Chimney cap height

[Hoytman]

Does anyone know if there is any significant differences in chimney caps height?

I’m referring to how high the top piece of steel is in relation to the cap base, or how high the top plate is designed to sit above the liner outlet.

I wonder if this height can have any effect on chimney draft or even cause some back pressure to warm and increase draft? Or does it make a difference how high it is?

Just something I’ve not seen talked about much and I’ve noticed some caps sit really low above the outlet and some average and some really high. Was wondering if others have noticed this.

 

[webby3650]

According to an engineer from blaze king, the cap can make a big difference when they test stoves.

 

[Hoytman]

According to an engineer from blaze king, the cap can make a big difference when they test stoves.

Which should theoretically tell us there is a range of optimal height that the top of the metal on a cap should be.

Too low or too high causing draft and stove performance to decline. I’d like to see some of these test numbers in writing though. Surely this information is available somewhere.

 

[armanidog]

I remembered reading this several years ago:
"
Will a chimney cap interfere with draft?

A properly sized chimney cap should not interfere with draft. Draft is that which controls how well smoke is drawn up and out of your chimney. The lid of the cap should be at least 5” above the top of the any flue tiles it is covering. Having more clearance than 5” is a good idea.

http://www.homesaver.com/content/tenquestions_caps.aspx?AspxAutoDetectCookieSupport=1#:~:text=A%20properly%20sized%20chimney%20cap,5%E2%80%9D%20is%20a%20good%20idea

.

It also depends on how much windspeed is present. The main drawback to having one too high would be the wind blowing it off or letting precipitation get into the flue.

 

[Hoytman]

I remembered reading this several years ago:
"
Will a chimney cap interfere with draft?

A properly sized chimney cap should not interfere with draft. Draft is that which controls how well smoke is drawn up and out of your chimney. The lid of the cap should be at least 5” above the top of the any flue tiles it is covering. Having more clearance than 5” is a good idea.

http://www.homesaver.com/content/tenquestions_caps.aspx?AspxAutoDetectCookieSupport=1#:~:text=A%20properly%20sized%20chimney%20cap,5%E2%80%9D%20is%20a%20good%20idea

.

It also depends on how much windspeed is present. The main drawback to having one too high would be the wind blowing it off or letting precipitation get into the flue.

This post gets to the heart of what I am questioning.

The link did not work for me.

Is the main purpose to prevent rain and downdrafts, or is it to improve draft? In the last case, a cap too high or tall would have no effect on draft and one too short would seemingly diminish draft. That stated, surely there is a sweet spot and it may or may not be the 5” mentioned. That is the information I would like to see and/or discuss.

Not too interested in debating a caps ability to keep rain and critters at bay, or the issue of preventing down drafts. Obviously, there are specific caps built for down draft and windy conditions. I am speaking specifically about the ability of a cap to increase draft and how design plays are part in that. Surely there must be a “range” or sweet spot that a caps top must be from the chimney outlet in order to have an effect.

 

[armanidog]

If you had still air, you might try various heights of the cap and see if the smoke was just hanging around inside the cap or if it was exiting as fast as it came out of the flue. Mine is about ten inches above the flue end and even on very still days the smoke never hangs around inside the cap.

 

[bholler]

This post gets to the heart of what I am questioning.

The link did not work for me.

Is the main purpose to prevent rain and downdrafts, or is it to improve draft? In the last case, a cap too high or tall would have no effect on draft and one too short would seemingly diminish draft. That stated, surely there is a sweet spot and it may or may not be the 5” mentioned. That is the information I would like to see and/or discuss.

Not too interested in debating a caps ability to keep rain and critters at bay, or the issue of preventing down drafts. Obviously, there are specific caps built for down draft and windy conditions. I am speaking specifically about the ability of a cap to increase draft and how design plays are part in that. Surely there must be a “range” or sweet spot that a caps top must be from the chimney outlet in order to have an effect.

Caps do not increase draft under normal conditions. There are a few when combined with winds will increase draft. The purpose of almost all is simply to keep water out. Some have screens to keep animals out. And yes if the height is to.low it can hurt draft

 

[ABMax24]

Assuming a perfectly calm day I don't see how a chimney cap could increase draft. The way I see it the ideal way to have the most draft is with no cap at all, a cap of any kind adds restriction (maybe extremely small though) to the gases attempting to exist the flue.

