Tom's Blog

Tom Urban is an industry educator, having taught for the Chimney Safety Institute of America, the National Chimney Sweep Guild and numerous regional chimney groups. He has received numerous industry awards. Tom has been a guest on This Old House, NBC's 20/20, interviewed by CNN and writes for industry publications. Tom has over 42 years worth of education in the hearth industry and is a well known expert who has helped create education programs and additions to chimney standards

​How Gaps in Chimney Liners Affect Chimney Draft
By Tom Urban


Don’t you love it when you pull away from a drive-up restaurant, and the straw bends and cracks – it just isn’t working? It has a hole in it — it’s pulling air in with a little bit of your drink.  A drinking straw is an excellent example of suction. In our industry, it’s a good thing if the chimney sucks. In this case, I’m talking about negative pressure. Gaps in flue joints have often been a concern of an unsealed liner system. But do you understand the significance of how it affects the operation of the chimney?

First, you need to understand this before you try to sell a product to your customer. Good sales training starts with product knowledge AND HOW IT AFFECTS THE CUSTOMER! Good salespeople do their homework on ways to relate to their customer’s needs. Poor salespeople just wing it.
We all understand that draft is supposed to go up and carry with it the products of combustion. Additionally, natural draft appliances depend on the amount of draft to function correctly – like a fireplace or appliance. So, what happens when there’s a broken link – like gaps in the mortar joints?
Like a straw, any open spots in the flue weaken the draft. Instead of pulling warm flue gasses up and out, it also pulls dilution air into the flue, cooling and slowing the flow of the flue gasses. Now, this lower functioning chimney starts to affect how the fireplace or appliances function. Instead of running like a well-oiled machine, it’s now running like a gas engine with a broken spark plug.
Let’s look closely at the numbers
If you have a chimney liner with 7″ x7″ interior dimensions and a 1/16th inch gap between the next liner, the open area is close to three square inches. Place nine more liners on top of this, with the same 1/16th of an inch opening, and collectively, you have 30.38 square inches. This opening is larger than a 6″ hole, which is about the size of a dessert plate. So that “not so bad mortar joint” may not be serving the chimney/fireplace/appliance well.
In another example, let’s ramp it up a little by taking a 13″ x13″ liner – typical for a fireplace chimney flue size. Consider a one-inch gap between the liners, which is not uncommon. We have 52 square inches of open area in one liner joint. That’s larger than an 8″ hole, which is equivalent to a small plate. Add four more liners to the top of this one, and you have a whopping 260″ sq. inch opening, larger than an 18″ round hole— the size of a turkey platter! Now, do you understand how important this is?
But what about an appliance?
Woodburning stoves that vent into flue tiles run the risk of having draft problems too. Imagine a woodstove with a 6″ outlet venting into an 8″x8″ flue with multiple gaps. The gaps combined could easily be larger in area than the 6″ flue collar at 28″ of area. Hmm, this could provide the answer to why there is so much soot and condensation. Maybe it’s not the homeowner’s bad burning practices.  Change the appliance fuel to gas, which has cooler flue gasses, and with dilution air, there are even more issues and condensation.
How do you pinpoint the joint gaps and the size?  
Your inspection documentation is only as good as the quality of your photographs. The old saying a picture is worth a thousand words rings true when educating your customer. A picture of a dark, fuzzy gap with blurred lines is doing your customer a disservice. Poor quality pictures can easily mislead. How can you verify the damage when the evidence is in question? What if you are dealing with an adjustor or code official who has a little more knowledge than a homeowner. What if you have to go up against a competitor and they have quality pictures that put yours to shame? Be very careful what you write and point out if you are using flawed photos. You can have a good argument with clear, crisp pictures; if the pictures are flawed, there can be no defining evidence of crisp lines, textures, patterns, and shades such as smoke damage and condensation creosote, and flame paths. A camera with advanced lighting and high resolution can pinpoint small defects not visible in lower quality cameras. Multiple viewing angles with the ability to adjust the viewing angle at any time provides an immense advantage over your competitors because you can identify defects they miss.
Get the idea? Here’s something else to keep in mind.
There are state and national building code recommendations concerning gaps in flues. Even though codes explain in detail how to join clay liners, both NFPA 211 and IRC codes understand that the smallest hole can create a loss in performance when trying to exhaust smoke and gases from a fireplace or appliance. Gaps can create uncertainty on how the fireplace or appliance operates under all conditions. The chimney structure is designed to support the liners, not exhaust the hot smoke and gases.
Uncertainty is not your friend. 
And do you think your customer is going to understand the codes? Most likely not, or do they care, or can they relate? Again, probably not. But they do know what it is like to have a hole in their straw, sucking their brains out trying to taste the beverage. That is something they can relate to and may come to understand why their chimney is not performing.
Remember, there are various solutions to stop the leaking, whether a chimney reline, resurface, or joint repair. If you were wondering if gaps are a big deal, yes, they are! Even though the chimney may appear to operate, it is doing so with limited capacity. The chances are that the fireplace or appliance is not performing at its full efficiency. Also, premature deterioration in the chimney happens at an accelerated rate when there are gaps in the flue joint. This issue is why the code bodies specify that chimney flues need to be a sealed system. It’s that important.
Today’s technology and increased demands for efficiency and performance are higher than ever. Today’s appliances need a top-performing chimney. 
Chimney Gaps and Moisture
Even though there may be brick or block surrounding the liners, air could still leak into the chimney envelope. This leaking during the burning season can contribute to the summertime damp/smoky odor your customer may experience from time to time. Instead of chimney drafting, the home’s needs and other appliances overpower the draft dumping the smell into your home.
When the smoke and hot flue gasses cool, they condense on the flue liner’s walls and roll down to the gap and enter into the interior chimney cavity between the outside of the flue liner and the inside of the chimney walls. (Not good)! Explaining this to your customer can be quickly done by visualizing a sweating icy cold drink on a hot day. This visualization is a lot easier for them to understand than the above paragraph. YOU NEED TO UNDERSTAND THIS! I think this industry has forgotten some basic principles on the physics of chimneys and finding ways to relate to their customer a complex process without jumping on the “it does not comply with NFPA 211. Do not use your chimney, and it is a hazard.”
I especially love the folks who use this language on a site-built chimney or fireplace in a state that adopted IRC!  States that adopted IRC do not recognize NFPA 211 as code. NFPA 211 is the national inspection standard for the levels of inspection. People send me reports to review, and it amazes me that they use NFPA 211 on-site built chimneys or fireplaces in IRC adopted states. By submitting reports based on NFPA, sweeps tell the Authority Having Jurisdiction the sweep does not understand the state/local building codestherefore, their inspection reports have little value.
Communicating the Options to Your Customer
What is your communication style? How do you think your customer would respond to these scenarios below?
Option 1: Our interior chimney inspection camera revealed open mortar joints between the chimney liners’ sections during the inspection process. The protective nature of the liners is compromised. This condition may reduce the chimney’s capacity to vent or allow gases and condensates to escape the flue liner to the chimney’s exterior. If these conditions remain, the gases or condensates can prematurely deteriorate the chimney. Or in other cases, it provides areas for dilution air, reducing draft and limiting its capacity to vent all combustion products properly. Depending on the amount and exposure rate, condensates or creosote may produce a potential hazard to the structure. There are repair solutions that can correct and elevate the problem. We recommend completing the repairs as soon as possible to bring your chimney to full operational ready condition. Or use this – sweeps choice – We recommend completing corrective actions before further use of the fireplace and chimney.    
In option 1, the customer is educated, knows the problem’s location, what’s happening with their chimney, potential problems, and choice(s) to correct it. Plus, I’ve used prewritten explanatory paragraphs. These are ready in advance (from my library), easy to plug into my report, and I don’t have to waste time writing in the service truck, then waste their time trying to explain it. They can read it while I explain it to them. Then they can reread it later. They can show it to their spouse when they get home. I just saved 10-15 minutes of their time and mine. By the way, the paragraph from Option 1 is from the Urban Inspection Language™. We included both “recommendation” sentences because some circumstances may require one or both sentences. Now let’s look at option 2.
Option 2: In the process of our inspection, we determined your chimney has gaps between the mortar joints. NFPA 211 and the International Residential Code both state that your chimney needs relining with a UL Listed Liner because of the gaps. These gaps could result in death or property fire. If you have people staying in your home, you could be responsible for not taking action to correct this problem if they are present at the time of the fire. We can’t predict when the fire could happen, but because pyrolysis works over a period of time, nearby combustibles could spontaneously ignite. You have to stop using the chimney, and we need to reline it before you use it again. If you fail to do this, you and your family could perish, and you could lose your home. Sign here.
With option 2, I polled some friends. They felt this language was harsh and intimidating. They voted to get a second and maybe third opinion (which means you lost the job). Intimidation rarely works. Intimidation and fear most likely isn’t the intention, but it comes across this way. When buying, people like to hire transparency, communication, value, and trust. When they hear or read this type of language, most people start to blank out. They feel intimidated by legal

