Code question of the day all 6" corrugated liners are sized improperly?

[elkimmeg]

If effort to generate a hot issue debate here, place this one into I did not know category. Recent post advocated reducing the 6” requirement to 5.5” liners which only reduced the area from 28 sq “ to 23” not knowing the friction characteristics of corrugation, in effect they are telling that poster its ok to reduce it by 1/3 . The code is not the only source noting friction drag. The manufactures of the products themselves admit it. The testing labs have proven it. Many installers probably do not know it or ignore it and most who have corrugated liners do not want to hear about it from me This will be blast the messenger again. Then someone will point out, fuel collar reductions are common in Europe. The question of the day is, as an inspector, knowing other code prevent the reduction requirements of the exit fuel collar, How can I accept a 6” corrugated liner attached to a 6” flue collar?

Taken from the latest international Gas and Mechanical Codes
All fireplaces and wood heaters are classified as category I appliances. The Categories are determined by heat ranges

Chapter 5 : Chimneys and Vents
Section 504 Sizing of Category I Appliances Venting Systems
•Where a corrugated liner is used the maximum capacity is reduced 20%
•504.3.1.7 Liner System Sizing
Added reference to Table 504.3.5

We have a lot of post here where stoves with full liners do not draft correctly. How could they if their liner restricts draft 20%. Hopefully some installers will read this post and start sizing corrugated liners correctly.

This is one of the many questions. I will be asking all the building inspectors and state inspectors in attendance to my seminar in a couple weeks. I wonder what there reactions will be? Is a bad or un-enforceable code, meaning physical dimensions of existing 8” flues make it impossible to correctly line existing flues and forget if they are required insulation. It is impossible to get a 6.5” liner, which is what this, code says, is needed with ¾ insulation jacket? Who is right here? The inspector for failing every 6” corrugated liner? The installer, for installing them? The manufactures, for not disclosing the info or purposely hiding it? The code writers for not taking account real world field conditions? I’m leaving my opinion up till this issue is debated a while, but I have some solutions and suggestions

 

[Todd]

Good point Elk,

But how old is that code? Do they need to compare the stoves max BTU's to the BTU ratings of the 5, 5.5 or 6" liners? I remmember reading that the BTU's ratings of those liners are more than twice the BTU rates of the stoves. Maybe they need to change the code?

I will admitt my liner is 5.5", but the old 6 1/2 x 10 1/2 flue was a poor drafter, and stove performance suffered. Now the stove burns like a champ!

 

[webbie]

Chapter 5 : Chimneys and Vents
Section 504 Sizing of Category I Appliances Venting Systems
•Where a corrugated liner is used the maximum capacity is reduced 20%
•504.3.1.7 Liner System Sizing
Added reference to Table 504.3.5

We have a lot of post here where stoves with full liners do not draft correctly. How could they if their liner restricts draft 20%. Hopefully some installers will read this post and start sizing corrugated liners correctly.

Notice that it says "Chimney Capacity" not draft. These are two different things.

As a for instance, large European wood stoves used to have flue sizes of approx. 4.7" (Jotul, Lange, Morso, Etc) and still do in Europe, they just put larger ones on for the usa.

Here is a older chart of chimney capacities:
Height 19 28 38 50 78 113 (square inches)
------------------------------------------------
6 ft. 45 71 102 142 245 NR
8 ft- 52 81 118 162 277 405
10 ft- 56 89 129 175 300 450
15 ft - 66 105 150 210 360 540
20 ft. - 74 120 170 240 415 640
NR means not recommended
--------------------------------------------------
*From National Fire Prevention Association
------------------------------------------------------
So a 20 foot chimney that is 6" (28 sq inches) would have a capacity of 120,000 BTU. This is approx twice the average input of most wood stoves. A taller chimney, say 25 feet, can take much more.

Even a 5" chimney of 25+ foot height (typical 2 story reline) can accept about 90K BTU.

