Chimney Cap question

[paulslund]

Hi all.

This year I made a few changes to the evaporator. I have a 7 inch stack vs 6 inch from last year (actually used woodstove chimney pipe I got for a steal) and then also got a chimney cap to keep rain out. This is the typical chimney cap for woodstove applications (has about 3 inch gap then the top/cover part). Same length as last year (8 feet long chimeny, on a 4 foot evap)

Noticed this year that when I put the AUF on, I'm getting fire coming out the gaps in the front door to the evap (now this is a homebuilt file cabinet arch..it's not airtight by any stretch) but it didn't do this last year. If I seal it up it pushes smoke out between the pans (they actually start to move off a bit so I know the pressure is being moved to there) and then eventually pushes through the door again.

So I'm getting ready to take off the chimney cap as that is really the only change from last year..figuring it is constricting the flow too much, but wanted to pose this question before I do:

Would lengthening the chimney, thereby increasing draft, counteract the restriction that the cap is providing? I have a few extra 3 foot pieces, so if I add an extra 3 feet, or even 6 feet, would the extra pressure be enough to push past the cap properly? It's easier for me to add a piece than to get on the roof to take the cap off, but I also like the cap to keep rain out so if I I take it off I need another solution.

Just wanted to see if anyone has tried this before I proceed with one of the options..

Thanks!.
Paul.

[Dlee]

I used a cap the first yr worked fine for a few boils and started smoking when I went to check it out found that the screen around it was starting to plug took the screen off and has worked great

[BCPP]

What is clearance from top of chimney to underside of cap? If it's less than radius of chimney then the cap itself will cause pressure increase. For 7" chimney need at least 3.5" gap, preferably a little more because of change of direction. Also if there is screening on it, take the screening off. Not needed in sugar house application and screening can greatly reduce flow.

[paulslund]

Ok, so I've taken down the chimney cap. The clearance from the top to the underside of the cap is about 5 inches, so that should be good. I've also removed the screening from it as well. I'm also thinking I should remove the horizontal support that was behind the screening, and just leave each vertical support that holds the cap on (the red circled area in the pic). Does that make sense?

Thanks for the advice!
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[maple flats]

On an evaporator it is very uncommon to have a cap when boiling. Some use a flip open cap so it can be closed when not boiling.
If you still get smoke, you may just need more height. Yes, 2x the evaporator length is the guideline, but several things can affect what you need. The surrounding hills, buildings, trees and other things often make more height necessary. The space near the back of your evaporator (inside the arch) can also affect it. I suggest you just add 2' more, using single wall pipe on top of what you have, if that doesn't correct it, add 2' more.
Also, you say smoke, if you get smoke out once you have a good fire going, your wood is either wet, not split small enough or punky or for some other reason it is not burning hot enough. On my evaporator after the initial lighting you can not see any smoke, it is clear once the fire gets going hard. Once going hard, even punky wood, if dry will not smoke.

[Dlee]

Hey Paul all I did was remove the screen and it has worked great ever since. I see the part you are talking about I would try it like that first .

[Big_Eddy]

Increasing stack length increases draft fora natural draft arch.
As soon as you add a blower, increasing stack height has NO real effect on the draft. (If anything it increases restriction - but it's negligible)

[mellondome]

With forced draft, stack height is no a factor in the burn. If you are getting smoke and flames out of the cracks and under the pans, you have created a pressurized arch.
Remove the stack cap. Remove any dampers that are in the pipe.

[paulslund]

Thanks for all the advice everyone!

I don't have a damper so that's not an issue. I've removed the stack cap to take the screen off.. I may just cut the connections and add a hinge to convert it to a flip-open cap.. Or I'll just leave it off and put it back on as needed (i.e. when it rains and during the off season)..

The way the weather is going here I may only 1 or 2 more boils left anyways.. :-(

Happy sugaring!
Paul.

[paulslund]

Just following up for everyone. I put the chimney cap back on with the only modification being the removal of the bug screen. That's all it took for the evaporator to no longer be pressurized. The bug screen was providing too much restriction. I was happily boiling in the pouring rain today (well.. as happy as one can be in that kind of damp weather!).

Thanks again for all the advices provided!
Cheers!
Paul

[Aaron Stack]

Came across this thread after my first boil challenges. Smoke? Oh there was smoke, and lots of back pressure.. Took apart the stack yesterday and the cap was chock-full of gunk, restricting flow. Took it off entirely and will rebuild the stack today sealing all the seams with furnace cement and changing from a 2' length out the back directly to the 45, then three feet pipe, the other 45 and keep the four foot upright with no cap. Hoping for the best. Ordered a new/longer setup yesterday where the upright will be 2, 4-foot sections with a different style cap. Think I'll try a boil without the new cap first but don't really have ladder long enough to get me up there so might just pull out the screen before installing. I have time to think on that one.

[eustis22]

Why do you have a cap? It's my understanding a cap will impede the draft

[woodguyrob]

I tried adding a screen at the top of my stack once as a spark arrestor. It sooted up quickly and stopped the boil...
never again.

[Aaron Stack]

I tried adding a screen at the top of my stack once as a spark arrestor. It sooted up quickly and stopped the boil...
never again.

That was my thought process on the cap - a spark arrestor. Had a couple campfires get out of hand as a kid and have since been extra cautious as a result. But yeah it sooted up and while I did have a "boil" it wasn't rolling. Hoping the removal makes the difference.

[therealtreehugger]

Think I'll try a boil without the new cap first but don't really have ladder long enough to get me up there so might just pull out the screen before installing. I have time to think on that one.

My plan was to fashion a cap that I can put on and take off with a really long pole. Haven’t gotten to it yet, but that was the idea. Primarily to keep the rain out and keep it from rusting.

[eustis22]

I throw an old galvy bucket over the top of the chimney between boils.

I suspect you're going to get rust anyways unless the stack is stainless. Remember, Rust Never Sleeps.

[Aaron Stack]

New chimney is dripping in via FedEx one piece at a time and have sap to boil (though it might be a block of ice in the morning). Had a small hour and a half boil Thursday without the cap and with the blower. Much better results. Hit my ten GPH goal and probably exceeded that so boiling-wise I was pretty happy. Temp got only as high as 450 and the exit run was cherry red with flames out the top and eventually split at the seam (it's one of those snap connection pieces from Depot). McGiever'd three hose clamps to close that seam for now. The new one is 24 gauge solid-piece single wall stainless so expecting better results once it's in place and reconfigured.

No syrup yet, but the steam smells right and the boiling temp got to 2 degrees over boiling so almost there!

[Aaron Stack]

What a difference taking off the cap (and using the fan). Boiled off around 60 gallons in 4 hours yesterday and got my first gallon before I ran out of sap in the tote. Looks to be Amber Rich in color using the tester.

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