This is a pic of the furnace and water heater exhaust flue. What are the possible causes of the water stains.
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Rain leakage- even if it has a rain cap, wind driven rain can get into the flue.
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Is AL for Alabama or Alaska? This one may be more applicable in Alaska: Condensation from flue gases being too cool for the local conditions. In cold climates, this can be very dangerous as in the coldest weather the condensation may start to freeze in the top section/s of the flue with the possibility of blocking the flue completely. This leads to combustion gas spillage into the home.
In Saskatchewan about 7-10 years ago, a family of 4 died from this phenomenon. A high efficiency furnace was installed and vented into the original large flue. (1)With the flue gases temperature now being lower, (2) lower volumes of flue gases produced due to the better mixing of hydrocarbons and air/oxygen…less excess air needed for full clean combustion and (3) and venting into a now oversized original chimney/flue, conditions are set up for condensation, at least. In this case a severe cold snap tipped the system over the line…condensation turned to frost and ice.
Was at a house Thursday and watched an oil furnace being serviced and tested by the oil supplier. The flue gas temp was 250F leaving the furnace breech and then going into a massive, cold (outdoor temp: -12C or 10F) outside masonry chimney!!! The serviceman was grinning from ear to ear…87.5% efficiency…but he didn’t realize that this chimney condensed and ran water at times!!! The gas temp needs to be as much as 50-70-100F higher for safety reasons (my oil code book is out in my car). I will be speaking to my clients about this on Monday.
Edit: Just did some checking and found that you’re in Alabama. Condensation could still be an issue with low flue gas temps and cooler weather. Better chance of rain leakage though.
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Note that the stains are coming from the joint where the elbow (approx. 90 deg bend) ties into the straight double wall flue going thru the roof. I do not see any streaks or stains above the connection. I do not know how much flue pipe you got above that. One thing to remember for every 90 deg bend it is equivalent to adding 10 ft of flue pipe. I suspect it may be the moisture from the moist flue gases condensing as Brian stated. This moisture also contains acids from condensing flue gases and can easily and quickly etch the pipe. Is the furnace a draft induced or does it have a forced venting fan? How long is the horizontal run of flue. I can’t tell if the 90 is a Type B flue pipe but all of these considerations will make a difference.
“Moisture from the moist flue gases” condensing inside the pipe is normally a positive sign that the flue is not drawing properly.
Even plain water discolors galvanized pipe but if it was rain water it would be on the double wall pipe above, not just leaking out of the adjustable elbow joint.
Hi Brian
Are you sure this was a high efficiency furnace. High efficiency furnaces remove most of the water vapour from the exhaust.
Could it have been a mid efficiency furnace?
Looks like B Vent all the way up?
I don’t see Dbl wall. If it is, it’s the wrong size. Gas is going up the outside section.
Condensation of combustion gases.
No this most likely was not a higher efficiency condensing furnace but it was efficient enough to lower flue gas temps to condensation and then freezing.
[quote=sparksnmore “Moisture from the moist flue gases” condensing inside the pipe is normally a positive sign that the flue is not drawing properly.
Even plain water discolors galvanized pipe but if it was rain water it would be on the double wall pipe above, not just leaking out of the adjustable elbow
joint.[/quote]
The chimney liner temps and flue gas temps are considered the “motor” of any naturally aspirating combustion system. If the gases are not hot enough they will not rise quickly and exit the flue fast enough leading to poor draft. Most days chimney “draw” is an actually interplay between light hot bouyant gases rising (called “chimney” or “stack” effect in building science) and draw caused by air moving over the top of the chimney. Hot gases have to be the dominant mover or producer of “draft”, as some some days and nights there are no winds and, hence, there is no help in developing draft. If the system needs the help of the winds for draft, then it has a high potential of failure, leading to combustion gas spillage or backdrafting…not a good situation.
Hot gases and chimney liners produce the draw/draft; cool gases/liner/masonry causes poor draft/condensation…poor draw does not cause condensation. If there are not design or deterioration problems, blockages or other unknown openings/equipment venting into the flue, with hot combustion gases , draft should be no problem.
I sat on the national board and used to do local guest training for the Canadian WETT (Wood Energy Technology Transfer) certification program. My 1990’s 2.5 hour talk on house pressures/smoke spillage/backdrafting has now been expanded to two 1 day courses. One thing this association would like to see is: outside chimneys banned. Think of it…a chimney has to be hot to work properly yet we put them outside so they’ll be cold when we need them the most…winter!! How crazy is that?? How smart are we?? Are we doomed as a species? In my area, if you look at older rural homes, you’ll almost never find on outside chimney…most are dead center of the house. With an inside chimney, it is always at least warm when it begins venting and…most parasitic heat loss is to the interior of the house.
In 1988, I worked locally on a cross Canada Indoor Air Quality research project. One section I did not work in was studying oil and gas furnaces/boilers/domestic hot water heaters for combustion gas spillage upon startup. They found that almost 40% of these appliances had varying amounts of spillage!!! Very scary when 40% of appliance/chimney systems are potential failure candidates.
I guess I should have said “not working” ,…no that would be wrong also according to you, wouldn’t it. 
It’s sort of like we all say “flashlight batteries” when they are not batteries at all, they are cells.
In my world “not drawing properly” is used, and has been used forever as a way to communicate that there is a problem with a flue.
Nobody really cares about all that other stuff so there is no reason to make it so much more complicated to the home owner.
Just like when they take their car in for a transmission rebuild, they don’t want to hear how it all works inside, just fix it.
“Hot air rises, taking it’s humidity up and all the way outside in a properly working flue, otherwise the gasses condense within the pipe” has always been a plenty good enough simple explanation (IF they are even interested.)
I figured that would be a good enough answer to the question asked here also and wasn’t trying to say how to fix the problem without knowing WHY this particular flue isn’t “drawing properly”
As a HI you better know the whys,and hows and if you are smart enough, you would thank Brian for giving you a mini lession!!
Read these (Typical) installation instructions, they should verify most of what Brian has stated. And if you are a HI you should know this info.
I haven’t trained as a gas tech or oil burner mechanic but have, at the request of a local furnace manufacturer’s engineer, given instruction on these issues to a class of training burner mechanics and their instructor. This field has gotten much more complex since the advent of high efficiency equipment, higher volume exhaust appliances in homes and airtight, higher efficiency buildings. Codes have not changed quick enough or have **made blanket, knee jerk reactive changes that were reversed in the next issue of the code!! **This is not the way we should be solving the problems we find.
Thank-you, Mario!! Just read your 2 posts after composing my last post.
Thank You Brian for allways providing great info!!!
OK, gang up on me now, I can take it 
Sorry I certainly didn’t mean to come across that I was questioning anyone’s knowledge or credentials OR that Hi’s shouldn’t be educated on the subject.
I apologize if that’s how my post was interpreted.
My point was just that different people from different regions say the same things in a different ways.
I was criticized for using the wrong words but I have found in dealing with homeowners and agents, repairing and redoing these things for over 40 years that keeping explanations simple has always worked best for me.
They understand “the flue isn’t drawing properly” and don’t really care to know anymore than that (except for what’s it going to cost)
After lurking here for a couple of years or so, I have gained a lot of respect for today’s Hi’s with all the knowledge they have to know in so many different fields. I’m amazed at how much so many of the people here know.
Back when I did do inspections, I only did the HVAC and plumbing part, never even thinking about what else was involved.
Here’s a good page from the Chimney Safety Institute of America (CSIA):