Residual Heat from Tile Flue vs Lined Flue

[Longboat]

Hello, forgive me if this question has been addressed before. I wasn't able to find it in my searches. BTW, I've been lurking for years, and have found your forum to be a treasure-trove of knowledge.
Here it goes: I have a Drolet Legend stove, installed in an uninsulated basement in 2013. I previously had an older gas furnace venting into a 31' brick chimney lined with 7x11" clay tile. I had it inspected and was cleared to vent my wood furnace into the now-unused chimney. I know the 7x11 is a bit too large for this stove, and the recommended chimney is a 6" round. My draft is acceptable, but I would like to install a liner in anticipation of better efficiency. Here's my dilemma: The stove makes the brick (center-of-the-house) chimney very nice and warm. This heat is held and slowly warms the house kind of like a masonry fireplace. This effect alone keeps my main source of heat, a new high-efficiency gas furnace, from running most of the day. If I drop an uninsulated liner through, and insulate just the attic-on-up, how much heat can still be captured by my brick chimney? Do you think I will lose the heat that was captured before the liner was installed? Would that loss be offset by the better efficiency gained by my stove with the recommended diameter flue?

 

[EatenByLimestone]

Welcome to the forum!


A bit large is an understatement. A 6" flue as an area of just over 28 in square. Your current flue has 77 in sq.

My fear is in marginal weather you could lose draft and carbon monoxide could fall into the home. The stove willbemuch easier to start with the smaller flue also.

 

[bholler]

Well the first question to ask is do you have the proper 2" clearance between the outer layer of masonry and any combustible materials? If not you need a full insulted liner and the chimney should have never been cleared for use with a wood stove. You already know that masonry heats up considerably over time that will lower the kindling point of the surrounding wood and if there is a chimney fire there is a possibility of starting a fire. Not to mention the fact that loosing all of that heat from the exhaust hurts efficiency and increases creosote buildup.

 

[Grisu]

I hazard the guess that you get that additional heat from the chimney because you are leaving the air control of the stove more open to keep the fire going. Thus, you are stealing the heat from the chimney from the stove. Once you have the liner you will be able to close the air more, which will result in a hotter stove and a more efficient burn. A liner will give you a better performing stove, an easier to clean flue, less creosote formation, and an added layer of safety in case you have a chimney fire. Given the length and dimensions of your chimney an insulated 5.5" liner would probably work and be a bit easier to install.

Btw. With the stove in an uninsulated basement you are losing a ton of heat to the surrounding ground. I suggest to either insulate the basement or move the stove to the actual living area.

 

[Longboat]

Yeah, the fire goes out when I close the air off all the way. It needs to be open a quarter inch, so the effect is that it cruises real hot and then dies right down once it turns to coals. I have 1year old maple, and the moisture is measured at 20 to 25 percent. I know- should be better seasoned, but it's my first full year with a wood stove. I get some black seepage at the bottom of the chimney when it gets above 30 outside, so there's the creosote issue too. I'll not question it any further and just get the liner and drop it thru the tile. I like the recommendation of getting 5.5 inch liner for ease of installation, if it's okay to go smaller than the standard six inch lining. The clay tile has proper clearance from the brick chimney, so that's okay. Regarding insulation in my basement, one step at a time here. I'll get there as I move down the list of gottagetterdones.

 

[bholler]

The clay tile has proper clearance from the brick chimney, so that's okay.

you also need clearance from the outside of the chimney to combustibles just the space between the liner and the masonry is not enough. You said it was an interior chimney so you would need 2" from the outside of the masonry to any combustibles in order to be ok without insulation. Here is a link to a copy of the applicable code
http://www.rumford.com/code/clearances.html

 

[Longboat]

Thanks bholler, I checked it out and that part seems to be okay, at least. Seems the previous owner burned wood in an old wood furnace up until the late '80s, and then stuck a gas furnace in it's place.
It's funny, but I took the salesperson's word for it (A local shop called L&M Supply) when he said this stove would be fine on the 7x11 flue and not to worry about the 6" recommendation with a 30' chimney. They usually know their stuff, but I guess sometimes not as much as the experts. I'm going to put the liner in. One final question: Grisu suggested a 5.5" liner for ease of installation. You think a 6" flexible liner would be a problem to pull through? Is 5.5" still good? If I'm going through the time and expense of doing this job, I want to do it right the first time!

 

[bholler]

If i was doing it i would break out the old liners and put in an insulated 6" round but a 5.5" will work at 30' but i still strongly recommend that you insulate it

 

[Nick Mystic]

I have a somewhat similar situation to yours, except I have two stoves. I have a freestanding Jotul F600 in front of the fireplace on the main level venting with a 6" SS flex liner(no insulation) going up through a masonry chimney of about 20'. I have a second stove in my lower level that came with the house. It is a Woodstock classic and vents up through a clay tile lined 28' chimney running up through the center of the house. The stove is connected to the flue with a 6" metal stove pipe into a clay thimble. Even with that large clay tile flue (7"x11" - same as yours) the stove drafts fine and the flue stays very clean.

On the Jotul with the uninsulated flex liner I have a block off plate inside the fireplace and a sealed top plate on the top of the chimney. Enough heat comes off the stove and flex liner to heat the huge brick chimney to act somewhat like a masonry heater. I can shoot the chimney between the stove and the heat sheilded mantle with an IR thermometer and pick up temperatures over 200F when the stove is burning and still well over 100F the next morning. So, in answer to your original question I believe your thinking is correct in that you can still get some decent heat out of the chimney if you run the flex liner without insulation. Yes, you will get more creosote in the cooler running flue, but I haven't found that to be a problem during the two years I've had this set up so far.

 

[Longboat]

Thanks for the great replies. Nick, that's a great looking fireplace! I would rather not knock out the clay tile flue, not so much because of the effort, but I don't think I can justify the extra expense of running insulated rigid liners up the entire length. Glad for all your input. Cheers!

 

[bholler]

Thanks for the great replies. Nick, that's a great looking fireplace! I would rather not knock out the clay tile flue, not so much because of the effort, but I don't think I can justify the extra expense of running insulated rigid liners up the entire length. Glad for all your input. Cheers!

You would probably not run rigid most do not and we never install rigid anymore. But as long as you have that 2" gap between the chimney and any wood the whole way top to bottom there is no code saying you need to insulate it but honestly it will work much better

 

[branchburner]

Do you think I will lose the heat that was captured before the liner was installed? Would that loss be offset by the better efficiency gained by my stove with the recommended diameter flue?

I think it would be offset by gains in stove efficiency. When you think about it, one purpose of a smaller insulated liner is that you DO lose some heat, hopefully just enough to prevent creosote formation. If the gasses cool enough in the chimney to condense on the clay tile, yes, more heat is being transferred to the tile. But is all that heat then getting transferred indoors? No. And is that heat as useful as the heat directly thrown by the stove? I don't think so.

As for insulating your liner, so long as you have the clearances to meet code, I don't think it's vital for a center chimney. I did insulate mine, and even so the block chimney upstairs still throws some heat (too bad it's only helping to heat a closet).