Smoke Control

Sometimes homeowners contact me about smoke flowing out of the front of their fireplace rather than out the chimney.  I stand behind every product, so I traveled to two or three sites this summer to meet with homeowners to investigate what was going on, especially if my engineering was faulty or if there was a product defect.  In every instance, the fireplace smoke control problem was easily identified and addressed. One of my jobs is educating others about tips and tricks to make your outdoor entertaining more enjoyable, so I want to share want I learned with anyone. 

Every fireplace will tend to smoke out the front while the flue is warming up or cooling down.  That is a principle of thermodynamics. Heat rises, but smoke has some weight that is influenced by gravity and air pressure, rising heat creates an advantage at the top of the chimney to draw more smoke up the flue.  

Round Grove Products are engineered to industry-standard ratios of firebox-area-to-chimney-height to create positive pressure up the chimney, but even the best engineering cannot overcome limitations that are introduced at the homeowner’s location.  Here are some common limitations that homeowners and contractors need to consider before installing a fireplace:

1. Fuel – the best engineering cannot overcome the amount of smoke generated by green or damp firewood.  One simply does not put regular gasoline in a top-fuel dragster; nor should one burn wood that is damp or green.  One contractor admitted to picking up random twigs lying about the property and the resulting smoke was overwhelming.  Another dealer burned pallet wood and trash in their firebox. Both instances resulted in sooty smoke that blackened their exterior finish.  In each instance this summer, when I fired using seasoned hardwood, there was little to no smoke migration out of the firebox. Round Grove recommends burning seasoned hardwood.  Burning pine or other softwoods, bark, newspaper, or trash (do I really need to say this!?) will create undesirable smoke.  
2. Structures – the top of the chimney should be at least 24” higher than any other structure within a 10-foot radius.  This is a minimum recommendation. One of the locations I visited was a two-story home with a walk-out basement, so the fireplace was well below the roof ridge.  Air was flowing over the ridge and straight down the chimney. By extending the chimney higher than the minimum recommendation, the draw was improved. Ultimately, however, the homeowner opted for a chimney duct fan that addressed the downdraft condition from over the taller structure.
3. Tree Canopy – this same issue can occur near a wooded area.  Air flow cascades over the top of trees and straight down the chimney or in a wooded area a layer of still air under the tree canopy can inhibit air flow over the top of the chimney, holding the smoke under the trees.  I also observed this condition at a home located adjacent to an overhanging bluff. Smoke travelled up out of chimney, then immediately cascaded back down to the ground. Again, a duct fan was the alternative solution to address this issue.
4. Shear Airflow – I see this issue at my own home.  We have a U-shaped patio with the fireplace on the open end.  Airflow deposits leaves in the corners of the patio and airflow across the front of the firebox will occasionally draw smoke and ash out.  This is wind shear and should be addressed with a firebox screen or perhaps a smaller firebox opening. 

That’s an overview of how I addressed what I observed of the fireplace smoke control this summer.  Our product is engineered correctly, since every site has its own limitations, Round Grove’s material warranty excludes chimney draw performance or smoke from rolling out the front of the firebox opening.  The contractor and homeowner must identify site-specific limitations that might influence chimney draw, but there are several alternative solutions to address these limitations:  

1. Size the firebox based on air flow and wind shear—build a test fire on-site and watch how the smoke behaves
2. Round Grove recommends burning seasoned wood
3. Round Grove recommends chimney height at least 24” taller than any nearby structure
4. A firebox screen can inhibit wind shear
5. If seasoned wood is not available or if the amount of smoke from seasoned wood is unacceptable, then Round Grove recommends a gas log option or a gas burner feature
6. Alternatively, a chimney cap fan or duct fan will induce a stronger draw, but again this degree of engineering may not overcome poor fire craft.

I hope this information helps improve your understanding of fireplace smoke control and ultimately the enjoyment of your backyard entertainment space.