Wind related draft issue.

[kuma.jason]

I'd appreciate your input on this one. The homeowner has been very patient but I want to get this fixed up for him asap. We installed this about 18 months ago and everything works perfect as expected..... except when the wind gets going. Hopefully the 2 images attached will give all of the details needed but I'll answer any questions that might help identify the issue.

The issue that the wind is causing is down-draft. When the wind blows at these speeds, the homeowner says that it will nearly put the fire out and it causes a lot of smoking. He has no trouble with starting fires and no smoking during normal operation.

So far I have made 2 attempts to fix the problem. First attempt was adding another 48" to the chimney height. This brought the chimney top to about the same height as the peak but not above it. The second attempt was a vacu-stack or wind beater type cap with the additional 48" chimney removed. That's it, I haven't attempted the cap and extra 48" chimney together, nor have I tried a wind directional cap.

Thanks,
Jason

 

[jeff_t]

So, what's the issue?

Looks kinda short as is, just shy of 13'? What is recommended for whatever stove it is?

 

[kuma.jason]

Post was edited to explain the problem that the wind is causing, thanks for pointing that out.

The flue length of 13' does not include the attic, of which there is probably 2-3'. Total of 15-16', the recommended minimum is 14'.

As mentioned in the original post, the performance is perfect as long as the wind isn't blowing too hard.

 

[jeff_t]

Any topographical concerns? Mountains, ridges, hills? Is it only when the wind is from the direction indicated? Might look kinda silly, but I'd try getting over the peak.
I get the exact opposite in a strong wind.

 

[Crane Stoves]

wind directional cap (use your extra 48" if it there and set up already), im with jeff on this one and i dont like seeing the cap so far below the roof peak (wind firing off that opposite side prolly tails right down your stack).

 

[blades]

Jason, You are ignoring the number one rule - min 3 ft above highest peak with in a 10ft radius for the flue. This is only a rough guide number. In your diagram you are below the main peak. Until you extend the flue above the main peak your customer will continue to have difficulties. To accomplish this the flue may need external bracing, nature of the beast. It may take more than 3ft. above the main roof line. You are dealing with air currents caused by the roof lines. You need to get above that.

 

[nate379]

Pipe would need to be 12" taller to achieve that.

Jason, You are ignoring the number one rule - min 3 ft above highest peak with in a 10ft radius for the flue.

 

[Ashful]

Jason, You are ignoring the number one rule - min 3 ft above highest peak with in a 10ft radius for the flue.

Pipe would need to be 12" taller to achieve that.

Ditto. With 7/12 pitch, you need 10' * 7/12 + 3' = 108 inches exposure.

 

[Crane Stoves]

these guys are right, i guess if wind always blew in one direction 100% of the time and wind never tailed of a rooftop (like mountain tops) which it does, it would be alot less worrisome. here is a pic of wind off a mountain and you can see why its so important get that cap up over the level of wind whipping off the peak (maybe you can be equal to peak because your stack is pretty far away from it in this case) but i think that needs to be your first step.

(broken image removed)

 

[Jags]

I dealt with this early on when I first installed my stove. The addition of 2 ft AND a different cap solved it. I did not use a directional cap, but it is of a design that helps to deflect wind away from it.

 

[kuma.jason]

Any topographical concerns? Mountains, ridges, hills? Is it only when the wind is from the direction indicated? Might look kinda silly, but I'd try getting over the peak.
I get the exact opposite in a strong wind.

No topographical issues, this home is located on a prairie of sorts. According to the homeowner, the issue is directly related to the wind direction and velocity as shown on the diagram. As mentioned in the original post, a 48" extension had put the cap basically even with the peak of the roof. Though not over the peak, I am very surprised that this did not fix the problem.

wind directional cap (use your extra 48" if it there and set up already), im with jeff on this one and i dont like seeing the cap so far below the roof peak (wind firing off that opposite side prolly tails right down your stack).

My next move was to re-add the 48" extension along with the wind directional cap, if no other suggestions were made that seemed likely.

