Must the chimney be ALL metal lined?

[suemarkp]

I have removed an oil furnace from my basement and want to put a woodstove in its place. My brick chimney is clay lined and appears to be in good condition. However, it is a little large. It measures 5.75 x 9.75 inside and is exposed to the weather on 3 sides. This seems just barely legal for a stove with a 6" smoke pipe. Its is 25' high if that matters.

I had an installer out and he said they couldn't do a liner -- too hard as oval flex would be required, and the manufacturer said I'd have a lazy flue without a liner. I've since found a great online source who sells 6" flex chimney and can squareize or rectangularize it. That will give me a proper chimney that I can fit down my clay liner (there is an angled bend in it, so it must be flex). However, I don't know if I can fit the thimble down the masonry chimney or find a way to put it at the bottom first.

So finally now my question: Could I just run metal chimney flex pipe to near the bottom of my clay lined chimney and not use a thimble? I could fill in the area outside the flex with refractory cement to force the smoke into the liner instead of around it, and similarly put a 6" round stub into the concrete wall also sealed with refractory cement around the outside diameter. But I don't know if the smoke will eat at this cement or if this approach will cause problems.

Any comments? Should I just get a sledge and knock a big enough hole to get a metal thimble in there instead (that's a lot of work)?

Thanks.

 

[kevinmoelk]

I think you're asking if you can use the wall of the chimney itself as a thimble? If so I'd advise against it. Knock the hole in, and make the correct connection with the correct pieces. By the way, don't use a sledgehammer, lol. Get a hammer drill (you can rent these) and drill a bunch of small holes to create the proper shape you desire, and then knock out the small pieces with a regular hammer. A sledge would be too destructive in this case, and not offer enough control.

Others will chime in here, but I also have some reservations about using an old furnace chimney. These chimneys are often not well made, and I'm wondering about the safety of using one for a woodstove, even with a liner.

-Kevin

 

[berlin]

IF the chimney is in good condition i wouldn't hesitate to use it with a woodstove w/out relining, i have experience with stoves installed in "8x12" liners and they've been excellent even though the stove uses a six inch flue. however, they were interior chimneys, it would be ideal if it was in an interior location though. with the furnace removed how is the draft from the thimble? if there is still signifigant draft without the heat of the furnace supplied, i would suggest that it would be fine.

 

[suemarkp]

How do I check the draft? I can open the cover plate in the basement, and I assume I need to hold a smoldering piece of paper or stick there. How do you judge good draft?

I think the chimney is in good shape. It is roughly 2' x 5' with the 1st floor fireplace chimney on the left side and the oil chimney on the right. It's one huge brick structure on the side of my house, and it was built in 1966.

Yes, I want to use the clay chimney liner as a thimble. Since a clay flue in good shape should be OK, that is why I thought just a short section would be OK. The difficult part seems to be sealing it, which is why I brought up refractory cement. My intended use is to have a backup heat source when the power goes out, so I wouldn't use it much. I'd like to just use the existing clay liner, but I can't seem to find any installers that are willing to do that. If it doesn't work well, I suppose I could add the flex liner later, but I'd like to skip the metal thimble part of that solution because of the demolition required.

 

[PaulyV]

Dont assume the chimney is in good shape...have a qualified sweep come check it out. They can also use a little movie/ video camera to check the liner all the way up. Be safe.

 

[BrotherBart]

What is the setup on the thimble end down in the basement? That is, is it a round clay pipe through a solid basement wall ending at the chimney flue tiles? If so what is the diameter of the clay pipe? Is the pipe flush with the flue tiles where it joins the or does it protrude into the flue? Does it have an outside cleanout door and if so how far is it below where the thimble enters the chimney?

A few pictures would be helpful here.

 

[seaken]

Ovalflex would not the correct choice. Maybe he mis-spoke? Was he referring to RectangleFlex? That dimension is just about what RectangleFlex is so there is not room for pulling it through with the flue tiles intact. Either remove the flue tiles or look for another acceptable liner. Have you checked in to poured liners? Or a liner sealant treatment? There are more options than steel. If you want steel you will have to remove the existing flue tiles, then install the steel liner with insulation.

You might be okay with 5-1/2" liner, or the blocked liner you mentioned. But the risk is poor perfromance due to restricted cross-section. I would check with the manufacturer before you decide to use a smaller than 6" liner or a 6"liner blocked to fit.

The tees and thimbles on the best liner systems have removal parts that can be assembled through the existing thimble. But I would choose to open up the masonry and then patch it after connection, rather than do as you described with refractory cement.

 

[elkimmeg]

what about cuttin in an ash cleanout door on the opposide ot the thimble location. that way there yo could have room to work to transition the bend into the thimble
or cut out a work area and brick it back up after the project is completed on the opposite side. I know is more work but it gives you the oppertunmity to get your hands in there and install the elbow to the cleanout on the opposite side Many times this is the only way to get the job done. If a cleanout then what about a tee and actually use that cleaniout access just for that purpose.

Wrench had it right if cutting out either a grinder witha masonry wheel of drill a lot of holes and punch it out. Damage control

 

[suemarkp]

The oil furnace had a 7" metal pipe the just butted up against the clay vertical tile (the 5 3/4" skinny side of the tile). Then there is tile up the chimney for about 25'. There appears to be a concrete block wall below the flue and there is no cleanout, but the voids in the block appear to be open. The HVAC contractor jammed in a piece of 6" B Vent and cemented that to the wall when the furnace was removed. This is my thimble. I don't know what type of cement was used or how well sealed this is to the 5 3/4" side of the clay flue (but there's an obvious size mismatch and smoke could get between the clay liner and the B Vent).

Is a cleanout required in the thimble area? I thought there wasn't much ash build up there and you just disassembled and vacuumed out every so often? Installing a cleanout may or may not be practicable. The fireplace has a cleanout low in the block wall about 3' to the left of the oil chimney. If the holes in the block aren't filled in with mortar, a cleanout door could be installed below the thimble on the inside wall (just like the fireplace). But I thought the thimble downward part should be sealed or clay tile, and it couldn't be for this to work. Cutting an ash door opposite the thimble would be tough -- this is about 3' under ground and I don't want to compromist the chimney foundation (5' x 2' x 25' of brick is a lot of weight to be working under).

Isn't a 5 1/2" square steel liner equivalent to a 6" round in cross sectional area and performance? If I center this in the 9 3/4 opening, I could pour liquid insulation down two sides. The other two sides would be too close to the tile for insulation to work its way in. Should I bother insulating if that is the best I can do? It doesn't get that cold here (rarely less than 20F, and almost always over 30F during the day). Is insulating the liner mandatory or just a recommendation?

I have a hammer drill (good idea), and would use a chisel between the sledge and the wall... It just seems these thimble Tee's are so tall that I'd have to remove at least a 6x12 or larger area to get it in. And the tee would have to be squareized too in order to stuff it inside the 5 3/4" clay tile. I'll have to see if they can squareize the tee too.

I"ll see what I can do about a picture. It is not easy to take a picture of because the 6" cover has one screw to the B vent and I can only pull it out so far to peek inside.