Which chimney size, height, cap to prevent down draft?

[thijsatteiltje]

Hi,

where I live it is often very windy, even on nice, sunny days when you're likely to be outside.

I would like to do what I can to minimise the effect of the wind on the draft of the chimney.

Which of the following techniques will give the best results, and how would your order them in effectiveness/priority?

- make sure of correct chimney surface area
- make chimney as high as practically possible
- choose a chimney cap. Looking at this list: Flues - Spares & Additions ? Sigma Sheetmetal Products , which model helps best against down draft?
- insulate the chimney? (to maintain a high temp of gasses and so a stronger updraft?

Any advise would be greatly appreciated!

Cheers,

thijs

[brickie in oz]

Re: Which chimney size, height, cap to prevent down draft?

Id go with the directional cap.

[thijsatteiltje]

Re: Which chimney size, height, cap to prevent down draft?

Thanks Al,

the trouble is the chimney will be in a small patio and the wind will come from all directions.

I accept that it can not be solved 100% and some smoke will always be blowing, I was just wondering what the best methods are to minimise the effect of a breeze.

cheers,

thijs

[Greenman]

Re: Which chimney size, height, cap to prevent down draft?

When things get warmed up the burn will be clean and smoke will not be an issue. Well cured, dry wood is the way to go.

[texman]

Re: Which chimney size, height, cap to prevent down draft?

i also live in a very windy area, West Texas that is. Several things to consider from my experience of dealing with smoke from ovens and fireplaces with ever changing winds.
Questions for you to consider:
1. Location of the oven in relation to other structures and the height and roof slope of those structures as well as distance from them?
2. Will your oven be under roof or exposed?
3. Fences, trees and other items that can effect wind velocity as well as swirling
4. Oven opening in relation to the prevailing wind direction
5. Planned chimney height
6. Building code requirements
7. Will you be able to support your chimney against the wind pressure?
8. What chimney materials are you planning? Steel, clay, brick, etc.
9. neighbors?
10. Sorry to make this a test, but the better plan the better result.

Texman
Send some site pics if you can.

[thijsatteiltje]

Re: Which chimney size, height, cap to prevent down draft?

Excuse me my late reply.

first let me emphasise I fully expect to not completely control the wind and smoke. What I am curious about is what adjustment gives the most bang for the bang and be still practical.
I'm planning a small 35-40" oven so a 12 feet steel or brick chimney is just not an option.

Location of the oven in relation to other structures and the height and roof slope of those structures as well as distance from them?

Our garden is very small, on steep hill and enclosed on all sides by high bushes, the house and fences.

Will your oven be under roof or exposed?

exposed

Fences, trees and other items that can effect wind velocity as well as swirling

Yes ;-)
The wind can basically come from all directions.

Oven opening in relation to the prevailing wind direction

little room for manoeuvring. also, wind will come from anywhere.

Planned chimney height

My previous oven just had a 1meter 15cm diameter stainless pipe, so ideally something like that. What I am after is if it makes sense to heighten the chimney or use other techniques.

Will you be able to support your chimney against the wind pressure?

I think so. I could always make the top half removable since it would be used on few occasions anyway.

What chimney materials are you planning? Steel, clay, brick, etc.

Steel. I thinking of no more than a small 40" diameter oven.

neighbors?

Darlings. ;-) Wood burners and fireplaces are still the norm here so no-one minds a bit of smoke. it's more for myself and the quest hanging around in the garden.

Sorry to make this a test, but the better plan the better result.

no, no, right you are.

Personally I was thinking:
- a 6 foot / 1.8M stainless steel chimney, double walled, the bottom half insulated
- a rain cap like this one:
http://www.sigmasheet.co.nz/web_imag...s/rainhats.jpg

Cheers.

Thijs

[texman]

Re: Which chimney size, height, cap to prevent down draft?

THIJS

Sounds like your oven is protected from all sides by either structure or plants/bushes. I think that is good and helps the draw and the enjoyment of the oven area. As long as you can get the top of the chimney high enough to be in an uninhibited air flow, the better. Gable roofs seem to cause an airfoil and resulting downdraft. So even when the draw is good, the exhausting smoke can be pulled down into you area. My oven is covered and protected on all sides at least six feet from ground and when i get a northeast wind, it drops that smoke right back into the patio area. Oven draws great, just the exhausted smoke. And that is only while starting up. after the fire is going good, i never notice it. I don't have a directional cap, like you showed, and that might help. I just try and get the fire started early if i am worried about guests getting covered. most of the time, it isn't a problem, just when wind is from certain directions.

My best advice is chimney height is your best friend. The higher the smoke exits, the farther it is from guests and cooking area. I had a taller chimney and ended up removing a 3' section mainly because it wasn't needed and too difficult to support.

you mentioned a six foot chimney, which with a 3' base and roughly 2' of oven, that will put the top of chimney at 11'. If you can do that, i don't know what else you can do really other than plan according to wind at the time you are cooking. I think the upsloping wind since you live on a hill will be the friendliest wind for cooking. The downsloping wind and the vacuum created from that causes the smoke to fall and swirl the most i think.

HTH
Send some pics if you can.
Texman

[bobmurphy]

Re: Which chimney size, height, cap to prevent down draft?

A general rule of thumb for a standard chimney is that it must be 3' above anything within 10' of it. That is actually code in many areas for chimneys. I do not know how much this rule is applicable to WFO, but that is what is required for other applications to prevent down drafts in the chimney due to wind.