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Fireplace Refurbishment. getting ready for winter.
I like a good wood-burning fireplace in the house. True, wood smoke is one of the more noticeable local air pollutants, and the SF Bay area has no-burn days when the air is bad maybe a dozen times a year, but fire has a place deep in our human needs and a house really isn't home without a hearth. We all deserve to make a few cozy wood fires on cold nights with friends each winter.
The promblem was, decades ago someone painted over the bricks in our fireplace and the damper was completely rusted shut. The fireplace was used just a handful of times. I have never seen a flue so clean, except for all the cobwebs. Perhaps the flue didn't draw well; the smoke chamber was poorly shaped, and the scope and cost of this refurbishment task would have made it an unattractive project to anyone not in tune with their primal need to have fire. There are high temperature paints meant for use on fireplaces but I don't like the look, and I have no idea if the underlying paint was the right type. I wanted bare brick, and only the original brick - not a facade or total rebuild.
First, I cut out the rusted-shut damper. I used an angle grinder with a cutoff wheel (several, actually) to get through part of it and then for lack of access went out and bought a powerful sawzall and had the damper out in no time. At first I had envisioned building a new custom damper for the smoke chamber, but it would be very difficult to install (usually those dampers go in as the fireplace is being bricked up) and there is a superior product anyway for these high-flow fireplaces: flue-top dampers operated by a pull cable down the chimney. They are not adjustable (actually one model is, but I've read some poor reviews) but they keep out bugs, birds, and rain when shut and they provide a good air seal since silicone rubber can be used up there farther from the fire.
I bought a Lock-Top type top sealing damper with a spark screen and rain cap, ordering it online from a place in Michigan. I have never seen such shoddy construction combined with so clever and elegant a design! Some parts were bent out of shape and didn't fit, one piece was missing completely, the materials were poorly formed, and the spring tension device for the pull cable simply didn't work because a threaded shank bolt was meant to slide inside a tube and the threads kept catching. I bought a carriage bolt of similar size with an unthreaded shank, ground the head down to fit inside the tube, and reassembled the device. It worked beautifully. After a general tune-up the damper was ready to install.
In a cardboard mold I cast a cap of mortar at the top of the chimney to receive the sweeps' ring, a metal mounting ring that allows removal of the damper to access the flue for cleaning. The instructions said to adhere the ring to the chimney using silicone sealant, but I wouldn't trust that not to detach after a period of time so I drilled holes in the ring, set it on the mortar pad, and sunk flat head stainless steel wood screws into the freshly poured mortar to secure the ring to the house.
It was time to get the paint off the fireplace. I had begun by scraping paint off the hearth which took many, many hours of chipping away while wearing a respirator and eye protection, and I quickly found that I was unwilling to put in the time to scrape the rest of the fireplace. My hope was that with the hard paint scraped off, I could use a small air compressor to lightly sandblast the paint-flecked masonry to a fine finish rather than use a more powerful tool to strip the paint. Having finished scraping the hearth but given up on the firebox and facing brick I called a blasting shop and learned that they would not work in an occupied house due to health hazards from the dust. Their website showed a picture of a guy in a space suit, hauling an air line the size of a firehose, and from that I understood what they meant.
I knew that a properly built tent would contain the dust, if the operator knew what he was doing, so I decided to do the job myself. I bought a pressure-pot abrasive blaster and rented a big diesel trailer air compressor; they each ended costing about $150 and I sold the blaster afterwards. I even put the used paint-flecked sand up online for free, with attendant warnings about lead and paint, and a guy called within minutes and hauled it away to bed pavers!
The job took me six hours and was extremely dusty, but the containment tent worked great once I sealed myself inside. I opened a small hole in the tent so the chimney could draw air and purge the dust when the wind was right. I went through three sets of respirator cartridges and 250 pounds of kiln dried, mesh screened sand. I had tried re-using sand at first, but the second time through it was extremely dusty such that I had trouble seeing the work surface after just seconds of blasting. The sand stream was very powerful and it took practice not to erode the brick and mortar too much; also, my face shield became progressively more frosted over and it became hard to see. That is what eventually made me call it quits - not being able to see. I probably would have otherwise spent another hour going after little flecks of paint in the mortar joints.
I swept up the sand, vacuumed everything, and swept the chimney using a telescoping window-washer's pole with a bristle brush taped to the end. Then I took down the tent, installed the damper, and it was done! Almost. After I removed the old damper, I decided I would sculpt the smoke chamber to reduce eddies in the flow and help smoke go smoothly up the chimney. Before blasting I did light off a pile of crumpled newspaper to verify that the flue will draw well in the first place, but as with almost everything I had plans for improvement.
Despite what Vrest Orton said in "The Forgotten Art of Building a Good Fireplace", a book about the design of Rumford fireplaces, the smoke shelf does not promote circulation of air within the chimney. Its purpose is rather to present an obstacle to gusts of wind coming down the chimney so they do not scatter the ashes from the hearth into your living room. It also makes sure sooty rainwater drains into the rubble fill behind the fireplace rather than into your living room (note: if you don't have a rain cap, a smoke chamber like mine needs a drip edge feature inside or water will run down the inside wall into the fireplace). Otherwise, you would want as straight and smooth a path as possible for the smoke to vent from an open-hearth fireplace such as this. It isn't efficient because a great quantity of warm indoor air is entrained with the smoke, but this is what keeps smoke out of the house.
Smoke and air entering the fireplace will flow up the inside wall of the smoke chamber and accordingly this surface must be smooth and free of ledges and steps that will create eddies and impede the flow. I also believe that a sharp corner on the smoke shelf at the back wall of the fireplace will not perform as well as a generously rounded edge, so I custom cast bullnose bricks from refractory mortar. I used HeatStop 50, a costly dry mix refractory mortar containing water-curing cements. The 50-lb bag cost me almost $100 at a building supply in Redwood City, and I used most of it up. I smoothed all the sides of the smoke chamber and covered the exposed red brick with the same refractory mortar, and backfilled the smoke shelf with gravel. The space behind the fireplace is filled with rubble rather than mortared brick for low cost and also to allow water to drain down to the ground. Of course my new chimney cap and top sealing damper will keep rain out most of the time.
With the fireplace finished, the house can have a cozy fire for the first time in decades. As long as there is heat, i.e. flame or a bed of coals, this flue draws every wisp of smoke out of the house and when the damper is closed, the flue is nearly airtight which is important because my kitchen and bath vent fans would otherwise pull lots of air down the chimney.
Watching sparks racing skyward up the flue, I can see the path of the airflow. It glides up the back wall of the fireplace and separates cleanly on my new bullnose bricks, then skims against the inside wall of the chimney and swirls up the flue. A few sparks veer over the bullnose into the smoke shelf but then they quickly streak onward up the chimney. So much air goes up the chimney that it takes more than an hour for heat to be felt radiating from the masonry, but this firepalce was never built for heating. It provides a cozy crackling fire, a warm orange glow on a rainy evening, and then for days afterward, the toasted charcoal aroma of burned wood from the fresh cinders on the hearth. This is what I remember from winters back home when I was young, and it's nice to have it back again.
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