I would make the following points:

1. In general, I think flexible liners are the easy way out. Rigid are MUCH thicker and have more capacity.
2. For most chimneys (which are over 20ft) and most woodstoves (which are not gigantic), a 5.5", even with flexible reduction, can handle the output.
3. If you are buying a extra-large woodstove and have a one-story chimney, you might have problems even with a 6".
(Plug: Use one of my chimney extensions and run insulated liner up inside of it!).

To give an idea of BTU input with wood, 10 lbs of wood per hour would be 85,000 BTU input. That would mean you'd burn 240 pounds in a day, or about 2 cords of hardwood a month. Since most full time users of newer stoves use much less than that, we can assume people are burning at between 15,000 and 40,000 BTU and hour (typical).

So that's my take for now. Having installed thousands of stoves, I've found there are so many variables with this stuff that hard and fast rules often are tough to apply. The bottom line is that the stove should work well, respond to the draft control and not smoke out the door!

 

[Todd]

What about an oval exhaust? My stove has a 7" x 3" (approx) oval stove exhaust. I have a oval to 6" adapter, but the oval area is less than the 6" round area? Isn't it? What does the code say about that?

 

[elkimmeg]

Todd the code is latest 2003 NFPA 211 not out dated but recognizing more recent data and testing results.
On your ovalizing 3/7 Not interpreting code but drawing from real life experiences and knowledge Code requires equalvant meaning if the oval area is equal to the flue collar it is acceptable. Reality smoke travels in circular pattern as it ascends, meaning it does not use the outer perimeter of the oval area, it is reduced to a smaller center circular area. Discussed here and proven oval liners areas are actually reduced compared to the area of the original circle. As you can see, ovalization and corrugation compounds problems and not a very efficient solution to positive draft.

Craig great insight: I had not though of the length correlation to diameter. Actually this is already done in the codes concerning fireplace openings and flue requirements. We now have a graft to figure correct openings and flue volumes like a parabolic curb. This is why I posted this question.
First of all I have not failed a 6” liner for the corrugations reduction. So already I am interpreting code outside the box.
So what can be done to reflect real world conditions? One manufactures of liners have to step forward. They should test the limitations of their products and do as Craig suggested. Not one size fits all. These sets of conditions means the 5.5 diameter liner will meet or equal the 6” flue collar appliance requirements. They could improve manufacturing process to reduce the friction drag corrugations produces. Do you know code does not allow for extrapolation, when interpreting the graft? For most code officials, not dealing with complex Mechanical code issues, inspections are easy. If you don’t know the issues, it is easy not to worry about them. Most just mail in the inspections. Ignorance is bliss. It is a lot easier to pass an inspection than to take a stand and fail one.
Craig thank you for steering me in another direction, I had not thought of. I learn more here on this forum, than reading code or doing field inspections, pertaining to wood stove installations. You pick my brain I pick yours. This is a very analytical group of posters. I mean real world situations are discussed here.

 

[woodpile]

I would imagine that a gravity exhaust vent system (aka chimney) would be called "good" (draft well) if it did 2 things: Provided enough pressure difference to pull air through the appliance, and was capable of providing this pressure difference at the maximum designed volumetric flow rate for the designed range of operating temperatures. Too little volume, or possibly volume/area ratio, or too low a temperature, and there will not be enough bouyancy of the hot air to provide the pressure difference. A bigger hot air balloon will lift more, as will a hotter hot air balloon. Too small an area and the applicance will be restricted in its volumetric flow rate capacity.

I am guessing that BTU/hr is one way of implicitly specifying the volumetric flow rate and the operating temperature, though there are a lot of variables that this does not take into account, mostly related to temperature drop by heat loss.

I did not know, and would not have guessed that smoke spirals up the flue enough to effectively waste a significant fraction of the cross -sectional area of non-round flues. Does this mean that a 6" flue ovalized to 5.5" x whatever" is no better than a 5.5" round flue?

 

[webbie]

Craig great insight: I had not though of the length correlation to diameter. Actually this is already done in the codes concerning fireplace openings and flue requirements. We now have a graft to figure correct openings and flue volumes like a parabolic curb.