Jason, You are ignoring the number one rule - min 3 ft above highest peak with in a 10ft radius for the flue. This is only a rough guide number. In your diagram you are below the main peak. Until you extend the flue above the main peak your customer will continue to have difficulties. To accomplish this the flue may need external bracing, nature of the beast. It may take more than 3ft. above the main roof line. You are dealing with air currents caused by the roof lines. You need to get above that.

Not to disagree necessarily, but the peak of the roof is about 15' away. As installed, it meets all applicable requirements, such as 2' above any part of the roof within a 10' radius -and- a minimum exit of 3'. I know that these are safety codes and don't ensure proper draft and the rule you may be speaking of is more of a draft related rule of thumb. We have installed hundreds of chimneys where the cap did not clear the peak of the roof (where the peak is beyond the 10' radius), and never once had a problem like this, not once. As the flue already stands 94", an external brace is already installed so that isn't a big hurdle. Are you thinking that getting the flue over the peak would eliminate the need for a wind cap?

Ditto. With 7/12 pitch, you need 10' * 7/12 + 3' = 108 inches exposure.

I'm pretty sure that the rule is 2' over any point within the 10' radius and minimum exit of 3', and remember, a 48" extension has already been tried which put the total height well over the rule whether it's 2' or 3', thanks for your input.

these guys are right, i guess if wind always blew in one direction 100% of the time and wind never tailed of a rooftop (like mountain tops) which it does, it would be alot less worrisome. here is a pic of wind off a mountain and you can see why its so important get that cap up over the level of wind whipping off the peak (maybe you can be equal to peak because your stack is pretty far away from it in this case) but i think that needs to be your first step.

(broken image removed)

I'm guessing the wind in that picture is coming from the right. If so, then I would expect a huge problem if the chimney was on the left side and lower than the peak. With the chimney on the same side that the wind comes from (see the diagram again), the wind would have to be curling back around to hit the chimney top. I did already try a 48" extension that placed the top of the chimney even with the peak. I have considered re-adding the 48" pipe along with a wind directional cap. In my mind, I see a lot of turbulence near the chimney that would only confuse a wind directional cap on which way to turn. Thanks for your input.

Thanks, everyone for your input. I'm not trying to be argumentative here, just looking to make sure you're all aware of the steps that have already been tried. From everyone's input, I gather that going back to even with the peak and using a wind cap may solve the problem, or adding even more length to clear the peak (this would then be 6 feet over the 2/10 rule).

Many thanks,
Jason

 

[TradEddie]

Maybe it's not the chimney at all, something else in the house could be causing negative pressure when the wind blows, or at least contribute to the problem. Any vents, cat flaps or open windows/doors/skylights on the leeward side of the house? OAK on the leeward side? If there is an attic ridge vent, and not adequate soffit or gable vents, maybe that could do it too if the access hatch to the house isn't sealed. (Curious about those bumps near roof ridge).
Any chance that wind from that direction means rain, so they use the clothes dryer only on those days?

TE

 

[kuma.jason]

Maybe it's not the chimney at all, something else in the house could be causing negative pressure when the wind blows, or at least contribute to the problem. Any vents, cat flaps or open windows/doors/skylights on the leeward side of the house? OAK on the leeward side? If there is an attic ridge vent, and not adequate soffit or gable vents, maybe that could do it too if the access hatch to the house isn't sealed. (Curious about those bumps near roof ridge).
Any chance that wind from that direction means rain, so they use the clothes dryer only on those days?

TE

Excellent points. The bumps at the ridge are the attic vents. If the wind is passing over the peak of the house it could be causing severe negative pressure in the attic. Though it seams slim, the attic access could be the culprit. On the picture you can see the back patio that connects with a sidewalk. There is a sliding door there on the windward side. If, during the severe winds, the sliding door was open and all other doors and windows on the leeward side were closed, that would force a positive pressure, right? Do you think that this would be conclusive enough to rule out a chimney problem, or confirm it?

