First Fireplace - Chimney Design Question.

[BillConn]

Hi All,
This is my first post here. I am in the planning stages of installing a new woodburning (open hearth) fireplace in our home. I will be installing/framing the fireplace myself as well as assembly/installing the chimney pipe.

First a little background:
My wife really wants a fireplace in our house. It was one of the concessions we made when purchased this home 5yrs ago. Our previous house had a decorative wood-burner that we used quite often for extra ambiance in the winter months. So the purpose of this new fireplace is purely for ambiance and aesthetics. We have a furnace & central air for our actual heating needs, so efficiency is obviously not a concern (Hence the open hearth).

For budget and design purposes we will be going with a factory built pre-fabbed unit. We are still deciding between the 36in Majestic Sovereign or the 36in Majestic Biltmore fireplace, but it will be one of these two most likely.

So now, for the question/concerns related to the chimney height/design. The location we plan to install the fireplace is not 100% ideal as it is not directly in the center of the house and as such the chimney will not exit the attic at its highest point. After a good bit of research online, I felt pretty good about our plan and understood the trade-offs in draft performance. The fireplace will be installed against an interior wall within the heated/insulated envelope of the house. The chimney will go straight up (with the possibility of one single offset pair to clear a truss) and enter into the un-insulated attic space and then protrude out of the roof. The challenge with our location is that the chimney protrusion will be offset more towards to the exterior of the house as opposed to the very center (highest roof point). Also to maintain proper min. chimney lengths as well as the 10ft-2ft rule, we will need approximately 6.5-7.5ft of chimney extending above the roof protrusion.

When I discussed this location/plan with the fireplace shop they seemed to imply that this location was terrible and that the chimney protrusion above the roof was WAY too tall and that a chase would 100% be required on the roof and even then, it would probably smoke a ton inside due to poor draft. Unfortunately there is no other location for this fireplace that makes sense in our living room, so picking a more central location isn't really a possibility. I also would prefer to avoid constructing a roof mounted chase as it will add good bit of additional cost to the project and isn't needed for aesthetics as the chimney will be on the backside of the house and not visible from the street or front of the home. If it matters, I also plan to duct an outside air intake kit to help supply the firebox with air and improve draft.

I have attached a simple graphic (drawn to scale) highlighting the details of my planned chimney layout/dimensions. Do you hearth.com experts see any significant concerns? There are no vaulted ceilings to contend with, the chimney leaves the insulated home envelope at the highest point, the only real issue is the rather long portion of chimney "exposed" above the roof surface. Will 6-7ft of "unprotected chimney ruin my draft?

 

[xman23]

What's there reason for the poor draft? Not enough chimney length and to much not within the house?

 

[jetsam]

The proposed flue may be too short. It needs to be at least 2' taller than the tallest point of the roof within 10'. (The masonry chimney does not need to be this tall if all it's doing is containing the stainless flue.)

Not sure about the chase, but 8-9 feet of flue pipe above the roof is a large amount and will at least require some support, which is going to be visible no matter how you do it.

The exhaust system in the drawing poses no draft concerns to me, though.

 

[BillConn]

What's there reason for the poor draft? Not enough chimney length and to much not within the house?

He wasn’t super clear as to the reason. He alluded to poor draft due to too much chimney height not within the house.

The effective chimney length would meet the fireplace manufacturers “minimum” requirements.
He did say specifically that “The manufacturer (Majestic/Heat N Glo) doesn’t sell roof reinforcement kits for their fireplaces/chimneys because you should never have enough exposed chimney pipe to need roof support”.

The proposed flue may be too short. It needs to be at least 2' taller than the tallest point of the roof within 10'. (The masonry chimney does not need to be this tall if all it's doing is containing the stainless flue.)

Not sure about the chase, but 8-9 feet of flue pipe above the roof is a large amount and will at least require some support, which is going to be visible no matter how you do it.

The exhaust system in the drawing poses no draft concerns to me, though.

The flue as shown meets the 2-10 rule. It needs to be just over 6ft for that requirement. I then need an extra foot at most to meet the manufacturers min length requirement.

The max flue height above the roof surface will be 7ft. I would certainly add roof support struts as the manual says anything beyond 5ft outside the roof should be supported.
Will that much flue outside of a chase guarantee poor draft? Our Portland, Oregon climate is reasonably mild in Winter. Typically high 30’s Fahrenheit for the low temps....

