Is a 3-foot tall sheet steel chimney cap 'functional' or 'aesthetic' ?

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The three-foot tall, four-feet long sheet steel fabricated around my chimney is falling apart so I took it down.
All that is sticking up out of the chimney itself are the elaborate spark arrestors (multiple chimneys use the same concrete column) which look like 18" tall space ships sticking out of the top of the chimney.
I realize the cap I removed served an aesthetic purpose; but is that the ONLY purpose?
That is, with the spark arresters (or whatever they are) sticking out of the chimney still intact, what 'function' does the three-foot wide by four feet by two feet painted steel chimney 'surround' cap perform?
A secondary question would be advice for fabricating a new one if it's needed (it certainly would look better with it back on).
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Take your specs to a good sheet metal shop and have them build you one. There are nice coated sheet metal types available today that are perfect for painting. If you get one made with these modified galvanized materials it should last for many years.
Joe
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On Wed, 06 Apr 2011 15:44:07 -0700, Joe wrote:

The old one was (apparently) galvanized sheet metal because I don't see any rust but it was very heavy lifting off the chimney (which sticks up off the roof by at least four feet).
The problem is high wind. We get 100mph winds here on the mountain facing the ocean. Over time, the winds caved in the structure.
I'll take a measurement and post that later. I think it needs better cross bracing against the wind.
But, what I'm trying to find out is whether it's actually necessary. It's going to rain tomorrow but when it's dry, I'll try to go up and snap a picture or three to post so you can see what I'm talking about.
Thanks.
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On Thu, 7 Apr 2011 04:40:52 +0000 (UTC), Aaron FIsher

Of course that will make it a little heavier. REading all your posts, I'm amazed you got it down.

You don't say what you burn that needs a chimney, or what the chimneys are made of.
AFAIK the purpose of a chimney cap is to keep critters and rain out. Even the low sulfer oil that I burn in my furnace has some sulfer and soot inside the chimney can turn to sulfuric acid when it mixes with water. I think it still takes a long time to eat through a stainless steel chimney. Are their other combinations that might cause problems, I don't know. Maybe if it rains hard enough it can put out the fire...pretty much kidding. I doubt that.
As to critters, that includes squirrels, birds, raccoons?, and pterodactyls.
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I'd be more concerned about the flammability of the critters' nests than the critters themselves.
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wrote:

Well, actually, if the chimney is not used long enough for them to build that big a nest, what happens is that when you start the first fire of the season, it gets scary from there. If the nest is small enough, it is all incinerated. That would be in the fall after the young birds had fledged, so there would be no KFC extra crispy. If the nest is big enough to block the airflow, it's an "OH SHIT" moment. If the bird is a mud nester, it may not burn off at all, causing a partial or full blockage that cannot burn away.
All chimneys need a weather cap and varmint barrier.
YMMV
Steve
Heart surgery pending? www.cabgbypasssurgery.com
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After having a bird, then a squirrel, I agree wholeheartedly. Best it SS so it does not rust and discolor the side of the house.
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On Thu, 07 Apr 2011 04:58:04 -0400, mm wrote:

The chimney appears to be concrete (at least on the outside).
The only things, I think, that are burned up in this chimney that go out the three spaceship-like "things" on top of the chimney (spark arrestors?) are (1) the propane-fired water heater, (2) propane-fired fireplace, and (3) wood-fired fireplace.
Here, for example, is a picture I just took of the three 'things' sticking out.
http://www.upload3r.com/serve/080411/1302291372.jpg
What are these three 'things'?
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On Fri, 8 Apr 2011 19:48:40 +0000 (UTC), Aaron FIsher

As you suspect, they are parts of rockets. Civil Defenese workers and survivalists often included special preparations in their homes for times of unrest. The exhaust gases from the three things you mention are not sufficient to ignite the rocket engines until their ignition has been enabled. He must have left instructions to do that, right?
If not, try www.civildefense.gov/homepreparedness .
On second look, those are chimney caps. What you took off is just a cabinet, to cover from view your ramshackle flues** and non-matching chimney caps.
**Especally the middle one.
I'm not sure but I would ask someone who knows more if the cover, given how high it is compared to the chimneys, reduces chimney draw.
I guess if the flue is hot the chimney draw is mostly dependant on the height. Maybe I'm confusing it with turbine attic exhaust fans, which have to have a breeze to work, but before I built another decorative cover, I'd check with a chimney company. Here we have a couple chimeny sweeps who expanded to all aspects of chimneys (plus another one who never expanded and low-balls and then tries to sell unneeded $300 pipes for $700 dollars, but that's another story.)
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On Wed, 06 Apr 2011 15:44:07 -0700, Joe wrote:

Here (finally) is a picture of the ruined metal enclosure.
http://www.upload3r.com/serve/080411/1302291053.jpg
I was wrong on the specs - it's about 2 1/2 feet tall, about 5 feet long, and about 2 feet wide (that's a yardstick on the ground in the pic, for scale).
The enclosure appears to be of galvanized steel, riveted together.
What I would need, to withstand the frequent 100mph winds, is to reinforce or brace it a bit better.
But that still leaves open the question whether this chimney enclosure is merely aesthetic or functional.
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Aaron FIsher wrote:

i'd vote for cosmetic
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Post a photo, its there for keeping smoke away by being high. your setup seems humerous, or dumb.
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On Wed, 06 Apr 2011 15:58:36 -0700, ransley wrote:

I'm not sure what's humorous about the setup but when the rain abates, I'll post a photo so you can see what it looks like (or maybe I'll use a zoom lens from the ground to snap a picture for you).
The chimney, which is thirty feet above the ground and about five feet above the roofline should keep the smoke nice and high, shouldn't it?
Besides, the top of the enclosure is just a tad higher than the top of the spark arresters so I don't see how this enclosure does ANYTHING about smoke.
I'm confused. Can someone explain how the enclosure handles smoke?
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On Wed, 06 Apr 2011 15:58:36 -0700, ransley wrote:

Here is a photo I just took of the other chimney (with an intact metal enclosure). This enclosure is open to the sky. Is it aesthetic or functional?
http://www.upload3r.com/serve/080411/1302294051.jpg
Here is a photo of the three items inside the metal enclosure. Are they spark arrestors? If I have to guess, the three "spark arrestors?" seem to be for the wood-burning fireplace, the propane-burning fireplace, and the hot-water heater. One got tilted over when I took the enclosure down (it was very heavy and high above me on the top of the chimney so I wasn't graceful.)
http://www.upload3r.com/serve/080411/1302291372.jpg
And, here is a picture of the metal enclosure back down on the ground. It seems to be riveted together and is about 2 1/2 feet high, 5 feet long, and about 2 feet wide (see the yardstick on the ground for reference).
http://www.upload3r.com/serve/080411/1302291053.jpg
My questions: 1. How to fabricate a STRONGER enclosure (that can be assembled on the roof) on top of a chimney by 1 person?
2. What is the purpose of this thing? (Aesthetic or functional?)
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Aaron FIsher wrote:

actually, it might be functional. if you have a strong wind go over the top of that, it may create a stronger draw via the bernoulli effect than if it wasn't there. the vents on the bottom were the clue.
if the sides were bowed instead of flat, they'd shed wind load better.
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On Fri, 08 Apr 2011 13:41:30 -0700, chaniarts wrote:

clue.
Amazing! I didn't even 'notice' those 'vents' on the bottom!
http://www.upload3r.com/serve/080411/1302294051.jpg

Interesting. I'm not sure 'how' to do that, but I do agree.
I was thinking more about BRACING the heck out of a new one. And fabricating it on the ground, and assembling on the roof.
Of course, the problem is the chimney is something like six feet tall on the short end and something like 7 or 8 feet tall on the long (downslope) end, so, getting on top of it is problematic.
But, there must be a way to do this!
Anyone know of a DIY?
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Aaron FIsher wrote:

when you get it fabricated, you can specify not only the dimensions, but how much each side bows out as long as each side is straight at the bottom (hint: each side will be a section of a cone). also, how it attaches to the chimney will be a critical measurement unless you want to redo that also.

well, yes. you can put braces on the top and bottom going across between the stacks. you'd have to make good measurements of the top for someone to build this for you. i'd get them to use pretty heavy braces.

got a friend with a crane? you might try calling places that replace a/c units. in my area, most a/c units are on the roof, and they just crane off the old one and lift on the new one. takes about 20 minutes once they're all set up. it might cost you some for crane rental time.

you might ask on rec.crafts.metalworking. at the least, you're going to need some pretty heavy duty tools: metal brake, shears, welding and/or drilling and bolting it together, etc. it's going to be heavy too. this isn't a typical diy job.
in my area, they do this with either sheet metal, like yours, or by building a metal rod frame, tying on a heavy chickenwire or expanded metal sheet, and doing a stucco job over that in place.
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It may be functional and required by code. The chimney must extend some distance higher than anything with in some distance of the chimney.
I forget the exact figures, but it is easy enough to Google.
Any metal fabrication shop can make one for you Consider stainless steel so it last a long time.
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On Wed, 06 Apr 2011 22:50:10 -0400, Ed Pawlowski wrote:

That's what I'm trying to figure out! :)

The chimney is a good five feet above the roofline and the sheet metal enclosure is another 2 1/2 to 3 feet above that (which is just about as high as the spark arresters).
Plus it's a clay tile roof so there's not much nearby to catch on fire.

The one I have seems to be galvanized (no rust) but the wind, which gets to 100mph across the ocean, is what knocked it over. So, whatever I use must be able to withstand moderately high winds all winter.
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That should be OK. Three feet above the roof where it comes out Two feet above anything within 10 feet.

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