Most of the insulated chimney makers have standard graphs and tables in their instructions. I used to have them all handy when I sold boilers. I don't know if Elk goes back this far, but until recently (or still in effect?) Mass. actually had a code that ALLOWED multiple solid fuel or even TWO units, one oil and one wood into the SAME chimney! Yes, to my knowledge it was one of the only states that had this "over ride" code, and this was largely due to the fact that HS TARM HQ as well as other boiler makers were once in Mass. - They petitioned the state gov and showed where it was safe when the flue had capacity for both unit running at full blast.

So chimney capacity really means everything. One good stroke of luck is that quite a lot of reliners are two story - in almost any case, these are 24 feet or higher, which in general means that they have plenty of capacity even with flex liner.

As far as oval flue, we have discussed that many a time here. My experience is twofold:
1. A small amount of oval, such as about a 1" squash on 6" and a 2" on 8" is usually always OK.
2. deeper ovals must be done by machine so they are correct.
3. If the oval is at the flue collar of the stove, that means the stove was tested this way and it is surely OK....however deep the oval.

Many older stoves had ovals in order to save room for cooking and water pots on top of the stove. Even more modern stoves like Vermont Castings and Upland had ovals which were quite deep.

I think the common sense thing with all of this is that chimneys SHOULD not be minimum or marginal if people want to enjoy their stoves. It doesn't usually (in my opinion) come down to a fire safety issue, but to a performance one. Code is mostly for safety, as a chimney can easily meet height codes and not work well.

Although some homeowners or installers might resent it, they are getting a great deal when Elk is on the job because he is giving them pro advice that they would usually have to pay a lot more than the inspection price for. On the other hand, some people don't want to know what's best for them. They figure they paid good money...and, damn it, the stove WILL work like magic.....

An interesting thing is that a high % of homeowners will not complain if things don't work right...they just will put up with it or stop using the stove as often. Although I might have appreciated this as a retailer, as a consumer I look at it a little differently....we used to laugh at our shop and say "Heck, we hope people don't use their stoves! That means less headaches for us!" - and in relatively warm southern NJ, it was somewhat true - many people used their stoves only a couple hours a week.

I had some friends who owned a big stove shop in Maine. They said one day they would retire (they have now) and start 1-900-call flue or something like that! Well, that was before the internet, but I think the day may come when the industry can support people like Elk or myself as pure consultants....like home inspectors, but just for the chimney and stoves. $125 (beats the $25 elk is getting now) and we'll visit once and write up a full report....but, considering our advanced age (in the future) we will not climb up on high or steep roofs!

 

[webbie]

I did not know, and would not have guessed that smoke spirals up the flue enough to effectively waste a significant fraction of the cross -sectional area of non-round flues. Does this mean that a 6" flue ovalized to 5.5" x whatever" is no better than a 5.5" round flue?

This is an interesting thought. There are people who study this, but I don't think any of them are here. My guess is that, in normal operation you are correct - the smoke probably uses the middle of the flue, but when the chimney is pushed to the limit, more of the oval is used....a guess only.

I'm thinking of it like water (upside down)....I used to describe the flow of heat in a house this way....I'd tell the customers, think of it as if you turned the house upside down and filled it with water - that's how heat flows from a stove (to the ceiling first, then through doorways, etc.).

So, if we had a large hose and let it run into the top of a vertical oval pipe that was larger than it, the water would pretty much use only the center of the oval. But is we increase the water flow until it just about starts to fill the pipe as fast as it empties, the entire oval would be used.

Sounds good, anyway!

 

[elkimmeg]

I agree with your theory that when push comes to shove the outer oval sections are then used
I got good news, I think the other day. My town is going to raise the inspection fee paid to me to $30.
but not alot my milage rebate. They had been a warping $.25 per mile. I also found out, the carpentry
rates I charge are woefully cheap. Guys. doing the stair work and finnish carpentry are getting up to $75 per hr.
I charged locally $ 37.50 and I thought I was a bandit at $45 doing work in Weston (one very rich community)
I guess I have to re-think what I charge, got to get while still physically able to do so. It bothers me to charge that much money
I have made a living at what I charge. Maybe I should get into stove installations lord knows how much they charge. I could do 3 a week and make more money than 40 hours carpentry. I have seen $400 for delivery setup and install just a short connector pipe into a thimble What an hours plus work two guys.