Now on a breezy or windy day a properly designed chimney cap could increase draft, using the wind blowing around the cap to create a vacuum in the chimney.

 

[Hoytman]

Is it possible for the right amount of obstruction (top of the cap at an optimal range of height above the flue) to cause enough back pressure to increase temperature in the chimney and therefore increase draft?

I’ve had some very knowledgeable peoplme this can happen. It is sort of what caused me to start this thread. I’m going to keep asking and searching to see if I can find an answer to this. Post number 2 indicates I am not the only one to ask this question because apparently there are those that think a cap can help draft...even in calm conditions.

Well aware there are caps designed for windy conditions. That’s not what I’m talking about.

Spark arresting, water and animal infiltration are obviously benefits of using a cap. The question remains are those things unintended side benefits of using a cap? If so, then has the original intent sort of been lost over time? Maybe those benefits have overshadowed original intent.

 

[ABMax24]

Is it possible for the right amount of obstruction (top of the cap at an optimal range of height above the flue) to cause enough back pressure to increase temperature in the chimney and therefore increase draft?

I’ve had some very knowledgeable peoplme this can happen. It is sort of what caused me to start this thread. I’m going to keep asking and searching to see if I can find an answer to this. Post number 2 indicates I am not the only one to ask this question because apparently there are those that think a cap can help draft...even in calm conditions.

Well aware there are caps designed for windy conditions. That’s not what I’m talking about.

Spark arresting, water and animal infiltration are obviously benefits of using a cap. The question remains are those things unintended side benefits of using a cap? If so, then has the original intent sort of been lost over time? Maybe those benefits have overshadowed original intent.

Why do you assume back pressure would increase temperature? I don't see there being any increase in temperature of the flue with 2 exceptions; there is a leak in the chimney that allows in cold air, a reduction in excess secondary air from the stove also cooling the chimney. In both of these cases if there is significant cooling air being pulled in either through secondary tubes or a leak sufficient draft in the chimney already exists and increasing draft is pointless.

My assumption is post 2 refers to various cap designs creating differing amounts of turbulence affect resistance to flow and therefore perceived draft. But in no way creating draft, some caps just reduce draft less.

The only possible design I can picture creating more draft is a divergent nozzle, high velocity gas goes through a nozzle and is slowed down, creating a vacuum in the chimney. Think the second half of a venturi tube. I don't believe this would work, chimney flow rates are far too low for this to be effective. I think I've read somewhere in the range of a dozen cfm with a stove turned down to low. It also requires moving flue gases to operate, and would do zero to start a draft. Not to mention it would make an excellent rain catcher. Installing an additional length of chimney sure seems like the fool-proof way to add draft.

I really think you are overthinking chimney caps. When both peasants were heating their huts and the nobles heating their castles all they were after was a method of keeping rainwater out to keep the living space dry, and the fire from being extinguished. I doubt draft was a topic much thought about, never mind tried to be increased by a different rain cap.

 

[Hoytman]

You’re reading too much into me reading too much into this. LOL! I’ve made no assumptions. I asked some questions. Big difference. I’ve not asked any questions that I’ve thought I’ve had the answers to, so there are no loaded questions from me. Just genuine questions.

I would ask any of you to consider the words used in Webby3650’s post. Apparently an engineer at BK has noted differences in draft using a cap versus not using one. Maybe there is something to it or he would not have noted such.

I’m not assuming there is back pressure...I asked a question if there could be back pressure.

I’m not in either camp on this. Just asking questions.

 

[ABMax24]

I'm just trying to provide feedback to your posts in the same order.

I questioned the backpressure topic because its been asked twice now, and ask your understanding to why you believe that would be the case.

 

[bholler]

Is it possible for the right amount of obstruction (top of the cap at an optimal range of height above the flue) to cause enough back pressure to increase temperature in the chimney and therefore increase draft?

I’ve had some very knowledgeable peoplme this can happen. It is sort of what caused me to start this thread. I’m going to keep asking and searching to see if I can find an answer to this. Post number 2 indicates I am not the only one to ask this question because apparently there are those that think a cap can help draft...even in calm conditions.

Well aware there are caps designed for windy conditions. That’s not what I’m talking about.

