Is that open space between chimney liners that big a deal?

Is that open space between chimney liners that big a deal?

While scanning chimneys, and Chim-Scanning on a day-to-day basis, you've most likely come across chimney liners not sealed to each other. You may think “well that’s not that bad, I don’t want to alarm anybody over a 1/8” gap. Let me finish sweeping here, and I will give it some thought on how I can address this. Well, I know it’s another hole in a poorly constructed chimney, I don't know how to tell them, so I’ll let it go.”

Think again.

Let’s look at this differently. If you have a 7x7” liner and a 1/16th inch gap between the liners, the open area equals 3 square inches. Place 9 more liners on top of this with the same 1/16th of an inch opening in each joint and collectively you have 30.38 square inches which is larger than a 6” thimble hole. So that “not so bad mortar joint” may not be serving your customer very well.

In another example, let’s ramp it up a little by taking a 13 x13” liner and have a one-inch opening between the liners. We have 52 Sq. inches in just one joint area. That’s larger than an 8” thimble hole. Add 4 more liners to the top of this one and you have a whopping 260 square inches of opening - larger than an 18” round thimble. 

Get the idea?

If chimneys are to be under a negative pressure or vacuum, then how can we expect the chimney or fireplace to operate in an acceptable manner?

If that doesn't make you ponder, place this example chimney in newer construction or renovation of an older home and this could become a real performance problem. Even though there may be brick or block surrounding the liners, air will still leak into the chimney. Conversely condensates and or creosote can leave the liner area and enter into the chimney cavity.

Something to keep in mind

Just apply the recommendations from the codes (which is the minimum standard) to your thought pattern and understand this principle. Even though the codes do explain in detail how clay liners are to be joined. (NFPA 211 or IRC) Both code bodies 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 will operate under all conditions. Smoke and gases can move into other areas within the chimney cavity creating more uncertainty (Uncertainty is not your friend).

Remember, you have a variety of solutions to stop the leaking, whether it is a reline, resurface, or joint repair. If you are wondering if this is a big deal, yes, it is.

What to tell your customer:

*Their chimney falls below recommended building codes.

* Even though the chimney may appear to operate, it is doing so in a limited ability. And the condition will only prematurely deteriorate the chimney at an accelerated rate.

*If they're asking you “is it safe to use?” You may respond, "I cannot offer you any peace of mind in its current condition".

Today's technology and higher demands for efficiency and performance are now ramped up higher than before. So, yesterday's thought process will not work with today's reality.

Here’s a demonstration to easily explain this problem.

· Take a beverage straw and make a small hole in it.

· Place straw into beverage and draw on it. 

· Once the beverage drops below the hole, there should be a sound coming from the straw plus you'll have to work twice as hard to draw the beverage through the straw.

· This simple concept is the same with the chimney.

Now Mr. & Mrs. Smith, do you understand why there’s smoke staining on the front of your fireplace?