 

[jetsam]

He wasn’t super clear as to the reason. He alluded to poor draft due to too much chimney height not within the house.

He did say specifically that “The manufacturer (Majestic/Heat N Glo) doesn’t sell roof reinforcement kits for their fireplaces/chimneys because you should never have enough exposed chimney pipe to need roof support,"

Will that much flue outside of a chase guarantee poor draft?

Use insulated flue pipe and heat loss will be less of a problem.

I'd agree with him that a chase or tall masonry stack will give the flue better protection from wind, and mechanical support.

I don't think that 7' exposed automatically means bad draft, but it will definitely run cooler than a flue that is out of the wind. A cooler pipe does mean less draft and more frequent sweepings.

If I had 7' of uninsulated pipe above the roof and typically poor first-season-burner wood, I'd probably start out sweeping it once a week.

So yes to poorer draft due to exposed pipe, but it could be mitigated by insulation, dryer wood, hotter burns, and windproofing (a chase or chimney).

 

[xman23]

All of our chimney's end up with a good amount of chimney above the roof line. Mine is about 10' of double wall class A, no chase. There are 2 braces about 6'. It might not be the prettiest, but for my county cabin it's fine. We don't even notice it. There are very few issues with this type chimney. For me its function over form. A chimney chase on the roof adds a bigger roof penetration, flashing and water dam for the water above it. Plus maintenance. But if you need it for cosmetic reasons go for it.

If your going straight up I doubt you'll have a draft issue. The guys here that have startup draft issues have multiple bends, and all outside chimneys. Fireplaces being open, sends all the heat uncontrolled up the chimney. You will have a much hotter chimney than us wood stove burners. That said have you looked at a stove? They are the fireplace you want and heats the house. All with far less wood consumption My wife wanted a fireplace, and now loves the stove.

 

[BillConn]

Use insulated flue pipe and heat loss will be less of a problem.

I'd agree with him that a chase or tall masonry stack will give the flue better protection from wind, and mechanical support.

I don't think that 7' exposed automatically means bad draft, but it will definitely run cooler than a flue that is out of the wind. A cooler pipe does mean less draft and more frequent sweepings.

If I had 7' of uninsulated pipe above the roof and typically poor first-season-burner wood, I'd probably start out sweeping it once a week.

So yes to poorer draft due to exposed pipe, but it could be mitigated by insulation, dryer wood, hotter burns, and windproofing (a chase or chimney).

Unfortunately true insulated flue pipe is not an option with these open hearth pre-fabricated units. It seems you are pretty much locked-in to using the matched flue system from the same manufacturer (air-cooled double wall). Otherwise I’d be considering something Dura-tech insulated Class-A.
Luckily, I have access to plenty of well-seasoned, dry wood... and since our use will be limited, I won’t be burning a ton of it anyhow.
I think I will only pursue a chase if I suffer from draft issues after we use it for the first winter. It isn’t too much too much harder to add a chase after the fact, compared to doing it from the get-go.

All of our chimney's end up with a good amount of chimney above the roof line. Mine is about 10' of double wall class A, no chase. There are 2 braces about 6'. It might not be the prettiest, but for my county cabin it's fine. We don't even notice it. There are very few issues with this type chimney. For me its function over form. A chimney chase on the roof adds a bigger roof penetration, flashing and water dam for the water above it. Plus maintenance. But if you need it for cosmetic reasons go for it.

If your going straight up I doubt you'll have a draft issue. The guys here that have startup draft issues have multiple bends, and all outside chimneys. Fireplaces being open, sends all the heat uncontrolled up the chimney. You will have a much hotter chimney than us wood stove burners. That said have you looked at a stove? They are the fireplace you want and heats the house. All with far less wood consumption My wife wanted a fireplace, and now loves the stove.

I sorta figured this chimney layout situation cannot be that unusual. I was pretty comfortable with it until I chatted with the guy at the fireplace/stove store.

I agree with all your “negatives” regarding the chase which is why I want to avoid it! Our house backs ups to 40-acres of forest so I don’t really care how the chimney looks on the backside of the roof.

As for a straight shot up, further measuring in the attic has indicated that I will need a single short offset/return (no length between elbows) to provide proper truss clearance.

I also tried the stove approach as well, but the wife demands OPEN hearth with a large view (and the sound) of a crackling fire. We really don’t need supplemental heat as our winters are so mild so efficient is not enough to sway her mindset.