help with chimney cap

[mnt]

It is about time I replaced the chimney cap or spark arrestor. My question is; is there any reason I couldn't have a foot or so of steel "extension" then expanded metal with a cap on top of that?

I am thinking a sheet metal box on the bottom 8-12 inches solid, then the next 8-12 inches expanded with a top covering the hole thing.

Can anyone think of a reason not to do that?

Thanks,

[Blacksmith]

I don't know enough to give a definitive answer, but I know improper caps can cause draft and smoking problems. If the cap you have was fine in that respect, except it's rusting or otherwise needs replacing I'd just copy it. Trying to improve something when I don't know the underlying principle that makes it work hasn't always worked well for me. Your mileage may vary.

[urch55]

mnt,
Is there a reason why you want/need to extend the chimney?? If not,, I agree with Blacksmith, If your old cap/arrester worked fine just copy it. Changing the height will affect the draft/draw.

[mnt]

I just copied it the last time. I agree with you guys on knowing why something was built the way it was. I have heard from some of the old timers in the neighborhood that the chimneys were built a bit too short, and therefore when it is windy the smoke gets pushed back down the chimney. I can attest to the fact that smoke does get pushed back down the chimney when it is windy.

That all being said, the reason may not be chimney height, and may be something else entirely. In which case, if I build a short extension it may or may not work. I have a bit of time before it gets anywhere near cold enough for a fire, so I may have to do a bit of research on teh dynamics of wind and roof pitch and chimney height.

Thanks guys.

[Blacksmith]

There are formulas (don't ask me where) regarding roof pitch, distance from the ridge of the roof and chimney height. Going down that road is a lot better than just adding 8-12 inches because I think it's okay.

[Roger]

Those formulas give minimum chimney height. Not a problem making chimney higher other than strength or rigidity to resist high winds. A tee at top of chimney with slanted pipe at each end of tee /---\ is old design to reduce down drafts from winds problems.

[ct01r]

Just some feedback on a problem that I had several years ago: I added about a foot of stove pipe to a masonary chimney to get it above the roof peak. When the temperature was low, it affected my draft. Your draft works best when the whole chmney is warm/hot. I used uninsulated pipe, and the outside temperature on cold days (single digit digits to the teens) would reduce the draft enough that the sceen on the cap was actually collecting soot. I cleaned the cap three or four times that winter before I figured it out. The soot would collect on the cooler cap, and the clogged cap would affect the draft. I just took the extension off to fix the problem. If you use insulated pipe, I would think that you'd be ok. Just my two cents. Curt

[canoecruiser]

Back in the day a friend in the Shenandoah mountains of VA had a two-flue chimney for his wood stoves. One in the main part upstairs and one in the finished basement. They never drew well, especially if the stoves were banked back resulting in the flue(s) not getting very warm. One night after we were, ummm, out howling at the moon and having a fine old time I crashed on the couch in the basement. The stove upstairs was running and the house was fairly tight, so the basement stove was pulling air (and smoke) down the other flue. This made it worse since it cooled the already too-cold chimney. I was about gassed which compounded the hangover. Not saying you'll have that problem but a good air supply is a beautiful thing.......