Height below the thimble...does it affect draft?

[avc8130]

Curious, does the distance BELOW the thimble have an effect on draft? My chimney goes from basement up through the roof of my ranch. My stove thimble is roughly 60% up the chimney on the main floor. There is a cleanout about 3' off the floor in the basement. So there is approximately 10' of chimney below my thimble. Would Roxul insulation below the thimble provide any benefit?

ac

 

[begreen]

If there is air leakage below the thimble it will affect draft. Roxul might help, but it's not really an airblock. Gasketing the cleanout door sometimes makes a big difference. Try sealing it up with duct tape. If that makes a big difference, you have your answer. Use some silicone on the door sealing edge, let it set for 5 minutes, then close the door and latch it to form a gasket.

 

[fossil]

If you're exhausting your appliance through the thimble and then directly into the masonry flue (no liner), then anyplace beneath the thimble where the chimney structure is not airtight could affect the draft, but I can't imagine it would be a major factor. More significant would be the change in cross-sectional area of the flue between thimble and chimney. If your chimney flue has a liner installed from the thimble up to daylight, then your draft shouldn't really be affected by anything down in the chimney. I'm (obviously) making some assumptions here, but an informed answer to your question begs for more information about the flue/chimney arrangement you're operating. Rick

 

[avc8130]

If you're exhausting your appliance through the thimble and then directly into the masonry flue (no liner), then anyplace beneath the thimble where the chimney structure is not airtight could affect the draft, but I can't imagine it would be a major factor. More significant would be the change in cross-sectional area of the flue between thimble and chimney. If your chimney flue has a liner installed from the thimble up to daylight, then your draft shouldn't really be affected by anything down in the chimney. I'm (obviously) making some assumptions here, but an informed answer to your question begs for more information about the flue/chimney arrangement you're operating. Rick

Rick,

Thanks for the info. Really this was more of a "mind exercise".

My personal setup is as follows:

Masonry chimeny from basement to 3' above the roof in the center of the ranch house.
Flue is 6.5x6.5 internal clay tile.
Cleanout is in basement ~3' from the floor with a steel door.
Jotul 500 is on the 1st floor.
Jotul has ~4' of single wall to a 90 fixed elbow into a 6-8" expander through the thimble and into the masonry chimney.
Approximate height BELOW the thimble is 10'.
Height ABOVE the thimble is 12'.

I am not 100% happy with my draft, but it isn't miserable with good wood.

ac

 

[fossil]

Interesting. That means your flue gases see a conduit of ~28 in² for 4 ft, then a 90° into ~50 in², then another 90° up into ~42 in². My uneducated guess is that the flue path has more to do with your occasional draft concerns than anything going on beneath the thimble penetration. Just make sure the ash cleanout door is well sealed up. I don't think there's really anything else to be done down there provided the masonry's all in good shape. Rick

 

[avc8130]

Interesting. That means your flue gases see a conduit of ~28 in² for 4 ft, then a 90° into ~50 in², then another 90° up into ~42 in². My uneducated guess is that the flue path has more to do with your occasional draft concerns than anything going on beneath the thimble penetration. Just make sure the ash cleanout door is well sealed up. I don't think there's really anything else to be done down there provided the masonry's all in good shape. Rick

Rick,

That's an interesting point. I never actually looked at it that way. I wonder what it would be like if I ran the 6" pipe into the masonry flue area and packed Roxul or something inside the "expander". Then it would be a straight change from 6" round to the 6.5x6.5 clay.

ac

 

[fossil]

In general, I'd say anything you can do to provide as smooth and straight a path as you can with a cross-sectional area as close to equal that of the flue collar on the appliance as feasible is the direction in which to focus whatever efforts you choose to put into seeing what you can do for draft. After you've you've done everything you're going to do in that arena, then adding more height to the chimney is about all I can think of that's left. Rick

 

[avc8130]

In general, I'd say anything you can do to provide as smooth and straight a path as you can with a cross-sectional area as close to equal that of the flue collar on the appliance as feasible is the direction in which to focus whatever efforts you choose to put into seeing what you can do for draft. After you've you've done everything you're going to do in that arena, then adding more height to the chimney is about all I can think of that's left. Rick

Rick,

Trust me, I realize a 6" SS liner would be my best setup. I know you must be biting your tongue not to suggest it.

There is a chance I might be moving the stove to the back side of this chimney to get it in the living room rather than the "den" when I redo my kitchen. That change is keeping me from lining now. When I have a firm decision on that, I WILL be lining the chimney.

I do have to re-do the fake stone on my chimney this summer. I am also considering adding another tile or 2 to the total height just for kicks.

ac

 

[HotCoals]

Getting rid of the 90 and using two 45's may help draft also.

 

[avc8130]

Getting rid of the 90 and using two 45's may help draft also.

I've considered that. Do 2 45s take up more horizontal room than 1 90? I don't want my stove jutting out into my room any more than it absolutely must.

ac

 

[fossil]

...I know you must be biting your tongue not to suggest it...

Carry on, you know where you want to go and how to get there. Thanks for being a contributor here on these forums. Rick