Large masonry fireplace to alcove conversion

[rustyneil]

Greetings, I'm a new member converting a large masonry chase into an alcove for a freestanding wood stove. I'm sharing my project here in hopes of getting feedback on several considerations: 1) stove position and insulation, 2) protecting a wooden mantel, 3) venting the lid, and 4) stove pipe clearances.

I purchased a Hearthstone Green Mountain 60 to heat less than 1200 sf of our 1980's home. Professional installation will include the stove, class A chimney and boiler flue and a new cap. I'll be doing most of the rest myself as part of a larger renovation including all interior and exterior surfaces. Aesthetic goals and the need to repair extensive water damage to the fireplace, framing, and masonry motivated us to gut the chase completely (see picture). The interior of the chase is approx. 20" deep by 80" wide.

I plan to keep everything in the walls noncombustible below the mantel and behind the columns. The walls will be thin stone veneer and plaster on durarock, supported by metal studs. The walls will be filled in with mineral wool insulation without any air gap. I'll describe some of the constraints, but I'd welcome any input on how to get more insulation directly behind the stove (or how big a concern that really is). I'm happy to have the stove set forward and use a pad for ember protection on the floor. As shown in the section, assuming an offset, the rear wall thickness seems to be constrained more by the stove pipe than the stove itself. It'd be nice to build the new walls plumb and level, but I could slope the rear surface parallel to the existing wall for a little extra space at the bottom (see note below). At the lid, the pipe will transition to a class A chimney within a 12" square chase extending about 2' up to a platform at the base of the second floor (from there it will continue to the cap through about 12' of unconditioned space in the chase). I plan to frame the lid structure of the alcove with wood unless noncombustible materials at this height would serve a purpose.

The biggest issue seems to be the wooden mantel. We would really like to use some sentimental old white oak beams reclaimed from a family home for the mantel and side supports. A metal heat shield should help us achieve enough vertical clearance above the stove (about 24"), but the mantel might be within 10" of the stove pipe. If the heat shield extends up the back as shown, is clearance still a concern? The overall mantel width is over 60" and the stove is 30" wide. How wide would the heat shield need to be?

The top of the alcove has nearly 16" of vertical space behind the mantel. I plan to install horizontal vents at each end of the alcove which will extend upwards to exit the face near the ceiling. By default, the grills would each be 6" tall by 14" wide, but the throats would only be 3-1/4". Does this seem like enough ventilation? I could widen the intakes in the alcove, but aesthetics are a constraint on the face.

Thanks in advance for any input. I've really benefitted from the knowledge already shared here and tried to summarize what I've learned about alcove construction in a separate post to offer an easier starting point for others.

Note: the exterior wall of the chase is only 4" block with cultured stone veneer and was built as much as 5" out of plumb over 20'. This seems to be because the house itself was built almost 3" out of plumb and they bellied out the wall in the middle to accommodate the masonry/tile chimney. Removing this weight and the 8" concrete cap was another reason for gutting everything. The chase is built on a poured concrete foundation wall. The chase is not mechanically connected to the framing, yet there was no separation between the masonry and house siding. No one we've consulted has raised enough concern not to leave it as is (apart from some repairs to the upper courses) and to run new fabricated flues through it.)

 

[MedicineMan4040]

True masonry heater, as in 2-4 tons in weight? I'd only consider a true masonry stove if building new and could put it in the center of the house AND even if I had a masonry stove (most you can only fire up 2 times a day) I'd still want a wood burning stove.

 

[FTG-05]

Don't do it. It won't work. You'll end up burning your house down and killing your wife and kids.

Proof:

It's 23 American degrees outside right now and I've been rocking this Englander since around 6 am this morning. Around 78 inside, nice and toasty!

 

[Cyounce1]

Don't do it. It won't work. You'll end up burning your house down and killing your wife and kids.

Proof:

It's 23 American degrees outside right now and I've been rocking this Englander since around 6 am this morning. Around 78 inside, nice and toasty!

This is kind of how my set up looks right now. I only have durock installed now. It was done by a local company. My only issue right now is floor to ceiling clearance with all the stove brands. Most stoves floor to ceiling clearances are like 84". Did you durock a ceiling in the opening you have pictured or is your ceiling higher up inside the wall? I think my opening ceiling height is right at 64". If going by stove manuals only the green mountain 40 will work in my house. Any input helps, thanks.

 

[stoveliker]

Durock on the ceiling with wood joists behind it won't help with clearances because clearances are to the nearest combustible. So it would increase the distance by 1/2" by removing the (paper facing of the) drywall.