Spark arresting, water and animal infiltration are obviously benefits of using a cap. The question remains are those things unintended side benefits of using a cap? If so, then has the original intent sort of been lost over time? Maybe those benefits have overshadowed original intent.

Those that think that are wrong. Even if it did increase temps which I seriously doubt the backpressure would negate any gains from that small increase in temp. If you are increasing back pressure you would have a pretty serious obstruction at the top and would be reducing draft

 

[Hoytman]

I'm just trying to provide feedback to your posts in the same order.

I questioned the backpressure topic because its been asked twice now, and ask your understanding to why you believe that would be the case.

I never said I believed it. I am asking if it is possible. Apparently, from at least one poster here, there may be an engineer at BK who might have evidence that it works. At least that’s how I understood the post.

 

[bholler]

I never said I believed it. I am asking if it is possible. Apparently, from at least one poster here, there may be an engineer at BK who might have evidence that it works. At least that’s how I understood the post.

A cap can absolutely make a big difference. But in my experience only in reducing draft when to restrictive. Or helping to control the effects of wind. That post only said caps can make a big difference. Not that they increase draft.

 

[Hoytman]

“That post only said caps can make a big difference”...

...”when they test stoves.”

We have to keep the entire sentence in proper context.

Those words didn’t infer windy conditions, just that caps helped when testing stoves. “How” is the answer I am searching for. Maybe it was meant during windy conditions, but it wasn’t implied. Helped during testing was implied. Yes, that is being picky with the wording.

 

[bholler]

“That post only said caps can make a big difference”...

...”when they test stoves.”

We have to keep the entire sentence in proper context.

Those words didn’t infer windy conditions, just that caps helped when testing stoves. “How” is the answer I am searching for. Maybe it was meant during windy conditions, but it wasn’t implied. Helped during testing was implied. Yes, that is being picky with the wording.

It didn't infer that it increased draft at all either just that caps can make a big difference. And there would be no wind involved during lab testing of stoves anyway.

 

[Hoytman]

It didn't infer that it increased draft at all either just that caps can make a big difference. And there would be no wind involved during lab testing of stoves anyway.

I took it that it did...at least in the context of a response to my original post.

 

[bholler]

I took it that it did...at least in the context of a response to my original post.

Your original question was.

Does anyone know if there is any significant differences in chimney caps height?

I believe he was responding to that. And the answer to that is absolutely. But I have never heard of or experienced any cap that increased draft without wind effect. I have been doing this a long time dealt with lots of vendors been to lots of trade shows conventions etc. Never heard a manufacturer make that claim.

 

[Hoytman]

Fair enough BHoller, but don’t you think it best to let the person who made the post clarify, if they can? If I took post 2 in the wrong context, then I am fine with them clarifying that. Or at least someone from BK.
I don’t have solid proof, and neither do you. Your experience speaks for itself, but even in that regard I speak with men of old age often...have all my life. Some of which their experience and their words to me HAVE made that claim with very good reason/debate/conversation, while others are adamantly against a cap and make a good argument. All I am asking for is if someone can provide concrete evidence in the form of paper/pictures, links, graphs, charts, etc., if a cap can work in this manner. If an answer is not out there, then fine, but let’s give it a chance. You obviously can’t provide the info...and for that matter neither can I. Your comments that it can’t have been understood for several posts now. I am openly asking for comments from those who think it can/does. It doesn’t hurt to ask.

I will point this out...
One poster has already made the claim their cap top plate was 10” above any flue outlet. Sort of what I am asking is if there is evidence that shows when a caps top plate only served to deter rain water coming down...not even to account for wind driven rain...and loses it’s ability to help prevent down draft. To those less familiar than forum members this all points to: not all caps serve the same purpose, or even more than one purpose, and to those who wouldn’t know a difference, they can be sold anything and waste their money. Anyone can make a simple cap. Some makers I am certain take it further than that.