 

[Jags]

Do you think that this would be conclusive enough to rule out a chimney problem, or confirm it?

Probably neither. The problem is recreating it in the real world, not "Look what happens when I open this door". But TE sure has some valid points. The whole house pulling a slight vacuum during certain conditions is a valid point. I can tell what direction the wind is blowing from just by opening and closing the door between the house and garage. I could see the same effect happen on the stack.

 

[kuma.jason]

I just talked with the homeowner and had him do some simple tests as the wind is currently blowing (though not at the same severity) and he has no fire. With all of the doors and windows in the house closed, he has a very slight down draft when the stove door is open. He can feel some cool air entering the stove and see a little movement of the ashes in the firebox. When the door on the windward side is opened, there is no noticable change. When the door on the leeward side is opened, there is a significant change in the velocity of the cool air and ash movement. As stated by Jags, this is not exactly going to prove anything. The outside air is being drawn from the crawl space. What are the chances that the wind is causing a significant negative pressure in the crawl?

 

[Jags]

What are the chances that the wind is causing a significant negative pressure in the crawl?

It could happen. Anytime you have a leading edge into a wind, the trailing edge has less pressure (negative). Just a rule of physics. The only way to stop that from affecting the home is to tighten it up. Sounds like it goes way beyond your job as an installer.

What about extending the outside air to the outside??

 

[Backwoods Savage]

So with only a slight downdraft, having a fire in the stove should warm the chimney enough to negate that. Which begs the question, is the chimney insulated? Probably a silly question but you have to look at all possibilities. Another: When the problem exists, is this perhaps during a warm spell or falling temperatures? Another: Has the homeowner tried opening the draft more during these winds? Is there an OAK or is draft from interior? These type of problems can sometimes be puzzling but it can be costly to just guess and, say, add chimney or cap.

 

[TradEddie]

Are there soffit/eave vents for the attic? Assuming that's a cape-cod style house, there also needs to be passages for air to move between the soffit/eave vents up along the roof to the top crawl space, see (broken link removed to http://www.houserepairtalk.com/f106/proper-way-insulate-finished-cape-cod-attic-9704/) . If there are no other vents, or the channels are blocked, the negative pressure intentionally induced in the attic by the ridge vents will pull air from the house instead. In winter, this causes warm moist air from the house to condense in the attic, resulting in mold issues. I've also read that an OAK on a leeward side can cause exactly the problem you describe.

Disclosure, I know nothing about installing stoves other than what I read here, but my job includes troubleshooting pressurization in clean-rooms, and I own a cape cod with every imaginable ventilation problem.

TE

 

[TradEddie]

The outside air is being drawn from the crawl space. What are the chances that the wind is causing a significant negative pressure in the crawl?

Whoa, I missed that the first time. Do you mean outside air (OAK) for the stove is drawn from the crawlspace? By crawlspace, do you mean an attic, or do you mean a space below the ground floor? If OAK is from an attic space, or any space connected to those roof vents, that's almost certainly the problem, and possibly even dangerous. Attic and kneewall crawlspaces are designed to have negative pressure, you should not have an OAK there. If the OAK is to a crawlspace below the house, consider disconnecting the OAK, or if an OAK is really needed, extend the OAK with two branches, one to front, one to back of the house. There was a post here somewhere before about a problem like this. Of course, extending an OAK, especially vertically brings its own problems.

TE

 

[blwncrewchief]

I definitely concur with where TradEddie is going here. Let me also reiterate not to terminate an OAK in a crawl space or an attic. I would also recommend bringing out the OAK on the back (flue side) of the house to keep it on the high pressure side when you are having the problem. With an OAK (given it is a sealed setup) your problem is either a pressurization problem at the flue exit or a negative pressure where the OAK is located. Do keep in mind this is a relationship between the two. If you changed the chimney height by 4' and changed caps with no noticeable change I would definitely be looking at the OAK location. Once the chimney is up to temp something has to force it to loose draft. With an OAK the condition in the house (weather pressurized or in a negative pressure) should have no effect on the stove when the door is closed.