language, our technical industry language they don’t understand and may see you as a bully telling them what they have to do. Customers may feel themselves losing control and feel that you are trying to use scare tactics, which rarely works. People don’t like to hear legal language unless they are an attorney. Who wants to hear hard to understand legal language? They are hiring you, not a lawyer. The best salespeople understand this principle; it’s not what you say or what you do. It’s how you make them feel and understand. Sell the defect first, so the customer understands the issue, and the product sells itself. So if you wish to have the customer trust you, you need to do your homework and understand these basic principles. Then find a relatable way like a straw and sweating glass so they can relate to it. You need to know the codes; you need to know the inspection levels; your customer doesn’t. If they ask, then tell them.
A Resolution for Your Customer
The next time you come across a fireplace with smoke stains at the top of the opening or a customer who complains that the fireplace is smoking or not drafting well, scan the chimney for openings in the flue. Remember that there may be a large, combined opening space in the flue where dilution air affects the draft. Keep in mind that this type of issue can also affect appliances, not only fireplaces. Appliances generally require a smaller flue, which can exacerbate the problem when the dilution hole combined areas are roughly as large as the appliance flue collar. Gas and oil flues run at a lower tem

perature and may exhibit more problems than flues venting solid fuel appliances. When communicating these issues to your customer, I suggest doing it in such a way as to educate them, so they understand the defect, and then the solution (product) sells itself. Regardless, now you have another tool to put in your toolbox when diagnosing venting problems and learned a few new ways to explain them. Please use the copy if you like, I’ll write more!