Switching from Wood Stove to Pellet Stove - 10" Pipe to flue

[nhmtb]

Hi,

I am looking to vent a new to me pellet stove where my old wood stove was located. There is a 10" round hole to the flue lined chimney.
Wondering if anyone has any experience or suggestions reducing the 10" down to 3" or 4" for the pellet stove vent.
I see a lot of 6" to 3" but figuring I am going to need a couple of reducers to accomplish venting. Alternatively it might be easier to run flex pipe through a collar up the chimney - 15 or so feet rise.

Pellet stove is a Harman XXV -

Thanks much,
Bob

 

[Washed-Up]

Here’s an install I did of a P61-A using a 10” existing setup

 

[Oregun]

The XXV stove has an option that bolts to the back of the stove you can get that vents vertical to a 6" pipe.
You can see it in the install manual.

 

[Dataman]

I switched from Wood Stove to Harmon XXV in Spring 2018. I just used chimney that was there. 8" works fine for pellet stove. Clean it once per year.

 

[nhmtb]

Hi,

I am looking to vent a new to me pellet stove where my old wood stove was located. There is a 10" round hole to the flue lined chimney.
Wondering if anyone has any experience or suggestions reducing the 10" down to 3" or 4" for the pellet stove vent.
I see a lot of 6" to 3" but figuring I am going to need a couple of reducers to accomplish venting. Alternatively it might be easier to run flex pipe through a collar up the chimney - 15 or so feet rise.

Pellet stove is a Harman XXV -

Thanks much,
Bob

 

[Whitenuckler]

You will need 4 inch anyway all the way up. There is someone else posting right now that is doing it, and he is adding his OAK cold air too.
The latest post was about a custom transit plate.

F

Post in thread 'Ussc 6041i'

Sep 8, 2025

so I've got the old wood stove out, planning the block off plate. ill be running the 4" ss flex along with a 3" aluminum fresh air line up the chimney. from all the stuff I've seen for block off plates people just cut the smallest hole possible to allow the liner though, I'm thinking about welding a 4" and 3" section on pipe into the block off plate. the top part of the liner would attach to the top and then the lower part that connects to the back of the pellet stove would attach to the bottom part of the block off plate. hopefully that makes sense?

Edit:added terrible drawing

• Fordguy64

• 

 

[Dyammons]

I switched from Wood Stove to Harmon XXV in Spring 2018. I just used chimney that was there. 8" works fine for pellet stove. Clean it once per year.

Nice to see you can connect into an existing 8” pipe. What adaptor did you end up using to connect the pellet stove to the existing 8” flue?

 

[Whitenuckler]

Nice to see you can connect into an existing 8” pipe. What adaptor did you end up using to connect the pellet stove to the existing 8” flue?

I'd check your manual to see what size it says you need.

 

[Dyammons]

I'd check your manual to see what size it says you need.

I can’t find any clear recommendations.

I am trying to connect a Harmon p68 with 3” duravent to an existing chimney with an 8” single wall flue.

Previous owners had 3” duravent going to 8” insul-flue thimble which does not seem appropriate. I am thinking of running 3” duravent through a Dura Vent DuraPlus 8 in. Diameter Wall Thimble then extending 3” duravent into the existing 8” single wall flue until a few inches before the flue goes vertical. Does that sound reasonable?

 

[Whitenuckler]

I can’t find any clear recommendations.

I am trying to connect a Harmon p68 with 3” duravent to an existing chimney with an 8” single wall flue.

Previous owners had 3” duravent going to 8” insul-flue thimble which does not seem appropriate. I am thinking of running 3” duravent through a Dura Vent DuraPlus 8 in. Diameter Wall Thimble then extending 3” duravent into the existing 8” single wall flue until a few inches before the flue goes vertical. Does that sound reasonable?

https://downloads.hearthnhome.com/installmanuals/p43_61a_68_i.pdf

Starting at figure 4.3 and then 4.4 ect they do have options for you to use an existing chimney but recommend a 4" liner. I'd have 4" all the way to the top in the chimney. Then use double wall 4" duravent pellet to get into the liner.

It might be hard to install this way and clean. I had a P43 installed in my basement with all new 4 inch excelpellet. It worked out well as they sell an adapter to go from 3 inch to 4 inch. They went through the basement wall with the pipe.

I think that last OP Fordguy64 would be the one that can help. I do know people have a hard time connecting their 3 inch stove to duravent 4" double wall. I don't think they have the Harman adapter that I was able to use.

 

[Washed-Up]

You’ll need a clean out T off the back of the stove to the flex pipe through a collar/thimble up the chimney with either a modified cap or a prefabricated reducer. I’ve used a 4” hole saw with a solid cap in past installs

 

[Joe.G]

My Buddy tried to run his pellet stove using the wood stove chimney, we had an adapter that matted the two, We could never get the stove to work right.

 

[Whitenuckler]

My Buddy tried to run his pellet stove using the wood stove chimney, we had an adapter that matted the two, We could never get the stove to work right.

Pellets stoves have a combustion blower which is totally different than a natural draft woodstove. I'm not sure of the science behind it, but it might be like the exhaust on an ICE. The engine/valves ect expect a certain back pressure. Best to do it right the first time.