Free standing Old Timer 6in to 8in chimney increaser help!

[aspenbjune]

HELP!

Hi y’all, I really don’t wanna burn my house down. But if I get another 400$ electric bill I’m gonna lose my mind. SO. I have an old timer, set me back 100. She’s got a 6in chimney. My house is equips with an 8in double wall pipe from 4in below the ceiling up threw the roof. (I’ll clean it prior to installation) I can’t figure out what would be the best air flow. Option A) get like 20ft of single wall pipe to put threw the entire length. (Making above the crawling triple wall) and then just slapping an adapter on to close up the gap. OR. Option B) put an adaptor for the 6in-8in difference. The idea is Option A would be easier to clean and extra safe. But Option B might be better for the efficiency of the stove. What do you think?
TIA,
Sincerely,
A DIY loving first time home owner in over her head.

 

[coaly]

There are a few reasons why you should not and cannot run a inner flue pipe inside the existing chimney.

First, it would need to be stainless steel. It will condense water vapor from combustion inside rotting away very quickly. Black connector pipe is only for connecting stove to chimney.

The inner pipe would be air cooled, allowing hot flue gases to cool excessively. This reduces draft and forms creosote.

Code requires a UL Listed chimney tested to UL 103 HT which the proper chimney becomes a Listed assembly only when installed and used as installed by manufacturers installation instructions. Modifying it would lose the UL Listing wether safe or not.

The only bad thing about increasing from 6 to 8 is allowing the hot exhaust gases to expand in the larger diameter pipe. This cools the exhaust gases containing water vapor from combustion. Wet flue walls allows smoke particles to stick forming creosote.

The object is keeping flue gases above 250*f to the top while smoke is present. Use a magnetic thermometer on pipe as high up as practical to read it keeping it well in the burn zone. The temperature shown on thermometer reads surface temperature, which is about 1/2 the inside flue gas temp. So 250 is actually 500 internal assuming cooling back down to 250 at top. This is only a guide since all venting systems cool differently. Since you are aware of some extra cooling, keep the flue temp higher than the low minimum and check creosote formation frequently until you know how much you are forming.

It is advisable to leave the flue damper open and control the stove with the air intake on stove. The flue damper is a chimney control to slow the draft of an over drafting chimney. It is easy to slow the draft too much with overuse forming creosote.

It is not against code to increase this much. It is against code to decrease, or increase more than 3 times the outlet diameter of appliance.

3 screws are required at each black connector pipe joint.

 

[coaly]

I never saw a chimney support box like that for Class A high temp rated chimney pipe. As large as the outer cover appears it could be triple wall air cooled chimney?? We don’t know what this chimney was installed for. Freestanding fireplaces do not require the high temp rated chimney a stove with controlled air capable of forming creosote does. So make sure this is the proper type chimney by the labels on it that will specify being UL 130 HT rated.

If no labels, pictures of the outside and termination will probably be recognized by the chimney pros here.

 

[rwhite]

It does say Type HT on it. The only thing I could see that was similar was a Selkirk wall thimble. Time to take a trip to the attic and see what you got.

 

[aspenbjune]

Wow, thank you all for your help! This is what I’ve got from the attic. (We plan on sending someone up to move the insulation away prior to burning)

 

[aspenbjune]

And here’s some from when we got our roof inspection,

 

[aspenbjune]

Did regard 3rd photo, I didn’t mean to add that one. 😊

 

[rwhite]

Yeah, I don't see a ceiling support box. I'm leaning towards they used a wall thimble in the ceiling. The chimney looks to be triple wall. Maybe duravent. You may get away with using the chimney but it will definitely need a ceiling box and support.

 

[coaly]

Yeah, I don't see a ceiling support box. I'm leaning towards they used a wall thimble in the ceiling. The chimney looks to be triple wall. Maybe duravent. You may get away with using the chimney but it will definitely need a ceiling box and support.

Yes, the big HT on it should have been my first clue!

Looking close, you’re right about it being a wall thimble instead of a support box. No wonder it looked like a strange large trim ring around a support box.

OP will have to figure out the manufacture of the chimney pipe and get the appropriate support box, and insulation shield. Then the support and chimney system will make sense. (And become a Listed assembly as required)

 

[aspenbjune]

Okay, sounds good. This might be a dumb question, but how would I go about finding out the manufacturer? Is there a typical place that it’s listed? Thank you!

 

[coaly]

Okay, sounds good. This might be a dumb question, but how would I go about finding out the manufacturer? Is there a typical place that it’s listed? Thank you!

The chimney pipes in attic should have labels on them. They normally have an arrow for direction of flow, manufacturer name, and model or part number. As an example, Dura-Vent brand double wall is Dura Tech, compared to their triple wall called Dura Plus.

You cannot mix and match chimney parts from different manufacturers. (The finished chimney becomes a Listed assembly when installed as tested)

The support box will have a starter ring in the bottom of box that matches the top of your chimney pipe. So the first chimney section sets down in the box and supports the weight of chimney. Since all chimney brands are different, and won’t connect together, only the proper one will allow the chimney pipe to twist lock into it.

When the box is installed in ceiling, and pipe is twist locked into it, the insulation shield is similar to your roof flashing the covers the top of the open support box to prevent anything from getting into box (mouse nest) or insulation coming in contact with pipe.

The instructions for support box will give how much the support box must be installed below the finished ceiling. Most are 2 inches. This way the chimney pipe starts below ceiling level and is the only type pipe that can go through a ceiling, wall, or roof. The part you have there is for a wall pass through called a thimble. When chimney pipe is installed in it, this maintains the 2 inch minimum clearance to combustible wall. No idea what is supporting the weight of your chimney. That’s why it’s called a support box. Some manufacturers offer round or square. Square is more common as it is easier to nail or screw to ceiling joists.

 

[aspenbjune]

Gotcha. Thank you guys so much, you've been so helpful. just reiterate and insure I'm understanding correctly, step 1) figure out the manufacturer and model of the chimney. step 2) install the correct chimney support box. step 3) put in a 6-8 increaser, then attach the stove as normal with class A black pipe?

 

[coaly]

Yes. But the black pipe is called connector pipe. It can be single wall if you have 18 inches to combustible clearance. It must be 24 ga. minimum thickness with 3 screws at each joint. Double wall black connector pipe is for close clearance down to 6 inches. Class A is the chimney pipe from support box up.

 

[webby3650]

This could be roof supported and not need reworked. Get up there and moved the insulation away from the pipe. Look up at the flashing from below. If it has the proper 2” clearance you will be able to see a support mounted to the roof deck. Get a piece of sheet metal and make an insulation shield while your up there.