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[mlappin]

I know it's possible to get your flue temp too low and have condensation issues when burning wood. What's the lowest a person could get away with say burning waste oil?

I've googled it and can't find anything about waste oil flue temps.

I have a waste oil burner in the shop that I placed in the old wood stove, before next winter I was going to build a simple tank in tank boiler to place it in then use a plate exchanger to hook the waste oil boiler to the wood boiler.

It's rated at 180,000 BTU an hour by strictly that there is x amount of btu's in a gallon of waste oil and it can burn up to a 1 1/4 gallons an hour, most of the time I keep it around one gallon/hour as any  more oil and it does't seem to heat anything up any warmer except the stack going out of the shop.

Be a shame to over build it then have to plug some heat exchanger area off if the temps are too low and condensation becomes a issue.

This is where I picked just the burner kit up, like everything else it's gone up quite a bit in the last ten years. http://www.wasteoilheater.net/Wasteoil.pdf

[BoilerHouse]

I am guessing the issues with waste oil would be similar to fuel oil.  The three things to watch out for would be water, sulpher and soot.  If temps are too low they can create a corrosive soup, unless the system is designed for it.  To minimize any problems,  the minimum stack temp should be about 300 deg F. 

[mlappin]

I wondered about the water part as it seems unless it's stored in a sealed drum, from burning it for almost fifteen years it seems waste oil attracts water.

I've thought about making the secondary exchanger out of stainless even.

[BoilerHouse]

Yes condensation can be a problem.  Another part of the issue is when burning a hydro-carbon fuel like oil.  The carbon burns to form CO2 and the hydrogen burns to form H2O aka water vapour.  If you can design the system to condense the water and minimize corrosion, you will capture a lot of extra heat that would otherwise be lost.  High efficiency gas furnaces take advantage of this principle.  There are also high efficiency oil furnaces but they are not as common.

[mlappin]

Got to talking to father a bit about this as well. I imagine waste oil would have residual acids in it as well from the combustion process while it was still in the engine?

Almost sounds like under certain conditions if not most with a little moisture or condensation waste oil by products could be more corrosive than wood?

Like I mentioned before I was thinking of a simple tank in tank. For the secondary tubes I've even thought about buying 409 SS exhaust pipe and having the local shop bend em up.

Come up out the back, do a 45, go a foot or so and another 45 then a straight shot to the front, something like the NC 6000. If I still felt the stack temp was too high I could always build another heat exchanger in the stack before it leaves the shop. Something like the multi tube sidearm heat exchangers and could either run return water thru it or just mount a small squirrel cage fan in the top and exhaust the air out the bottom.