Properly Sizing a Chimney Liner for Gas or Oil Appliances

Properly Sizing a Chimney Liner for Gas or Oil Appliances

What Causes Moisture in my Gas / Oil Chimney?

Is your chimney leaking when it doesn't rain? If it is, it's likely caused by excessive condensation from a gas furnace or oil boiler. If the flue size is too large for an appliance, it will cause the exhaust gas to condense more quickly creating a lot of moisture in the chimney.

Over time, this excessive moisture can cause the flue to deteriorate. The easiest fix for this is to install a stainless steel chimney liner. But not just any liner will work for your appliance. An undersized chimney liner for a boiler or furnace will cause a limit switch to trip, turning the appliance off. Understanding what measurements and what information you need is crucial for a safe and successful installation.

Measurements Needed for a Properly Sized Chimney Liner

The first item on the list that we need to determine is what type of appliance you have and how many appliances vent into the chimney. It is very common for a gas furnace and hot water heater to exhaust into the same chimney. While oil appliances generally vent into the chimney alone. Note: It is never a good idea to have two different fuels exhaust into the same chimney.

The next item to identify is the BTU capacity for gas appliances and finding the gallons per hour (GPH) firing rate for oil appliances. This information is generally located on the specification panel on the appliance. Most of the time the specs panel is on the access door to the burners. This is the absolute most important piece of information in order to size a liner for a gas or oil appliance. Usually furnaces and boilers will have an exhaust hole that is much smaller than what liner size is required. TIP: Never size a liner based on the size of the exhaust for gas or oil appliances.

Once we have the BTUs and the GPH, the next item to address is to count the amount of elbows that are in the connector pipe that goes from the appliance to the chimney. Each 45 degree elbow in the connector pipe will reduce the max BTU and GPH capacity by 5%. For each 90 degree elbow in the connector pipe, it will reduce the max BTU and GPH capacity by 10%. A connector pipe should never have more than 3 total 90 degree elbows in the system. We also need to measure the lateral length form the appliance to the chimney.

The final measurement we need to determine the correct size liner is the height of the chimney. Measure from the top of the chimney to the top of the thimble and add 3.5" to accommodate the liner clamping to the top plate. Next, measure the exact inside flue measurement that you will be installing the liner down. This will tell you if the correct size liner will fit down your flue. Never make an assumption of the inside measurements of your flue. Always measure the flue.

Measurement Form for Parts List

Here is a link to our measuring form. You can fill this form out and it will provide us the information to put together a parts list for you. Or, call a Rockford Chimney Supply Certified Technician to assist you in selecting the correct size liner.

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Our 316Ti flexible chimney liner and chimney liner components have passed the rigorous testing at the Underwriters Laboratories with best in class status and are UL Listed. So if you are looking for UL listed chimney liner sold directly to homeowners, then look no further than Rockford Chimney

Customer Testimonials

Back in July you gave me excellent advice re. sizing/purchasing a flue liner for the chimney associated with my wood stove. I'd like to thank you once agai... More

—AE Northfield, MN

A few weeks ago I ordered a chimney liner system for my fireplace insert. Your salesman recommended the liner system plus the insulating blanket. I just wa... More

—AH Happy customer

Just got around to installing my new smooth flexible liner and deluxe terra cotta cap (wow). The quality of your product is second to none. Very pleased an... More

—B.B. from Massachusetts

Thanks so much for the great tech service from Dennis, very knowledgable, friendly and proffessional.Your website is the best, beats your competition by fa... More

—Bill in Knoxville

Sue and Dennis, I received the final part of my order yesterday (the chimney cap that actually fits over the chimney chase). It looks wonderfu... More


Hello, I was shopping for an fireplace insert and called RCS to ask why they were so much cheaper than my local guy, the reply was " we sell thousand of th... More

—Chris, PA.