Rockford Chimney Supply: Gas Heating Appliance Measurements
If you’re installing a chimney liner for your residential gas furnace, boiler, or water heater, this guide will help you tremendously! We’ve narrowed in on all of the measurements you’ll need to gather.

One more thing to keep in mind before you move on to the guide - this particular guide is for relining the masonry chimney to vent a residential gas burning furnace, boiler, or water heater.

Notes: Rockford's smooth wall flex liner or rigid chimney liners are recommended for gas appliances. The smooth interior wall will allow up to 20% more BTU's for the liner as opposed to the standard corrugated liner.
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A1: What is the BTU input for each appliance?
A2: What is the efficiency rating of the appliance? If you are unable to locate the efficiency rating, you can calculate the efficiency by dividing the BTU output by the BTU input.
A3: Is the appliance natural draft or fan assisted?
B1: How many 90 degree elbows are in the connector pipe that connects the appliance TO the current chimney?
B2: How many 45 degree elbows are in the connector pipe that connect the appliance TO the chimney?
B3: What is the lateral/horizontal distance from the appliance to the side of the chimney?
B4: What is the diameter of the connector pipe that is connected to the chimney?
C1: What is the depth (or thickness) of the chimney thimble?
C2: Does the current chimney have a terra-cotta clay flue inside? If so, does the flue extend above the top of the chimney?
C3: What are the exact inside measurements of the existing chimney flue?
C4: What is the height of the chimney from the thimble hole to the top of the chimney?
D1: Every installation is different and nothing is standard in the chimney world! Please provide any additional information about your setup here if needed.
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