Correctly Sizing Chimney Liner for Propane Furnace Exhaust

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Caleno22

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Oct 19, 2015
7
Nebraska
I upgraded to a newer high-efficiency furnace (not over 90%) which exhausts into my existing chimney. Last winter I had moisture weeping thru the chimney block. In researching the issue, I need a liner to stop this issue. So my question is: How big a diameter liner do I need? I currently have a cap on my chimney so no rain or birds can enter. My furnace exhausts about 4 1/2 - 5' from the base of the chimney stack into an existing hole where an old cook stove used to connect. It is in the center of my house, all heated living space. My chimney inside diameter is: 11 1/8" x 6 5/8". Do I really need something that large - that sizing is quite costly to say the least. Could I use a 4" or 6". I still need enough room to connect a Tee to the furnace flue.

I see that most are round so wondering if they come in an oval shape too?
 
You need to look at the installation manual for the furnace that you have.
The venting specifications & restrictions will be there.
All we can do is guess & that's not a good idea when dealing with combustion &
its particular by-products.
 
Last edited:
You need to look at the installation manual for the furnace tha you have.
The venting specifications & restrictions will be there.
All we can do is guess & that's not a good idea when dealing with combustion &
its particular by-products.
Thanks. That's a huge help
 
Not sure if your reply is a snide remark or not, but I stand by my answer.
Your install manual will indicate what sizes are approved, & the minimum
& maximum venting configurations for those sizes for your particular heating unit.
You will also need to know the height of your chimney, as that too, will have to be
considered into the venting calculations.
 
It is usually the same size as the outlet on the furnace but that can change depending on the height. Like daksy said you need to check the manual
 
I upgraded to a newer high-efficiency furnace (not over 90%) which exhausts into my existing chimney. Last winter I had moisture weeping thru the chimney block. In researching the issue, I need a liner to stop this issue. So my question is: How big a diameter liner do I need? I currently have a cap on my chimney so no rain or birds can enter. My furnace exhausts about 4 1/2 - 5' from the base of the chimney stack into an existing hole where an old cook stove used to connect. It is in the center of my house, all heated living space. My chimney inside diameter is: 11 1/8" x 6 5/8". Do I really need something that large - that sizing is quite costly to say the least. Could I use a 4" or 6". I still need enough room to connect a Tee to the furnace flue.

I see that most are round so wondering if they come in an oval shape too?
Does your furnace use pvc pipe or metal pipe for the exhaust? Either way you just need to extend that pipe through the length of the chimney out the top.
 
Either way you just need to extend that pipe through the length of the chimney out the top.
not always it can change depending on the height
 
It is usually the same size as the outlet on the furnace but that can change depending on the height. Like daksy said you need to check the manual
Had carrier email me an install manual. It shows 4". My entire chimney measures 30' and the flue connects 4-5' from the bottom.
 
What does the max length say in that manual?
ok, that part may be my challenge. I found another area, discussing the ventilation "Table 2...Min Free Area Required for Combustion Air Opening or Duct to Outdoors" My furnace is 66,000 BTUH and for single duct work is states Round Duct 6" and Free area of Opening and Duct (sq in.) is 22.
It stated some models may require 6"; however, the initial flue pipe is 4".
 
ok, that part may be my challenge. I found another area, discussing the ventilation "Table 2...Min Free Area Required for Combustion Air Opening or Duct to Outdoors" My furnace is 66,000 BTUH and for single duct work is states Round Duct 6" and Free area of Opening and Duct (sq in.) is 22.
It stated some models may require 6"; however, the initial flue pipe is 4".
If you can upload the manual on here I'm sure one of use can help you figure it out.
 

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Looks like the exhaust should be 4"
I think the other sizes is for intake depending on how many other gas appliances you have and total btus.
 
Wow, that was a nice team effort! Nobody got their underwear bunched, and a 4" flex liner should be a relatively low cost solution in that good size tiled flue.

But looking back, it is predictable that a tile flue that big would weep water from day one. Especially if the furnace was short cycling.
 
But looking back, it is predictable that a tile flue that big would weep water from day one. Especially if the furnace was short cycling.
We put in allot of liners because of this. Older lower efficiency furnaces worked fine in clay chimneys but when the efficiency went up there is allot more moisture present and it tears the old clay up pretty quick. Also the old chimney are usually much to large which means the gasses expand quickly and condense more.
 
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Wow, that was a nice team effort! Nobody got their underwear bunched, and a 4" flex liner should be a relatively low cost solution in that good size tiled flue.

But looking back, it is predictable that a tile flue that big would weep water from day one. Especially if the furnace was short cycling.

I very much appreciated all the help. I will be picking my material by next weekend to install and have connected to my furnace flue (by a professional :) I had a 1989 furnace that came with the house and felt it was time to upgrade. My brick chimney never had a problem (with old furnace) until it was well into the winter months and it cried like a baby there was that much coming thru. Did my research, lots of Google help and it also led me here with even better results. Even the HVAC company didn't hone in on that one either! However, everything else was done to perfection and how I wanted it.

I appreciate everyone's time and effort to assist.
 
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I very much appreciated all the help. I will be picking my material by next weekend to install and have connected to my furnace flue (by a professional :) I had a 1989 furnace that came with the house and felt it was time to upgrade. My brick chimney never had a problem (with old furnace) until it was well into the winter months and it cried like a baby there was that much coming thru. Did my research, lots of Google help and it also led me here with even better results. Even the HVAC company didn't hone in on that one either! However, everything else was done to perfection and how I wanted it.

I appreciate everyone's time and effort to assist.
Insulation is not required for a furnace but it will reduce the condensation allot and make for more stable draft. So i would consider it we insulate all of our liners even for furnaces
 
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