 

[FTG-05]

This is kind of how my set up looks right now. I only have durock installed now. It was done by a local company. My only issue right now is floor to ceiling clearance with all the stove brands. Most stoves floor to ceiling clearances are like 84". Did you durock a ceiling in the opening you have pictured or is your ceiling higher up inside the wall? I think my opening ceiling height is right at 64". If going by stove manuals only the green mountain 40 will work in my house. Any input helps, thanks.

The inside is as high as the outside top of the Durock you see in the 2nd pic. Plus there is several layers of Durock on the inside plus the back and sides. The base has four layers to meet the 30-NC floor temp resistance requirements. The bottom of the hearth has 6 layers although the 2nd pic only shows two. Four more were added by the rock installer to match the rock layout.

The 30-NC is the only thing we use to heat our home.

 

[Cyounce1]

The inside is as high as the outside top of the Durock you see in the 2nd pic. Plus there is several layers of Durock on the inside plus the back and sides. The base has four layers to meet the 30-NC floor temp resistance requirements. The bottom of the hearth has 6 layers although the 2nd pic only shows two. Four more were added by the rock installer to match the rock layout.

The 30-NC is the only thing we use to heat our home.

Awesome thank ya

 

[Dieselhead]

Don't do it. It won't work. You'll end up burning your house down and killing your wife and kids.

Proof:

It's 23 American degrees outside right now and I've been rocking this Englander since around 6 am this morning. Around 78 inside, nice and toasty!

That came out very nice!

 

[begreen]

I was concerned about the heat trap created behind the mantel, but it looks like vents have been added. That will help. It's going to get hot up in that pocket.

Will there be power in the alcove? At some point you may want to add a blower to the stove to improve convection out of the space. Make sure that the boiler vent pipe clearance requirements are also honored. It might be good to chase it in 6" round pipe for better servicing in the future should that be required.

 

[rustyneil]

This is kind of how my set up looks right now. I only have durock installed now. It was done by a local company. My only issue right now is floor to ceiling clearance with all the stove brands. Most stoves floor to ceiling clearances are like 84". Did you durock a ceiling in the opening you have pictured or is your ceiling higher up inside the wall? I think my opening ceiling height is right at 64". If going by stove manuals only the green mountain 40 will work in my house. Any input helps, thanks.

Don't do it. It won't work. You'll end up burning your house down and killing your wife and kids.

Proof:

It's 23 American degrees outside right now and I've been rocking this Englander since around 6 am this morning. Around 78 inside, nice and toasty!

That's really impressive! Thanks for sharing it here. Seems like one of the more comparable installs to ours. Is the rear wall exterior? Nice adding some extra mass. I've had to keep the walls thinner than I hoped for fit and prioritized insulation where possible. Glad the new set up is working well for you!

 

[rustyneil]

I was concerned about the heat trap created behind the mantel, but it looks like vents have been added. That will help. It's going to get hot up in that pocket.

Will there be power in the alcove? At some point you may want to add a blower to the stove to improve convection out of the space. Make sure that the boiler vent pipe clearance requirements are also honored. It might be good to chase it in 6" round pipe for better servicing in the future should that be required.

Thanks for your input @begreen . Finally got to a point after the holidays where I can try to resolve the venting before the stove gets installed. I framed a new wall in front of the old so I could accommodate different ducting/venting configurations. I'm also keeping everything unfinished until the spring so we can easily adjust things as needed. I plan to monitor temperatures closely once we start burning so we can make any necessary changes. Obviously we want to make sure things are safe but satisfying my wife's aesthetic demands is also important. I'm interested in knowing how you would evaluate the options below.

Option 1 (strip vent immediately above mantel) for venting the air space behind the mantel seems like the simplest and most direct, but also the least desirable. I currently have the wall framed for this option so I could simply cut the drywall off to open up the vent and fire the stove.
Option 2 (side vent) with the vent(s) to the side is the preferred approach. The question seems to be how well the air would flow laterally (up to 8ft from far end to the vent). It would be nice to only put in one larger vent, but we could put one on each end if necessary. Also unclear is how large would the vent(s) should be.
Option 3 (strip vent at ceiling) would also be fairly simple but less desirable aesthetically. I also wonder how much placing the vent right at ceiling level would dampen the air flow - even though it would have the greatest chimney effect.
FWIW there will be passive ceiling vents just beyond each of the two ceiling beams. These will vent to two bedrooms on the second floor but may not always be open.

The air space behind the mantel is 12" high and about 9cf. The mantel is over 57.5" above the hearth. GM60 (30" tall) requires minimum 61.5" alcove height, thus the heat shield. Apart from the wood posts and mantel, all wall materials from surface to exterior block are noncombustible.

Also: Yes, I installed a receptacle to support a blower if needed. Regarding insulation around the boiler vent, I opted to completely fill the cavities with mineral wool based after talking with our stove installer. Our boiler is nearing end of life and the HVAC folks said any newer system could just as easily vent out the wall so they weren't too concerned either.