzle, not the reservoir. If there is a long distance between the pre-heating tank to the end of the nozzle, it doesn’t matter if the oil is 400 degrees in the tank, it’s got to be hot at the nozzle when it’s atomized. I think that’s the key and I’m not saying that’s your particular problem, or you’re doing something wrong, just that without seeing your setup, it’s hard to tell what the problem could be. Are you pre-heating your compressed air?

I was heating oil and air. I even had the nozzle heated. Your comment about the blower air is important. I needed a lot of blower air to keep the flame short enough for my chamber. I’m sure this was or at least part of my problem. I’m pretty sure the oil temp was good as it had fast ignition and stable ignition. I really needed to use a 1gph nozzle for the btu’s but that made my problems worse. I couldn’t run either nozzle at less than 10psi or it smoked, more like 13psi for the 1gph.

I’ve looked at your unit in your photos and I just don’t get it. It seems to me everything is in the right place… is your furnace a boiler of forced hot air and how big diameter in the combustion chamber?