Size matters... or does it? (flue size)

[embers]

I have a 7-inch SS flex pipe running up a 20ft masonry chimney. Two years ago I installed a Hearthstone Clydesdale insert and haven't been completely happy with the performance. The specs call for a 6-inch exhaust. Would I experience a noticable difference if I change to a 6-inch pipe? I think I have enough room to run a 6-inch flex SS inside the existing 7-inch, which would make the install pretty easy, but I was wondering if this is would be worth the effort. Disregarding the mathmatics of the situation, does anybody have a real-world experience of reducing their flue size by one inch?

I'm an experienced woodburner. I bought the Clydesdale to replace a 20-year Vermont Castings Winterwarm which was getting near end-of-life and had several catalytic replacements. The Clydesdale is not catalytic, but I am quite familiar with the pros and cons of each. I'm just wondering if a 6-inch flue run for the full 20ft would help to increase the heat output of the Clydesdale. I really like the new insert and will probably keep it for a long time.

 

[rdust]

How is the stove performing on the 7"? Does it get up to temp, does it get good secondaries, does it respond to changes in the primary air control? If the flue cross section is too large the stoves overall performance will suffer. If the stove is getting up to temp and performing I don't see a need to switch.

What exactly about the performance aren't you happy with? You mentioned "heat output" what type of stove temps are you seeing?

 

[embers]

Yes, good primary air control. The control is sensitive when coming off fully closed. There's a fine line between closed too much and open too much. It could just be the nature of this stove. Secondary burn is good. It does seem to eat up wood a lot faster than the old winterwarm, and the winterwarm seemed to crank out the heat at a much higher and consistent level, but I don't have good data for comparison. I had permanent installed pyrometer with a long probe installed on the old insert, which I used as a guide for engaging the catalytic at the appropriate temperature. I don't have a thermometer on the Clydesdale, so I can't give you numbers on the temps. I just had an expectation that it would kick out the heat just as well, or possibly better than the old Winterwarm as it has a slightly higher btu rating. I'll have find a thermometer I can use on the Clydesdale so I can monitor the temp. Sort of hard to find one that will work because of the insert design.

 

[DBoon]

While a 1" sized larger liner doesn't sound that much larger, the increase in area of the cross-sectional area is 36% - pretty significant. The description of the problem you provided sounds like it could be caused either by wood that is not quite dry enough or by a lazy draft that is not pulling air through the primaries and secondaries well enough.

 

[ScotO]

What size pipe does the Clydesdale specs call for? One thing to remember, as a general rule of thumb, is you would be better off with a stovepipe slightly smaller than called for rather than too big. Like DBoon said above, there is a significant increase in area in the 7" pipe over the 6" pipe. 36% is quite a difference.

 

[fire_man]

embers:

You definitely want to rule out the "easy" things first. Before you start swapping liners, make sure you are burning good dry wood. The older stoves could burn anything and put out good heat, not true with a new EPA stove, it needs good wood. I am surprised you are using more wood than expected, makes me think you have some kind of air leak- is your door gasket seated properly and door adjusted correctly? Is your 7" liner tight so it can't be sucking air anyplace - like at the "T"?

I checked a few reviews for that stove - it's a well liked unit so it looks like once you figure it out you will be happy.

Don't forget it's still kind of warm out, so draft suffers for everybody, especially those with marginal flue setups. If yours is an uninsulated liner, it does not help matters - but your tall 20" flue is a good thing.

So my answer is make sure you are burning well seasoned wood (>1 year unless oak, then >2 years), check for air leaks, and wait for colder temps to see if the stove burns better (and report back!).

 

[Blue Vomit]

I ran our Oslo with an 8" liner last year. A week ago I dropped in a new 6" liner in hopes of better performance. Last year wasn't bad, just looking for better.
If you do decide to drop the 6" liner in, I can tell you it was very easy for me... And I don't do DIY. If I did it, most can.
I plan on posting about the differences after a few burns.
Good luck.

 

[begreen]

7" is not a huge jump and the 20ft height is good. However, if you are in a warmer climate then you may benefit from a bit more draft.

Where are you located and what were the outdoor temps when you were burning? What kind of wood are you burning and is it fully seasoned?

 

[embers]

Yes, I live in a warmer, mediterranean-type climate, with no snow and winter temps 30-50 degrees. Yup, the wood is well seasoned. Been burning for a long time and that's one of the more important things I learned throughout the years, but I agree that is one of the first things to ask.

Specs for the Clydesdale call for 6-inch pipe. I also just found out the old insert (VC Winterwarm) called for a 8-inch, but I was running a 7-inch, so that probably gave a little extra advantage. I'll let you know if drop in a new 6-inch liner for the Clydesdale. I'm strongly leaning in that direction. Now I just have to research what brand liner I want to use. Will probably try to find a smooth-walled flex. Thanks for all the excellent comments and tips.