You also hit on another point, I had thought about consulting. The real worth of a professional inspection, that insurance companies would reconise and local states and towns as well. But hell why pay when some can post here and get the same advice free. There have been way too many instantances posted here where the consumer has been ill informed. What fustrates me is to read or see the poster does not want to hear it. or not willing to belive it. I have been told by two professionals________ that XXXX is correct
and I as the inspector must be wrong???? It will happen 5 times this week or next. I will admit when wrong and backit up when right

 

[webbie]

I agree with your theory that when push comes to shove the outer oval sections are then used
I got good news, I think the other day. My town is going to raise the inspection fee paid to me to $30.
but not alot my milage rebate. They had been a warping $.25 per mile. I also found out, the carpentry
rates I charge are woefully cheap. Guys. doing the stair work and finnish carpentry are getting up to $75 per hr.
I charged locally $ 37.50 and I thought I was a bandit at $45 doing work in Weston (one very rich community)
I have made a living at what I charge. Maybe I should get into stove installations lord knows how much they charge. I could do 3 a week and make more money than 40 hours carpentry. I have seen $400 for delivery setup and install just a short connector pipe into a thimble What an hours plus work two guys.

You also hit on another point, I had thought about consulting. The real worth of a professional inspection, that insurance companies would reconise and local states and towns as well. But hell why pay when some can post here and get the same advice free.

Well, notice how we always temper our advice here with "probably" "seems like" etc. etc, because we can't really see the job. Online advice cannot replace the person in the field, although it can educate the customer and the trade person.

Here's the deal - you sound like you know a lot more than how to swing a hammer. That makes you a general contractor, not a carpenter - that means you have value in how you direct the customer and you can save them money in a number of ways....

Stoves? well, you need a good helper and a strong back. My installer that really made good money also had a great quality - he rarely talked. You are either talking or working and these guys worked! Less than an hour for a perfect fireplace installation and about 3.5 hours for up through the roof with insulated chimney (a job that pays about 500-600 labor).

I probably mentioned this before, but common wisdom says that you can't every make money by the hour, because no one is paying for all those lost hours in traffic, at the lumberyard, paying bills, etc. etc.

There is a great book called "The Million Dollar Consultant" very readable although you might have a tough time finding it. It is very relevant to all these subjects although I have never become (nor do I want to become) a million dollar consultant. The point of the book is basically what I said above - piece work or by the hour one will never attain financial security (I don't know if Mass. is good enough to provide pensions or forever health care for inspectors!)....

I mention health care since that is a very important part of financial security!

In the end it somewhat depends on your goals and what you enjoy. I often thought about being a home inspector, which I would enjoy. But then computers and the net came along and I fell into this - can't beat it! The point is at sometime in your life, your knowledge becomes more important that you muscles...especially for people with experience in multiple fields....

I have a great friend who is into high end remodeling...he's 52, but loves to be involved and swinging hammers, etc. etc. - BUT, he takes only the best clients and the best jobs! He is in a suburb of a east coast city (not unlike Boston) and with a 3 man total operation he probably grosses over a million a year! He probably takes home 200K+ in total value or possibly more...heck, he's putting 4 kids through college and more!

My brother in law lost his job as a banker at 50. He knew very little about contracting, but moved to FL and studied and got his license. Now he is reworking high rises that are decaying because of the climate down there - a small operation but taking in a few million a year gross! You probably know a great deal more than he does (then again, you might not want to live in Miami!).....

So, yeah, although customers here might not want to hear it, you should charge what the market will bear. That doesn't mean you can't do favors for poor folks, but in the end I think you will do the poor more good by making enough money to take a week off and work for Habitat for Humanity and/or give some $$.

Well, that's a long story.....