Chimney in 1-story addition on 2+-story (LARGE) house

[selder]

I own a large brick Greek Revival home, built circa 1830. 35x45 footprint, peak of house is approximately 45' tall.

There are four existing chimneys that the previous owner used unlined, with green wood. Crumbling brick inside. I had one lined a few years ago and heated with wood as a bachelor. Upgraded to pellets (thanks to this forum) and now to a propane boiler system. No longer interested in using these chimneys because, in addition to poor condition, they are extremely tall (high draft / difficult to clean), and I've got little kids and would prefer a safer location for a woodstove.

I'm now looking to add a one-story addition which will contain a new kitchen and a wood stove (possible cookstove, but probably irrelevant here).

I'm aware of standard stovepipe placement advice / code for running up through the main roofline, but I haven't found much good information about a one story ell off a 2-2 1/2 story structure.

My concerns are

1) Safety, obviously. In most potential placement, the chimney would be at eye-level with a second story window, albeit 15-20 feet away. This is probably more of a psychological fear than a legitimate issue, I would imagine.

2) Draft. Again, in most placements of my addition, the chimney would be approximately 15-16 ft tall, 15-20 feet away from a 28 ft tall x 45 ft wall wide (eave end, north and south sides) or a 45 ft tall x 35 ft wide parapet gable end wall (east & west). All walls have windows. Given the size of the building, would I be creating some sort of dead zone of airflow that would cause smoke to linger or give draft issues? I've looked at placements where the chimney would be located 20" out from the corner of the main house, which would possibly allow better airflow, but it's an awkward set up.

My goal is to have a reliable source of back-up heat, and supplemental heat that will reach 1-2 rooms of main house. One story due to costs, but also would prefer to have a shorter chimney that I can maintain easier than the 35"+ dog-leg chimneys on the main house.

I hope I've over-thinking this and the house isn't a draft concern if the chimney is 20" or so out. Any advice is greatly appreciated.

Thank you.

 

[RockyMtnGriz]

I'd be mostly concerned with the airflow direction under the prevailing wind when it would be in use, as well as the other common wind directions in your area. Here, for instance, during the heating season, the prevailing wind is SW. Yet, often when it's stupid cold, the wind is out of the E. I'd want anything I built to be good in both, but I wouldn't worry so much about the rare N or S wind. Just a thought.

I wouldn't worry about a chimney cap 15-20 feet away from a window, unless it would typically be upwind, or the window open when the chimney is in use. Again, I'd worry more about the possibility of rotor or eddy currents that might bring an annoying amount of smoke around the building. Another consideration - a clean burning stove creates vastly less annoying exhaust, and might go almost unnoticed where an old school smoker would be a problem.

Draft is going to be the same issue. If something is creating downward airflow at the chimney, draft will suffer. Upward airfllow may increase the draft. It's going to be pretty site-specific, I would think.

Paying attention to how snow or BBQ smoke travels over and around your existing structure would be the first place I'd start.

 

[EatenByLimestone]

Another issue you might find is the house being a better chimney than your chimney is, drafting hard enough to reverse your woodstove chimney.

 

[selder]

Thank you, this is my thinking and the type of response that is helpful.

As you said, I'm hoping/expecting a new, efficient stove will alleviate most potential issues. I will also not be concerned about overnight burns (utilize my boiler system when fire not in use), so I don't see throttling down the stove to a low burn happening very frequently.

Prevailing winds come out of the west here. However, I'm located on a knoll in a somewhat forested valley so I don't usually suffer from high wind issues.

I'd be mostly concerned with the airflow direction under the prevailing wind when it would be in use, as well as the other common wind directions in your area. Here, for instance, during the heating season, the prevailing wind is SW. Yet, often when it's stupid cold, the wind is out of the E. I'd want anything I built to be good in both, but I wouldn't worry so much about the rare N or S wind. Just a thought.

I wouldn't worry about a chimney cap 15-20 feet away from a window, unless it would typically be upwind, or the window open when the chimney is in use. Again, I'd worry more about the possibility of rotor or eddy currents that might bring an annoying amount of smoke around the building. Another consideration - a clean burning stove creates vastly less annoying exhaust, and might go almost unnoticed where an old school smoker would be a problem.

Draft is going to be the same issue. If something is creating downward airflow at the chimney, draft will suffer. Upward airfllow may increase the draft. It's going to be pretty site-specific, I would think.

Paying attention to how snow or BBQ smoke travels over and around your existing structure would be the first place I'd start.

 

[selder]

I think I understand what you mean here -- wind coming against the building, hitting the building, and accelerating upward would create negative pressure, pulling air out of a leaky addition and therefore reversing draft of chimney in said addition?

New construction will be much tighter than the rest of the house, so with rest of house leaky to all directions (not just wind), this might mitigate / equalize pressure? Just a thought.

Another issue you might find is the house being a better chimney than your chimney is, drafting hard enough to reverse your woodstove chimney.

 

[EatenByLimestone]

If the older house us leaky and higher than the new chimney, it effectively drafts harder than your new chimney. Makeup has to come from something lower. This may be your new chimney.

 

[selder]

If the older house us leaky and higher than the new chimney, it effectively drafts harder than your new chimney. Makeup has to come from something lower. This may be your new chimney.

Could this be addressed, at least in part, by a wood stove with an outside-air-intake?

 

[EatenByLimestone]

Best way to address it would be air sealing the top of your house. When it gets down towards zero outside, your house is really moving a lot of air. The more air it moves, the more makup air is needed.

 

[begreen]

Yes, an outside air kit for the stove may help and as Matt pointed out, so will making sure that the upper story of the original house is well sealed. Whether negative pressure becomes an issue or not is to be determined. It could be that the area ends up being a neutral pressure zone. Is there a basement in the main house? Will there be one in the addition?