 

[bholler]

Fair enough BHoller, but don’t you think it best to let the person who made the post clarify, if they can? If I took post 2 in the wrong context, then I am fine with them clarifying that. Or at least someone from BK.
I don’t have solid proof, and neither do you. Your experience speaks for itself, but even in that regard I speak with men of old age often...have all my life. Some of which their experience and their words to me HAVE made that claim with very good reason/debate/conversation, while others are adamantly against a cap and make a good argument. All I am asking for is if someone can provide concrete evidence in the form of paper/pictures, links, graphs, charts, etc., if a cap can work in this manner. If an answer is not out there, then fine, but let’s give it a chance. You obviously can’t provide the info...and for that matter neither can I. Your comments that it can’t have been understood for several posts now. I am openly asking for comments from those who think it can/does. It doesn’t hurt to ask.

Fair enough. I talk to old guys allot as well. Some of what they say is very valuable information. And some is complete nonsense. But it is generally pretty interesting

 

[Hoytman]

I was trying to avoid mentioning this because it doesn’t matter and so my thread would stay focused. So much for that.

My chimney hasn’t had a cap on it in over 50 years. Just a tad longer than I’ve been alive. It was last re-built in 1976-78. From that point until the mid 1990’s it seen quite the use, then sat mostly dormant (used sparingly) until three years ago. Sweep checked it and couldn’t believe it’s condition for not having a cap. My grandpa was adamant about not using a cap. So much so he got mad at my uncle for wanting to install one.

Me...I could care less. Don’t have an opinion one way or the other.

I started this thread to ask the questions that I’ve raised because I was considering a cap. The reason I was considering a cap was because of speaking with other old timers, some of which were chimney masons that held a different viewpoint than my grandpa, to keep rain out...which was never a concern to those I’ve spoken with..., and to see if a cap would indeed increase draft even slightly as some had told me...now long deceased...decades. Even some of my forum friends have made this claim...and they to have 2-3 decades on me...and in one case 4 decades on me.

Not trying to discredit your experience. Sort of like the old logger whose never had a tree get him, or the plow driver that’s never been stuck or in a ditch. It’s just a matter of time...some just haven’t been at it long enough. One driver I know retired, got called back in to work part-time, went over an embankment. Never drove again it scared him so bad. Logger neighbor (50 yrs) cut trees and logs for 38 years before he got bit. Tree bit him on three separate occasions. Broken arm and two broken legs. Finally decided to call it quits. So, experience only takes us so far.

There...you have the reason for the thread and questions. Should be no need to have to explain all of that, but there you go.

 

[bholler]

I was trying to avoid mentioning this because it doesn’t matter and so my thread would stay focused. So much for that.

My chimney hasn’t had a cap on it in over 50 years. Just a tad longer than I’ve been alive. It was last re-built in 1976-78. From that point until the mid 1990’s it seen quite the use, then sat mostly dormant (used sparingly) until three years ago. Sweep checked it and couldn’t believe it’s condition for not having a cap. My grandpa was adamant about not using a cap. So much so he got mad at my uncle for wanting to install one.

Me...I could care less. Don’t have an opinion one way or the other.

I started this thread to ask the questions that I’ve raised because I was considering a cap. The reason I was considering a cap was because of speaking with other old timers, some of which were chimney masons that held a different viewpoint than my grandpa, to keep rain out...which was never a concern to those I’ve spoken with..., and to see if a cap would indeed increase draft even slightly as some had told me...now long deceased...decades. Even some of my forum friends have made this claim...and they to have 2-3 decades on me...and in one case 4 decades on me.

Not trying to discredit your experience. Sort of like the old logger whose never had a tree get him, or the plow driver that’s never been stuck or in a ditch. It’s just a matter of time...some just haven’t been at it long enough. One driver I know retired, got called back in to work part-time, went over an embankment. Never drove again it scared him so bad. Logger neighbor (50 yrs) cut trees and logs for 38 years before he got bit. Tree bit him on three separate occasions. Broken arm and two broken legs. Finally decided to call it quits. So, experience only takes us so far.

There...you have the reason for the thread and questions. Should be no need to have to explain all of that, but there you go.

No offense taken at all. I by no means pretend to know everything. But I grew up in this business and my father has about as much experience as anyone in the us sweeping field he has never heard it either. But who knows

 

[logfarmer]

I installed a “box store” chimney cap on my chimney some yrs back and I had some lose in draft and pulled it back off. Only installed to keep birds out of the chimney in the off season. So in my personal experience that is what happened. No I didn’t give you proof for your question but that is why I do not use one and my chimney is 35 yrs old and great shape.